The biology and control of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae)—A destructive storage pest with an increasing range

J. stored Prod. Res. Vol. 22, No. I, pp. I-14, 1986 Printed

in Great

0022-474X:86 $3.00 + 0.00 Pergamon Press Ltd

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REVIEW THE BIOLOGY AND CONTROL OF PROSTEPHANUS TRUNCATUS (HORN) (COLEOPTERA: BOSTRICHIDAE) A DESTRUCTIVE STORAGE PEST WITH AN INCREASING RANGE R. J. HODGES Tropical

Development

and Research Institute, Storage Department, Berkshire SL3 7HL, England

London

Road,

Slough,

(Received 25 June 1985) Abstract-A comprehensive review is presented on the biology and control of Prostephanus ttwncutu.s (Horn), a major pest of farm-stored maize and dried cassava. Originally indigenous to Mexico and Central America, P. truncafus has recently been introduced and become established in parts of East and West Africa where it has caused severe damage to farm-stored maize. It can cause far greater losses than those associated with “traditional” storage pests. Biological data show that P. truncufus has the potential to spread still further in Africa and to other tropical and sub-tropical regions. Every effort should be made to prevent this happening.

INTRODUCTION

For many years Prostephanus truncatus (Horn) has been known as a pest of farm stored maize in Mexico and Central America (Lesne, 1897; Chittenden, 1911) but only during the last decade has the true potential of this pest become apparent. Giles and Leon (1975) in Nicaragua, were the first to quantify the substantial weight losses that this pest can cause to stored maize cobs. This led British entomologists, concerned with tropical storage, to investigate aspects of the biology of P. truncatus (Shires, 1979 and 1980; Howard, 1983). These studies showed that climatic conditions and competition from other storage pests were unlikely to prevent P. truncatus spreading further in the tropics. This was confirmed by the outbreak of this pest in Africa during 1981 (Golob and Hodges, 1982) and its subsequent spread in Africa (Kega and Warui, 1983; Harnisch and Krall, 1984). The biology and control of P. truncatus was first reviewed by Hodges (1982), in response to the need for information to support studies directed towards the control and containment of the pest in Africa. In addition, an annotated bibliography on P. truncatus was prepared by Wright and Spilman (1983) but many of the 107 publications listed are only minor references to the pest. Since 1982 there has been a steady flow of fresh information on P. truncatus including a Ph.D. thesis (Howard, 1983), which gives considerable insight into the biology of the pest, and the proceedings of an international workshop under the auspices of the “Group for Assistance on Systems relating to Grain After-harvest” (GASGA). It thus seems an appropriate time for a further review which may encourage additional research and assist entomologists already engaged in the investigation of the biology and control of this serious pest.

SYNONYMY,

COMMON

NAMES

AND

IDENTIFICATION

P. truncatus was first described by Horn (1878) as Dinoderus truncatus Horn and has been referred to as Stephanopachys truncatus by Back and Cotton (1922). The genus Prostephanus was erected by Lesne (1897) to accommodate this and other species. P. truncatus is the only one of these species known to be associated with stored products. A number of common names have been given to P. truncatus. The first recorded English name appears to be the “larger grain borer” used by Chittenden (1911) in a paper on this species and dominica (Fabricius). The term “greater grain its relative, the “lesser grain borer” Rhyzopertha s PR 22:,--A

I

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borer” has also been used in the literature. Semantically, this gives a more logical distinction from the lesser grain borer, but unfortunately the Entomological Society of America has sanctioned the use of “larger grain borer” in its list of common names (Sutherland, 1978). In Spanish, the pest is known as “barrenador de 10s granos”, meaning grain borer. Tanzanian farmers gave the beetle the name “Scania”, drawing an analogy between the powerful trucks of that name and the rapidity with which the beetle converts grain into flour and perhaps also referring to the similarity in shape between truck and beetle; both are truncate. However, in Tanzania the term “Scania” has officially been replaced by “dumuzi”, a word in the Mwesi language, meaning robber. There are detailed descriptions of both the adult (Horn, 1878; Lesne, 1897; Fisher, 1950) and the larva and pupa (Spilman, 1984). The adults may be identified using the extensive key of Fisher (1950) or the shorter keys of Kingsolver (1971) or Hodges (1982). No keys are available for the identification of the larva or pupa. Adults may be sexed by reference to the clypeal tubercles (Shires and McCarthy, 1976) and pupae according to the size and shape of the genital papillae (Bell and Watters, 1982). GEOGRAPHICAL

DISTRIBUTION

Wright (1984) has reviewed the world distribution of P. tr~ncut~~ as it was in February 1983; the record from Togo marked on her world map was added at the time of going to press in 1984. In the New World, where the species is indigenous, there are records of the beetle from southern U.S.A. (Chittenden, 1911; Back and Cotton, 1922) Guatemala (Zacher, 1926), Brazil and Guatemala (Cotton and Good, 1937), Peru (Wright, 1984), Colombia (Posada et al., 1976), Costa Rica (Fisher, 1950) Panama, Honduras and El Salvador (McGuire and Crandall, 1967), Nicaragua (Giles and Leon, 1975) and Mexico (Chittenden, 1911; Delgado and Hernandez Luna, 1951) where it has been recorded on maize at altitudes up to 2249 m (Quintana et al., 1960). Howard (1983) has given extensive details of the distribution of the pest in Mexico and Central America and found a reasonable correlation between the maize growing regions and the occurrence of the pest. However, he warns that the distribution might reflect that of entomologists rather than insects. P. truncatus has been found on imports into Israel (Calderon and Donahaye, 1962) and Iraq (Al-Sousi et al., 1970). In both cases the beetle was introduced in maize but in neither country did it become established. It is recorded in a list of stored products insects of Thailand (Sukprakarn, 1976). However, this record may need confirmation since Dinoderus spp. have been confused with P. truncutus in northern Thailand (McFarlane J. A., pers. commun.). Previous to 1981 there were no records of this pest in Africa but in that year the beetle was identified as the pest causing severe losses in farm stored maize in the hot dry Tabora region of Tanzania (Golob and Hodges, 1982). Since 1981 P. truncutus has spread widely within Tanzania and has now reached southern Kenya (Kega and Warui, 1983) and Burundi (Tropical Development and Research Institute (TDRI), unpublished records). Early in 1984 a serious and sustained outbreak of the pest occurred in Togo, West Africa (Harnisch and Krall, 1984). HOSTS,

LIFE

HISTORY

AND

BEHAVIOUR

Host and host-related behuviour P. truncutus is a pest of maize infesting both the stored crop (Delgado and Hernandez Luna, 195 1; Ramirez Genel, 1960b; Giles and Leon, 1975; Hodges et al., 1983a) and the standing crop (Quantana et al., 1960; Giles, 1975). Attack in the field may occur fairly early when the drying maize still has a moisture content of 4&50% (Giles, 1975). The pest also thrives on dried cassava (Hodges et al., 1983a; Hodges et al., 1985). In maize the endosperm alone can provide an adequate diet. Howard (1983) found that larvae reared on germ-free flour, compared with those on whole maize flour, developed with about equal rapidity, suffered the same degree of mortality during development but weighed somewhat less on emergence. Torreblanca et al. (1983), in a study of changes in maize composition due to infestation, have suggested that P. truncutus may feed selectively on the germ. However, this seems unlikely as X-ray studies of the pest developing on several varieties of maize have indicated that it feeds indiscriminately within the grain (Ramirez Martinez and Silver, 1983; Howard, 1983). Further, the

