Effect of maize variety and storage form on the development of the maize weevil, Sitophilus zeamais Motschulsky

J. stored Prod. Res. Vol. 31, No. 1, pp. 29-36, 1995

Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-574X/9<$9.50+0.00

0022-474X(94)UOO38-7

Effect of Maize Variety and Storage Form on the Development of the Maize Weevil, Sitophilus zeamais Motschulsky K. A. VOWOTOR’, N. A. BOSQUE-Pl?REZ’*

and J. N. AYERTEYZ

‘Plant Health Management Division, International Institute of Tropical Agriculture (IITA), PMB 5320, Ibadan, Nigeria and 2Crop Science Department, University of Ghana, Legon, Accra, Ghana (Received 30 August 1994)

Abstract-The

effects of maize variety and storage form on