Review germ by itself provides an unsatisfactory

3

diet as few larvae reared exclusively on germ flour survived to the adult stage (Howard, 1983). There are varietal differences in the susceptibility of maize to P. trunca&s (Bell and Watters, 1982; Ramirez Martinez and Silver, 1983; Howard, 1983 and 1984) The harder, flintier grains suffer less damage although even the very flinty popcorn varieties are not immune to attack. Howard (1983) recorded a higher oviposition rate when P. truncatus was maintained on floury varieties and observed that the pest was more limited by varietal characteristics than Sitophilus zeamais Motschulsky. He thus concluded that varietal resistance is a potentially useful technique for the long term control of P. truncatus. The method of presentation of maize grain may affect its susceptibility to P. truncatus. Chittenden (1911) noted the pest’s “partiality for corn in the ear” and that it was “scarcely (being) at home in shelled corn”. This has been confirmed in field studies (Golob et al., 1985) in which maize stored on the cob suffered considerably more damage than shelled grain. In the laboratory, P. truncutus confined to simulated cobs or grains stabilised by being weighted down with glass beads, had a population growth rate up to three times that observed on loose grain and caused much greater damage (Cowley et al., 1980; Howard, 1983). Howard (1983) found only one exception to this, when large grained, floury varieties were host to the pest. Cowley et al. (1980) suggest that P. truncatus develops better on stabilised grain because it needs to brace itself against something that is fixed relative to the boring target. Bell and Watters (1982) are more specific and consider that the spaces between grains on a cob give P. truncatus a place to insert the hooked spines at the end of the first pair of tibiae so that the body can be anchored and a hole chewed. 0nce into the grain, the pest establishes itself by extension of the tunnels along the rows of tightly packed seeds. This would be more difficult in a discontinuous medium like loose grain. Howard’s (1983) observation on the higher susceptibility of loose, rather than stabilised, floury grains, is difficult to explain. Attempts in the laboratory to rear the species on cowpea, haricot beans, cocoa and coffee beans, hard winter wheat and rough rice have failed and resulted only in the commodities being bored. However, the pest can breed on a soft variety of wheat (Shires, 1977), to a meagre extent on chickpea (Cicer arietinum L.) (Hodges, unpublished) and possibly also on dried sweet potato (Mushi, 1984). Howard (1983) reared adult P. truncatus from eggs placed on artificial cereal grains consisting of a gelatin capsule filled with one of the following: finely ground sorghum, wheat, millet, maize, barley, rice or oats. All supported the development of P. truncatus although the development period was rather extended on the last three named cereals. Most cereal grains would seem at least nutritionally adequate for the pest. However, the ability of a pest to develop in a particular cereal may be determined by other factors. For instance, Tanzanian local red varieties of sorghum, with relatively small grain size, appeared to escape infestation (Hodges et al., 1983a). The dispersed nature of the grain on the stored sorghum heads and the small grain size relative to the size of the beetle would seem to save it from being a suitable host. The pest would appear to bore into solid substrates irrespective of their nutritional quality as the beetle has been observed penetrating a range of materials in which there is no evidence of breeding, e.g. wood (Chittenden, 1911; Hodges et al., 1983a), beans (Giles, 1975), groundnuts (Hodges et al., 1983a), perspex and polythene (Howard, 1983; Ramirez Martinez and Silver, 1983). When selecting a suitable location to bore the beetles may reject regions in a substrate that have insufficient depth. This has been suggested by observations of adult P. truncatus placed on an opaque plastic dish in which the upper surface was smooth and the lower had discrete, thickened ribs. Beetles placed on the upper surface proceeded to bore into the dish only at points where it was thickened below (Smith R. H., personal communication). This perhaps suggests that the beetle may have some accoustic mechanism for appraising the substrate. Many of the holes bored by the pest would seem to be exploratory, like the “trial holes” excavated in bamboo by the related beetle, Dinoderus minutus (Fabricius) (Gardner, 1945). When infesting stored maize cobs, with complete sheath cover, the adult P. truncatus usually initiate their attack by making an exploratory hole in the exposed core at the base of the cob. These holes are abandoned after reaching a depth of l--2 cm, possibly when no food is found. The adults eventually gain access to the grain via the apex of the cob by walking between the grain and sheath (Hodges and Meik, 1984). The maize core is presumably attacked first because it offers less resistance than the relatively hard sheath. Similarly,

4

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when the beetle is offered unfermented and fermented dried cassava it prefers to bore into the fermented material, apparently not for its olfactory properties but for its softer consistency (Hodges et al., 1985).

Conditions for development Studies of the life cycle of P. truncatus over a wide range of temperatures (124O’C) and humidities (30-90% r.h.) suggest that the optimum conditions for development on maize are 32°C and 70-80% r.h. (Shires, 1979 and 1980; Bell and Watters, 1982). The lower and upper limits for the completion of the life cycle are 25 and 32°C at 40% r.h., 18 and 37°C at 70% r.h. and 20 and 30°C at 90% r.h. (Bell and Watters, 1982). A physical limits diagram is given by Watters (1984). Under optimum conditions, on whole grain or on flour packed firmly into glass tubes, Bell and Watters (1982) observed that the life cycle was completed in 24-25 days. In contrast, Shires (1979 and 1980), rearing the pest in loosely packed maize flour, recorded a development period of 35.4 days. It is assumed that these widely different development periods related to the degree of packing of the development medium. Bell and Watters (1982) point out that in firmly packed flour the larvae made narrow tunnels against which they could brace themselves and hence were able to force their mandibles into the forward end of the tunnel for effective chewing. In low density media the tunnels were wider and larvae often twisted back and forth without moving ahead. This useless expenditure of energy may have retarded the growth rate.

Oviposition Adults bore into maize grains making neat round holes, and as they tunnel from grain to grain they generate large quantities of dust. After mating, adult females lay most eggs within the grain in blind-ending chambers bored at right angles to the main tunnels (Hodges, 1982; Howard, 1983). The eggs are laid in batches of up to 20 and covered with finely chewed maize dust. Howard (1983) noted that the number of eggs laid was proportional to the weight of flour produced by boring which suggests that flour may be produced by the adults for the offspring. This would certainly appear to be the case in R. dominica as only after mating do females produce large quantities of frass consisting of chewed but undigested grains (Potter, 1935). D. minutus also produces frass consisting partly of undigested fragments (Plank, 1942). The oviposition curve of P. truncatus includes an egg production peak at about 20 days and thereafter a gradual decline (Shires, 1980; Bell and Watters, 1982; Howard, 1983). The curve is similar to that observed for Sitophilus oryzae (Linnaeus) but more spread out than that for R. dominica (Birch, 1945a) and lacking the bimodal distribution observed for this species during the first 30 days of oviposition (Golebiowska, 1969). Shires (1980) found that when adult pairs were maintained on loose grains at 32°C and 80% r.h., females had a pre-oviposition period of 5-10 days, an oviposition period of 95-100 days and mean life-time fecundity of 50.5 eggs. Howard (1983) who studied oviposition in considerable detail, found a much higher rate of egg laying. He worked with adult pairs confined to damaged grains embedded in maize flour and maintained at 25°C and 70% r.h. Under these conditions the pre-oviposition period was about 5 days. The average length of the oviposition period was 14 weeks and the mean life-time fecundity was 114 eggs. Bell and Watters (1982) report an even higher figure for beetles confined to simulated cobs at 30°C and 70% r.h., when the life-time egg production averaged 430 and two individuals each laid over 600 eggs. In contrast, records kept for the first month after emergence have shown that females maintained on loose grain laid less than one-third as many eggs as those on cobs. This would seem to account for Howard’s (1983) observation of a higher population growth rate on cobs than on loose grain. The quantity of food provided for a female may influence the rate of oviposition. On maize grain, at 27°C and 70% r.h., over a 10 day period, females on 1 g lots of grain laid an average 1.4 eggs/day while 6 g lots elicited an average of 3.9 eggs/day. On cassava the oviposition rate is somewhat lower as on 1 g or 6 g lots of the dried root the mean daily fecundities were only 1.l and 2.3 eggs respectively (Nyakunga, 1982).

Review

5

Egg to adult development When eggs are maintained at 32°C and 70% r.h. larvae hatch after an average of 4.1 days and the mean larval period is 16.1 days (Bell and Watters, 1982). Using head capsule measurements, Bell and Watters (1982) determined that there are normally three larval instars. This has been confirmed using a technique involving fronto-clypeal measurements (Subrumanyam et al., 1985). In R. dominica four larval instars is the norm (Howe, 1950). The last instar larva of P. truncatus constructs a pupal case from frass stuck together with a larval secretion (Bell and Watters, 1982) either within the grain or in the surrounding dust. In contrast, D. minutus pupates in an unlined cell excavated in a suitable solid substrate (Plank, 1948) and the pupae of R. dominica lie free in the food medium (Potter, 1935). In P. truncatus at 32°C and 70% r.h., the mean pupal period lasts 4.7 days (Bell and Watters, 1982), so that an average development from egg to adult at 32°C and 70% r.h. takes 25.4 days. The life cycle may be completed a little more rapidly on maize than on cassava, as on yellow American No. 3 maize and blocks of dried cassava, at 27°C and 70% r.h., the mean development periods were 39.2 days and 43.1 days respectively (Nyakunga, 1982). It is not known how long after the completion of development adults emerge from the pupal case. In D. minutus, reared in uncontrolled conditions, the adults waited an average 2.5 days before leaving the pupal cell (Plank, 1948). Dobie (1978) has described a simple adult emergence curve for P. truncatus reared on maize flour. No sex difference was noted in the period to emergence. A later attempt by Shires (1980) to fit this curve, for beetles reared at 32°C and 80% r.h., suggested ,some sex difference although comparison of median development times from his earlier work did not confirm this. Subsequent studies have not clarified the situation. Nyakunga (1982) studying adult emergence from blocks of dried cassava or cassava flour compressed in gelatin capsules, maintained at 27°C and 70 or 50% r.h., found no sex differences in the time to adult emergence. However, Hodges et al. (1985) found some difference between the sexes developing on dried cassava maintained under the same conditions. At 70% r.h. the females emerged an average of about one half day after the males, while at 50% r.h. the difference was more substantial, females emerging, on average, l-2 days after males. In complete contrast, Howard (1983) has observed that females emerged 2 days before males when developing on maize grain at 25°C and 70% r.h. On emergence, female P. truncatus tend to weigh more than males. In a Mexican strain reared on maize flour, at 25°C and 70% r.h., the average female weight was greater by 7% (Howard, 1983) while the females of a Tanzanian strain developing on cassava, at 27°C and 70-50% r.h., averaged 13% heavier (Nyakunga, 1982; Hodges et al., 1985). Adults emerging from maize under the same conditions were about 40% heavier than those from cassava (Nyakunga, 1982). The sex ratio of adults reared on maize flour, maintained under a wide range of conditions, has shown no significant deviation from unity (Shires, 1979). Data on adult longevity has tended to be very variable and differences observed have failed to be statistically significant. Shires (1980) found that on maize flour at 32°C and 80% r.h., females generally outlived males, the mean life expectancies being 61.1 days and 44.7 days respectively. In contrast, of adults maintained at 25°C and 70% on maize flour and grain, males generally outlived females (Howard, 1983). The differences in female longevity observed in these two studies may be correlated with differing oviposition rates; the females in Howard’s studies having a much higher rate and hence perhaps shorter longevity. Humidity conditions in the range of 80-50% r.h. do not appear to greatly affect the pest as on maize flour at 32°C the lower humidity resulted in an increase in development period and mortality of only 20.2 and 13.3% respectively (Shires, 1979). Further substantial reductions in moisture may considerably limit the pest but even in maize flour at 32°C and 40% r.h. the pest completed development in an average of 38.1 days and suffered only 40% mortality. This tolerance of dry conditions has been confirmed during other laboratory studies (Young et al., 1962; Hodges and Meik, 1984) and in field studies in Nicaragua (Giles and Leon, 1975) and Tanzania (Hodges et al., 1983a) in which maize, with moisture contents in the range of 9-10.6% was heavily infested. L,imited observations suggest that adults may prefer dry conditions, as specimens placed in a choice chamber, one side of which was over water and the other over anhydrous silica gel, selected the dry zone (Meik, J., pers. commun.). Similar observations have been made for R. dominica (Roth and Willis, 1951) which is also able to develop under very dry conditions (Birch, 1945b).

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Review

Population increase

Estimates for the intrinsic rate of increase of P. truncatus under favourable environmental conditions, on stabilised maize grain (Bell and Watters, 1982) or maize cobs (Hodges and Meik, 1984) are in the range of 0.7-0.8/week. Under comparable conditions the intrinsic rate of increase for Tribolium castaneum (Herbst) is estimated at 0.72/week (Leslie and Park, 1949). Shires (1980) in a study of the development of P. truncatus on ground maize at 31°C and 80% r.h., recorded an intrinsic rate of increase of 0.36/week. This comparatively low figure appears to result from a low estimate for the mean number of eggs laid. Pheromones

Adult males and females may congregate at a food source in response to the male-produced aggregation pheromone (Hodges et al., 1984). The pheromone secretions of R. dominica and P. truncatus are each known to contain two physiologically active ingredients (Williams et al., 1981; Hall D. R., pers. commun.). In P. truncatus they have been given the trivial names “Trunc-call 1 and 2”. So far the chemical structure of only “Trunc-call 1” has been published (Hodges et al., 1984). It is chemically similar to both components of the R. dominica pheromone (Hodges et al., 1984) consequently P. rruncatus is attracted to the R. dominica pheromone (Hodges et al., 1983b) and vice versa (Hodges, unpublished). The significance of the dual nature of these pheromone secretions has not been established and neither has their site of secretion. Careful examination of the outer surface of male P. truncatus using scanning electron microscopy (Hodges, personal observations), has not revealed any of the obvious structures associated with aggregation pheromone released in other Coleoptera, e.g. setiferous sex patches or cribiform plates (Faustini and Halstead, 1982). Other boring beetles such as bark beetles (Scolytidae) are known to secrete their aggregation pheromones through the hind-gut (Vite et al., 1964). Plank (1942) observed oily secretions from the gut of D. minutus making its initial attack of bamboo. This suggests the gut as a possible pheromone source in this species and other Bostrichidae. There is very little precise information on the distribution behaviour of P. truncatus. The adults would appear to distribute themselves effectively by flying. In dense laboratory cultures, late in the afternoon, adults may be seen massing on the culture surface from which they fly. Tanzanian farmers have observed similar behaviour in heavily infested maize cribs when the beetles are said to “dance” above the maize (Hodges, unpublished). In a laboratory study, Hodges and Meik (1984) have shown that there is a tendency for adults of the first generation to disperse from a maize cob host, often boring out of the cob through the sheathing leaves. This did not appear to occur simply as a response to a high population density but probably included an element of exploratory behaviour. The distributing beetles also showed a tendency to eventually congregate at the same new host. This may well have been in response to the male-produced aggregation pheromone. RELATIONSHIP

WITH

OTHER

ORGANISMS

Infestations of P. truncatus may be found together with those of other storage insects. In dry conditions in Tanzania (Hodges et al., 1983a) and Nicaragua (Giles and Leon, 1975) P. truncatus was the predominant storage pest among at least seven other species. The ability of P. truncatus to develop in grain at low moisture may be one reason for its success. Under such conditions many other storage pests are unable to increase in number. For example, S. oryzae, a species occurring in the same ecological niche, needs a grain moisture content of at least 10.5% to be able to develop (Birch, 1945b). Thus, in dry conditions P. truncatus probably benefits from the absence of any significant competition from other storage pests. In Tanzania, maize cobs and shelled grain, maintained for a period of 10 months in the same storage crib, became heavily infested with P. truncatus and Sitophilus spp. The population of P. truncatus on cobs was always greater than that of Sitophilus spp., although with time the populations approached equality. On loose grain the Sitophilus spp. remained strongly dominant throughout (Golob et al., 1985). Some insights into the nature of the competition between these insects comes from the laboratory studies of Howard (1983) who investigated competition between S. zeamais and P. truncatus over a 70 day period. His work leads to a prediction that, on grain stabilised using glass beads and maintained at 70% r.h., P. truncatus would dominate the

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1

interaction with S. zeumais if the temperature was greater than about 28°C. Subsequent experimental work to investigate the effect of other factors on competition, such as grain stability, preoccupation of the grain by P. truncatus or the presence of flour were thus confined to the lower temperature of 25°C. The competitive ability of P. truncatus was improved by stabilisation of the grain so that on simulated maize cobs co-existence with S. zeamais was almost possible. When P. truncatus was placed on weighted grain 28 days in advance of S. zeamais the latter was still dominant. On simulated cobs P. truncatus fared better, although the outcome was more difficult to interpret. With a lead of 14 days P. truncatus did manage to dominate the relationship, while with a 28 day lead only co-existence was observed within the 70 day test. These two different outcomes were thought to reflect the result of competition between the different larval stages present at 14 or 28 days. It was anticipated that, at least initially, there would be a considerable degree of co-existence since the larvae would be spatially separated due to the differing oviposition behaviour of the species on maize cobs; P. truncatus females laying eggs in tunnels near the base of the grain and most female S. zeamais depositing eggs on the crown. Further, when at least 50% of the food medium was flour, which is generated in large quantities in a P. truncatus infestation, the species seemed to avoid some larval competition with S. zeamais by developing in the flour between the grains rather than in the grain. However, under natural conditions this would increase competition or predation from other storage pests such as Tribolium castaneum. In Tanzania, the numbers of this beetle in maize cobs showed a strong positive correlation with those of P. truncatus (Hodges et al., 1983a). It was presumed that they were feeding on the maize flour produced by P. truncatus and perhaps also on the young larvae and eggs. Indeed, it has been observed in the laboratory that T. castaneum can suppress the numbers of P. truncatus infesting maize cobs by as much as 50% (Rees D. P., pers. commun.). In Central America, the predatory histerid Teretriosoma nigrescens (Lewis) is associated with P. truncatus (TDRI unpublished records). In laboratory trials on loose maize grain weighted with glass beads, at moisture contents of 8.5 or 14%, 10 adult T. nigrescens were able to prevent populations of up to 100 adult P. truncatus from increasing (Rees, 1985). Work is in progress at TDRI to investigate the ability of T. nigrescens to suppress P. truncatus on maize cobs in the presence of other storage insects that occur in the same ecological niche, e.g. Sitophilus spp. and T, castaneum. In Tanzania, large numbers of Anisopteromalus calandrae (Howard) were associated with P. truncatus when relatively few other possible hosts were present (Hodges, unpublished). PEST

STATUS

AND LOSSES

P. truncatus appears to spread during the storage season. In a Tanzanian village during May, 1982, just after harvest, the pest could be found in 20% of stores while close to the end of the storage season, in October, 80% of stores were infested (Hodges, 1984). The timing of the onset of attack may vary from year to year. At one location in a particular year the infestation may be extremely severe but at the same place and time in the following year few, if any, beetles may be found (Golob, pers. commun.: Hodges, unpublished). This suggests that during the previous year the pest may either have exhausted its food source and undergone mass dispersal or that it may have suffered some form of density-dependent mortality. In Nicaragua, weight losses up to 40% have been recorded from maize cobs stored on the farm for 6 months (Giles and Leon, 1975), and in Tanzania losses as high as 34% have been observed after 3-6 months, with an average loss of 9% (Hodges et al., 1983). The most obvious cause of this loss is the conversion of maize grain into maize flour by adult boring; large quantities of maize dust are associated with infested cobs (Hodges et al., 1983a). Until a large population of larvae is established feeding activity may be a somewhat secondary source of loss. The composition of maize has been reported to change as a result of P. truncatus infestation (Adem and Bourges, 198 1; Torreblanca et al., 1983). Maize inoculated with 60 adults/kg, maintained at 27°C and 60% r.h., suffered 21.9% weight loss after 90 days. Proximate analysis showed a reduction in the ether extract and ash component by 20.68 and 28.38% respectively (Torreblanca et al., 1983). Larval infestation is apparently responsible for the greatest changes (Adem and Bourges, 1981). There was also an apparent decline in the concentration of the amino acids lysine and tryptophane, although this could well have been within the bounds of natural variation. Normally, maize composition changes

x

Review

are unlikely to occur as a direct result of infestation by pests that do not feed selectively on one part of the grain, e.g. equal weights of maize infested by Sitotroga cerealella, a non-selective feeder, and uninfested grain showed no nutritional differences after proximate analysis (Moore er al., 1966; Pandey and Pandey, 1978). Thus these results for P. truncatus, which would also appear to be a non-selective feeder (Howard, 1983; Ramirez Martinez and Silver, 1983), are somewhat unexpected. Losses in dried cassava roots can be very high. The dried roots are readily reduced to dust by adult boring and a weight loss of 70% has been recorded after only 4 months of storage in an experimental maize crib in Tanzania. Losses were lower in cassava that had not been fermented before drying. The higher susceptibility of fermented cassava appeared to be due to its softer and hence more easily penetrated structure. However, losses in both types of cassava were very high so that the storage of one rather than the other would not be a practical solution to the problem (Hodges et al., 1985). P. truncutus is able to sustain itself at an economically significant level under the hot dry conditions of Nicaragua and East Africa and causes similar damaging infestations in the hot, humid conditions of West Africa (Krall, 1984). It also has some tolerance of cooler conditions. In Mexico, Wilbur et al. (1962) maintained experimental cultures of P. truncatus at locations ranging from 16 to 2675 m above sea level to observe the effects of altitude and climate upon productivity. Each culture consisted of 25 kg of maize, and was initiated with 15 adults/kg. At the greatest altitude, 2675 m, where the mean temperature was 15.O”C, the beetle was severely limited during both summer and winter and apparently fared less well than either S. oryzae or R. dominica under the same conditions but cultured on wheat. At 2200 m, where the average temperature was 17.8 C, details of P. truncatus are given for only the winter period but the overall conclusion was that the pest has as much success in reproduction and causing damage as at lower more favourable altitudes. This comparison seems to be of limited value because the population growth at lower altitudes appeared rather feeble since even at 16 m, where the climatic conditions were more favourable with the temperature averaging 26.1”C, the population over 4 summer months never more than doubled its initial number. CONTROL In Tanzania maize is frequently stored by keeping cobs with sheaths in the roof space of a house, where they are subjected to the smoke of the cooking fire beneath, or by placing them on platforms or over racks in direct sunlight. These methods have proved effective against most storage pests with other traditional methods, but not against P. truncatus (Hodges et al., 1983a). Experiments such as the use of a 2.5 cm thick barrier layer of maize-core ash over the surface of shelled grain or admixture of this ash at the rate of 10 or 30% by weight, have demonstrated some degree of protection. After 27 weeks storage, 95% of untreated grain was damaged while with ash admixture at 10 or 30% or the use of an ash layer, 39, 46 and 28% of grains were damaged respectively. However, admixture of a dilute dust of pirimiphos-methyl at a rate of 1Oppm active ingredient (a/i) was considerably more effective with only 6% of grains damaged (Golob et al., 1983). The earliest records of insecticide testing against P. truncatus come from Central America and Mexico. Delgado and Hernandez Luna (1951) investigated the protection of seed maize using DDT (15-50 ppm), BHC (377150 ppm), magnesium oxide (1000 ppm), chlordane (50 ppm) or the fungicide ‘Arasan’ (thiram, 1000 ppm). All treatments gave good protection over IO months storage. Ramirez Gene1 (1960a) studied the efficacy of three insecticidal dusts for the protection of shelled maize after 4, 8 and 12 weeks storage in the laboratory. He claimed that 1% lindane applied at 25 ppm, which is an extremely high dose for application to shelled grain, was rather more effective than either bromocyclene applied at 25-75 ppm or malathion applied at 2-8 ppm and was still giving high kills after 12 weeks. Giles and Leon (1975) tested 2% dusts of malathion, tetrachlorvinphos, lindane and pirimiphos-methyl at rates of 12.5 ppm on cobs without sheath cover and 27.4 ppm on those with sheaths. Whatever insecticide was employed, cobs with sheaths were always more damaged than those without and, although malathion and pirimiphos-methyl gave some protection of desheathed cobs for up to 16 weeks, by 24 weeks the level of grain damage had reached extremely high levels (5&65%). Lindane, even after only 8 weeks, gave very poor

Review Table

I. Insecticides

investigated

by Golob

er al. (1985) for the control

Laboratory

Topical application

Insecticide Deltamethrin Permethrin (cis : lrnn.v 25 : 75) Fenvalerate Phenothrin (ci.3 : lrans20: 80) Lindane Pirimiphos methyl Chlorpyrifos methyl Methacrifos Malathion Fenitrothlon Carbaryl Pirimiphos methyl + permethrin (25: 75) Pirimiphos methyl + carbaryl Permethrin (25: 75) + carbarvl + Treatment --Treatment

study

of P.

trunt’atus

Field study

Filter paper

Dust on grain (PPm)

+

2.5 and 5.0 -

Dust on Grain Cobs (PPm) (PPm)

+ +

+ + + + + + + +

5 and IO 5 and IO 5 and IO -

_.. 20 20 20 24

IO IO IO I? IO IO

4+1 4tg

4+8

8+ I6

-

2+8

4+

I6

studied. not studied

results. This difference from Ramirez Genel’s (1960a) findings may well reflect the use of more normal dose levels, the difficulty of treating maize cobs which do not permit efficient admixture to the grain and the realistic conditions of a field trial in which the experimental material is subjected to the rigours of the climate. The first field trials in Africa tested the admixture of dilute dusts of pirimiphos-methyl, fenitrothion and bromophos to shelled maize at the rate of 10 ppm (a/i). Only pirimphos-methyl maintained the grain in good condition over the whole experimental period of 27 weeks (Golob et al., 1983). This study was followed by investigations of ten insecticides (Table 1) used singly or in “cocktails” of two compounds. The response of P. truncatus to some or all of these compounds was investigated in the laboratory by filter paper tests, topical application and exposure to small quantities of grain treated with insecticide dust, and in the field in Tanzania where maize cobs and maize grain were dusted with insecticide and exposed to natural infestation for 10 months in a wire-mesh crib (Golob, 1984a; Golob et al., 1985). These studies concluded that the treatment of shelled grain was considerably more effective than the treatment of the cobs and that in well ventilated conditions the synthetic pyrethroids would give better control of P. truncatus than the organophosphorus compounds. Permethrin was the only pyrethroid tested in the field trial but it was anticipated that other compounds in this group would be equally effective. Lindane was shown to exert good control on the beetle but widespread resistance to this compound in other storage pests would preclude its usage in the field. In a paper outlining a control strategy for P. truncatus i.n East Africa, Golob (1984a) recommends that maize should be shelled and a dilute dust of permethrin admixed at a rate of 3.0ppm. This recommendation would require some change in traditional farming practice and creates the need for provision of sacks or other storage facilities and perhaps also for mechanical maize shellers. It was recognized that alternative treatments which involve less drastic changes for the subsistence farmers would be particularly advantageous. The observation that P. truncatus initiates its attack on stored maize cobs with good sheath cover, by first boring into the core at the bases of the cob, has raised the possibility of controlling the pest by selectively treating this region with insecticide (Hodges and Meik, 1984). In a laboratory- study the bases of cobs were dipped in a 0.5% dilute dust of permethrin or a 0. I % aqueous dispersion prepared from an emulsifiable concentrate or wettable powder. This type of treatment was particularly effective in preventing infestation when the beetle was present at low population density. The technique is currently the subject of a field investigation and if successful would offer a method of control requiring minimal change to traditional storage practice, and avoid pesticide residues on the grain. sPK ?ZI -B

IO

Review

Hodges et al., (1985) have suggested that dried cassava may provide an alternative host of P. truncatus when little or no maize is present in store. In order to limit this source of infestation it may be worth leaving the roots in the ground for as long as possible before drying and storage, placing the dried roots in insect-proof containers, if these are available, or treating the cassava with a suitable residual insecticide at times when cross infestation to maize is likely to be significant. Such remedies could also be applied to reduce losses in cassava per se. The treatment of dried cassava with residual insecticide against P. truncatus has been investigated in the laboratory by Senkondo (1984). He dipped dried roots in insecticidal dispersions to give treatments of deltamethrin (0.5 and 1.0 ppm), permethrin (1.25 and 2.5 ppm) or pirimiphos-methyl (5.0 and 10.0 ppm) or dusted them with a dilute dust of permethrin to give a treatment of 2 ppm (a/i). Adults were then confined to the roots for 45 days, immediately after and 1 month after treatment. The same roots were similarly bioassayed after 6 months, outside of the period of Senkondo’s observations (Evans N., pers. commun.). Only the dipping treatments of deltamethrin, the permethrin at 2.5 ppm and the permethrin dusting remained effective at the end of the first month. These treatments still gave good protection after 6 months. It is not known whether such good protection would have been provided if the cassava had been infested prior to treatment. White (1982) investigated the use of emulsifiable concentrates of malathion and pirimiphosmethyl, applied to concrete or plywood, with a view to the treatment of empty storage structures or vehicles used for the transportation of grain. The concrete blocks used in the study were each divided into two portions, one of which was covered with a thin layer of maize dust whilst the other was dust free. The test materials were stored at 21°C and 30% r.h. and bioassayed at 4 week intervals up to 8 weeks on the concrete and 16 weeks on plywood. Malathion was found to be less effective than pirimiphos-methyl as a residual insecticide against P. truncatus, especially on the concrete surface, while the difference between the two on plywood was not great. On plywood both insecticides at 0.25-0.5 g active ingredient/m* may be considered reasonably effective if some anomalous results for pirimiphos-methyl at the highest dose are excluded, as after 16 weeks they were still killing 70-95% of beetles. On dust-free concrete even the highest dose, 2.0 g a.i./m*, gave only poor protection 1 week after treatment. It is well known that insecticides break down more rapidly on alkaline surfaces such as concrete (Parkin, 1966). The highest dose on concrete with a covering of maize dust gave reasonable protection, 7&95% kill, up to 4 weeks but thereafter the efficacy fell off rapidly. Under tropical conditions the periods of protection offered by these insecticides are likely to be somewhat reduced. Fumigation with phosphine is an effective means of eliminating P. truncatus. Giles (1984) reported fumigation of a consignment of 186 tons of maize using 600 tablets of “Phostoxin” for a period of 5 days. All stages of P.truncatus maintained in test cages, were eliminated. It appears that the pest has about the same susceptibility to phosphine as the susceptible strains of other storage insects (Harris A., pers. commun.). Krall (1984) has reported successful control of P. truncatus in Togo, where whole maize cribs were covered with thin plastic fumigation sheets and fumigated with phosphine applied at 5 g/m3 for an unspecified period. There appear to be no published reports on the susceptibility of P. truncatus to methyl bromide. The use of physical control techniques against P. truncatus has received some attention. Laboratory studies have been undertaken on the effects of y-irradiation (Ramos and Ramirez Martinez, 1979; Adem et al., 1979), and the action of laser light emitted from an argon laser (Ramos and Garces, 1980; Ramos et al., 1984) and a comparison made between the effects of accelerated electrons and y-rays (Adem et al., 1981). Such techniques can be used to suppress the pest but would seem to have their application limited to the disinfestation of shelled grain moving on a conveyor. MONITORING Unless beetles are present in large numbers the species is difficult to detect by visual inspection. Consequently, a trapping method has been developed for the pest. The traps consist of four layers of single layer corrugated cardboard, sprayed on the inner surface with permethrin (0.1 g a.i./m’), and baited with laboratory synthesised aggregation pheromone. Initial tests in farm stores in Tanzania employed components of the aggregation pheromone of R. dominica-‘Dominicalures

Review

II

1 and 2’,*ither singly or in combination. The pheromones were dispensed from rubber capsules impregnated with 5 mg of material and single traps were then located, for 2 weeks in farm stores containing 2-3 thousand cobs. It was found that traps baited with ‘Dominicalure 2’ were more effective than the other treatments and these traps demonstrated the presence of the beetle as frequently as thorough visual inspection of up to 200 maize cobs taken at random from each store (Hodges et al., 1983b). Subsequent to the ‘Dominicalure’ tests, one component of the maleproduced aggregation pheromone of P. truncatus, ‘Trunc-call I’, was identified. This compound was tested in maize cribs in Togo in order to compare its performance with ‘Domincalure 2’. As ‘Trunc-call 1’is rather more volatile than either of the ‘Dominicalures’, the compounds to be tested were dispensed from polythene vials, loaded with 2 mg of material, which gave a slower and more uniform release of pheromone than rubber septa. At 27°C and 8 km wind speed the half lives of ‘Dominicalure 2’ and ‘Trunc-call 1’were 12 days and 5 days in polythene vials and 1 day and < I day respectively in rubber septa. The traps were again deployed for 2 weeks and P. truncatus was detected much more frequently by those baited with ‘Trunc-call 1’(Hodges et al., 1984). At present, these traps are recommended for monitoring P. truncatus. Nevertheless, the male pheromone secretion of this species contains a second active component, ‘Trunc-call 2’ (Hall, D. R., pers. commun), and biological testing is in progress to determine whether this compound, alone or in combination with ‘Trunc-call l’, can provide an even more effective bait. Pheromone traps have so far been tested only in farm stores. However, it is likely that they could also be used to effect in larger stores when a suitable application rate for the traps has been established.

DISCUSSION As stated above, P. truncatus is principally a pest of the maize cobs and dried cassava stored on small-holder farms. Its recorded depredations on maize in East Africa are much more severe than those of the more familiar storage pests, e.g. Sitophilus zeamais, S. oryzae and Sitotroga cerealella, as weight losses in farm stored cobs in Tanzania averaged 9.0% after 3-6 months storage while those from other pests under similar circumstances, during an entire storage season, in Zambia (Adams, 1977), Kenya (De Lima, 1979) and Malawi (Golob, 1981) averaged 2-6%, 335% and 24% respectively. With the exception of very floury maize varieties, it is clear that P. truncatus is a more serious pest of maize on the cob than of the shelled grain. The efficient use of dilute dust insecticides against P. truncatus should reduce losses to economically acceptable levels but is never likely to achieve eradication of the pest since maize and cassava are stored in so many small and widely distributed farm stores that treatment of all infested premises would be unlikely. Further, beetles boring in wooden storage structures may escape treatment. Eradication programmes against the dermestid Trogoderma granarium Everts in large scale stores, using fumigation, have proved successful in East and West Africa (Salmond, 1957; Hayward, 1963). If P. truncatus is found in large stores it could be similarly eliminated but this would, of course, still leave the bulk of the population in farm stores. When the pest was localised in a relatively small area in southern Togo, in farm storage, the German aid agency GTZ had some success in fumigating the very compact Togolese maize cribs, which are traditionally built separately from human dwellings. However, the logistics of extending this technique elsewhere, to a myriad of farm stores, many of which form an integral part of human dwellings, would appear impractical even if the difficulties of fumigation in such awkward locations could be overcome. The further development of appropriate means of reducing losses caused by P. truncatus is a priority. Subsistence farmers in East Africa are already being advised to abandon traditional maize cob storage and to adopt grain shelling and the admixture of a dilute dust of permethrin. This situation needs to be carefully monitored as uneven admixture and inadequate dosing are likely to encourage the early development of resistance to permethrin. For the future, certain methods of biological control such as the use of the predatory histerid T. nigrescens or of more resistant maize varieties may offer alternative means of reducing losses. However, the introduction of more resistant maize varieties to replace the rather susceptible high yielding varieties grown for commerce

12

Review

is unlikely to benefit subsistence farmers who usually store local varieties that already have the desired resistance characteristics of complete sheath cover and flinty grain (Hodges et al., 1983a). On several occasions entomologists have asked why the pest should be of so much concern in its African habitat but of apparently only limited local interest in Mexico and Central America. Reports since the 1960’s have suggested that the importance of damage caused by the pest in this region may be increasing (Halstead, 1975). However, this increase may just reflect the recent interest in farm storage which in the past was neglected, presumably because of the low commercial value of the crops stored. If there is a true difference between the importance of P. truncatus in the New and Old Worlds, then this might be explained by the difference in maize harvesting and storage systems or possibly by natural suppression of the pest by predators or parasites in its native habitat. In Mexico and Central America cobs are frequently left in the field and are only picked when required; the cobs are thus maintained at a high temperature and kept very dry. There has also been a widespread introduction of storage bins, particularly in lowland areas, so that much of the grain may be stored shelled, i.e. in a less favourable form for the pest. The reality of any difference in the status of the pest will remain speculation until detailed investigations of P. truncatus are undertaken in its place of origin and its new habitat. Particular aspects of the beetle’s ecology that merit attention would include the relationships between field and store infestation, year to year variation in the onset and severity of attack, the interaction of P. truncatus with other storage pests, predators, and parasites, and flight behaviour in relation to its distribution. So far, P. truncatus has only been recorded as a pest of maize and cassava. However, in the laboratory it develops successfully on chickpea, a soft variety of wheat, and a range of cereal flours, when presented as large artificial grains. This suggests that there may be other potential hosts in stores, particularly among larger grained softer cereal varieties. In addition the pest is also likely to increase its geographical distribution. The ability of the beetle to establish itself as a serious pest in both the hot dry conditions of western Tanzania, the hot humid conditions of Togo and up to an altitude of 2200 m in Mexico suggests that it has the potential to spread far and wide in Africa, possibly to all areas where maize and/or cassava are stored, and to other tropical and sub-tropical regions. Its spread will no doubt occur both as a result of commodity movement in trade and natural dispersion by flight. Introductions into Israel and Iraq, in shelled maize, were eliminated by fumigation. Nevertheless, if P. truncatus has the opportunity to reach a favourable situation in farm storage there is a real danger of its establishment. Therefore plant inspection and quarantine authorities everywhere should be vigilant in preventing any further international spread of this serious pest. Acknowledgements-My sincere thanks valuable criticism of the manuscript.

are due to Drs D. G. H. Halstead,

D. C. Howard

and P. F. Prevett for providing

REFERENCES Adams J. M. (1977) The evaluation of losses in maize stored on a selection of small farms in Zambia, with particular Trop. stored Prod. ln$ 33, 19-24. reference to methodology. Adem E. and Bourges H. (1981) Cambios en la concentration de algunos componentes de1 grano de maize infestado por Prostephanus truncatus Horn, Sitophilus reamais Mots, and Sitotroga cerealella, Olivier. Archs lat. Am. Nulr. 31, 210-286. Adem E., Uribe R. M. and Watters F. L. (1979) Responses of Prostephanus fruncatus (Coleoptera: Bostrichidae) and Tribolium casteneum (Coleoptera: Tenebrionidae) to gamma radiation from “Co. Can. Ent. 111, I I I I-1 114. Adem E., Uribe R. M., Watters F. L. and Bourges H. (1981) Present status of corn grain disinfestation by irradiation in Mexico. Radial. Phys. Chem. 18, S-567. Al-Sousi E. J.. El-Haidari H. and Al-Ani J. N. (1970) Outbreak and new records. Plant Prof. Bull. FAO 18, 92--93. Back E. A. and Cotton R. T. (1922) Stored grain pests. Farmer’s Bull. No. 1260, U.S. Department of Agriculture, Washington D.C. Bell R. J. and Watters F. L. (1982) Environmental factors influencing the development and rate of increase of Prostephanus rruncafus (Horn) (Coleoptera: Bostrichidae) on stored maize. J. stored Prod. Res. 18, 131-142. Birch L. C. (1945a) The influence of temperature, humidity and density on the oviposition of the small strain of Culandra oryzae L. and Rhizopertha dominica Fab. Aust. J. exp. Biol. med. Sci. 23, 1977203. Birch L. C. (1945b) The biotic potential of the small strain of Calandra oryzae and Rhyzopertha dominicn. J. anim. Ecol. 14, 125-127. Calderon M. and Donahaye E. (1962) First record of Prostephanus truncafus in stored grain. Plant Prot. Bull. FAO. 10, 43-44. Chittenden F. H. (191 I) Papers on insects affecting stored products. The lesser grain borer. The larger grain borer. Bull. Bur. Ent. U.S. Dep. Agric. 96, 29-52. Cotton R. T. and Good N. E. (1937) Annotated list of the insects and mites associated with stored grain and cereal products, and of their arthropod parasites and predators. Misc. Publ. U.S. Dep. Agric. No. 258.

Review

13

Cowley R. J., Howard D. C. and Smith R. H. (1980) The effect of grain stability on damage caused by Prostephanus truncatus (Horn) and on three other pests of stored maize. J. stored Prod. Res. 16, 75-78. Delgado N. M. atid Hernandez Luna R. (1951) Control de1 gorgojo de la semilla de maiz (Prostephanus truncatus (Horn)). Inst. National Invest. Agric., Fol. Misc. 4, 2629. De Lima C. P. F. (1979) Appropriate techniques for use in the assessment of country loss in stored produce in the tropics. Trap. stored Prod. I$ 38, 21-26. Dobie P. (1978) A simple curve describing the development of some beetles breeding on stored products. J. stored Prod. Res. 14, 4144. Faustini D. L. and Halstead D. G. H. (1982) Setiferous structures on male Coleoptera. J. Morph. 173, 43-72. Fisher W. C. (1950) A revision of the North American species of beetles belonging to the family Bostrichidae. Misc. Publ. U.S. Dep. Agric. No. 698. Gardner J. C. M. (1945) A note on insect losses of bamboo and their control. Indian Forest Bull. No. 125. Giles P. H. (1975) Annual report 1974 on the activities of SEPRAL and the Grain Storage Extension Group. Rep. Section de Productos Almacenados (SEPRAL). Ministerio de Agricultura y Ganaderia, Nicaragua. Giles P. H. (1984) Summary of information of Prostephunus truncatus in Nicaragua obtained during a four year assignment at SEPRAL, La Calera, Managua, 1972-1976. Proc. GASGA Workshop on the Larger Grain Borer Prostephanus truncatus, 24-25 February 1983, TPI, Slough. Publ. GTZ, Eschbom. pp. 133-135. Giles P. H. and Leon 0. J. (1975) Infestation problems in farm-stored maize in Nicaragua. Proc. Ist Inr. Wkg Conf Stored-Prod. Ent., Savannah, Georgia, U.S.A. 1974, pp. 68-76. Golebiowska Z. (1969) The feeding and fecundity of Sitophilus granurius (L.), Sitophilus oryzue (L.) and RhJJzopertha dominica (F.) in wheat grain. J. stored Prod. Res. 5, 143-155. Golob P. (1981) A Practical Assessment of Food Losses Sustained During Storage by Small-holder Farmers in the Shire Valley Agricultural Development Project Area of Malawi I978/79. Report G. 154, Trop. Prod. Inst., London. Golob P. (1984a) Preliminary field and laboratory trial to control Prostephunus truncutus infestation of maize. Proc. GASGA truncatus, 2425 February, 1983, TPI, Slough. Publ. GTZ, Eschborn, Workshop on the Larger Grain Borer Prostephanus pp. 62-70. Golob P. (1984b) Prostephunus truncatus (Horn), the Larger Grain Borer, in East Africa: The development of a control strategy. Proc. 3rd Int. Wkg Conf Stored-Prod. Ent., Manhattan, Kansas, U.S.A., 1983, pp. 711-721. Golob P.. Changjaroen P., Ali M. A. and Cox J. (1985) Susceptibility of Prostephanus truncatus to insecticides. J. stored Prod. Res. 21. 141-150. Golob P., Dunstan W. R., Evans N., Meik J., Rees D. and Magazini I. (1983) Preliminary field trials to control Prostephunus truncates (Horn) in Tanzania. Trap. stored Prod. Inf. 45, 15-17. Golob P. and Hodges R. J. (1982) Study of an outbreak qfprostephanus truncatus (Horn) in Tanzania. Report G. 164. Trop. Prod. Inst. London. Halstead D. G. H. (1975) Changes in the status of insect pests in storage and domestic habitats. Proc. 1st Int. Wkg Con/ Stored-Prod. Ent. Savannah, Georgia, U.S.A., 1974, pp. 142-153. Harnisch R. and Krall S. (1984) Further distribution of the larger grain borer in Africa. FAO Plant Prof. Bull. 32, 113-114. Hayward L. A. W. (1963) Infestation control in stored groundnuts in Northern Nigeria. Wld Crops 15, 63-67. Hodges R. J. (1982) A review of the biology and control of the greater grain borer Prostephunus fruncafus (Horn) Trap. Stored Prod. Inf 43, 3-9. (Coleoptera: Bostrichidae). Hodges R. J. (1984) Field ecology and monitoring of Prostephanus truncutus. Proc. GASGA Workshop on the Larger Grain Borer Prostephanus truncatus, 2425 February, 1983, TPI, Slough. Pub]. GTZ, Eschborn, pp. 3248. Hodges R. J., Dunstan W. R., Magazini I. and Golob P. (1983a) An outbreak of Prosrephunus truncutus (Horn) (Coleoptera: Bostrichidae) in East Africa. Prot. Ecol. 5, 183-194. Hodges R. J., Hall D. R., Golob P. and Meik J. (1983b) Responses of Prostephunus truncutus to components of the aggregation pheromone of Rhyzopertha dominica in the laboratory and field. Ent. exp. uppl. 34, 266-272. Hodges R. J.. Cork A. and Hall D. R. (1984) Aggregation pheromones for monitoring the greater grain borer Prosrephunus truncutus. British Crop Protection Conference-Pests and Diseases, Brighton, Nov. 1984, pp. 255-260. Hodges R. J. and Meik J. (1984) Infestation of maize cobs by Prostephanus fruncufus (Horn) (Coleoptera: Bostrichidae)-Aspects of biology and control. J. stored Prod. Res. 20, 205-213. Hodges R. J., Meik J. and Denton H. (1985) Infestation of dried cassava (Munihot esculenta Crantz) by Prostephunus &ncatus (Horn) (Coleoptera: Bostrichidae). J. stored Prod. Res. 21, 73-77. Horn G. H. (1878) Revision of the Bostrichidae of the United States. Proc. Am. Dhil. Sot. 17. 54&555. Howard D. 6. (1483) The population biology of the greater grain borer Prosteihanus truncatus (Horn). Ph.D. thesis. University of Reading. Howard D. C. (1984) The ability of Prostephunus truncatus to breed on different maize varieties. Proc. GASGA Workshop on the Larger Grain Borer Prostephanus truncatus, 24-25 February, 1983, TPI, Slough. Publ. GTZ, Eschborn, pp. 17-31. IHowe R. W. (1950) The development of Rhyzoperthu dominicu under constant conditions. Ent. mon. Mug. 86, l-5. Kenya. Trap. stored Prod. Inf Kega V. K. and Warui C. W. (1983) Prostephunus truncafus in Coast Province, 46, 2. Kingsolver J. M. (1971) Key to the genera and species of Bostrichidae commonly intercepted in USDA plant quarantine USDA, Agric. Quarunt. Inspect. Memo. No. 697. inspections. Krall S. (1984) A new threat to farm-level maize storage in West Africa: Prostephanus truncutus (Horn) (Coleoptera: Bostrichidae). Trop. stored Prod. In/: SO, 2631. Leslie P. H. and Park-T. (1949) The intrinsic rate of natural increase of Tribolium castuneum (Herbst). Ecology 30,469477. Lesne P. (1897) Rtvision des Coleoot&es de la famille des Bostrvchides. Ann. Sot. ent. Fr. LXVI, 319-350. (Auril 1898). McGuire j. U. and Crandall B. S. (1$67) Survey of insect pests anh plant diseases of selected food crops of Mexico, Central America and Panama. U.S. Department of Agriculture. Moore S., Petty H. P., Lucknom W. K. and Byers J. H. (1966) Losses caused by the angoumois grain moth in dent corn. J. econ. En?. 59, 88&882. Mushi A. M. (1984) The larger grain borer (Prostephunus truncatus (Horn)) problem in Tanzania. Proc. GASGA Workshop on the Larger Grain Borer Prostephanus truncatus, 24-25 February, 1983, TPI, Slough. Publ. GTZ, Eschborn, pp. 71 87.

14

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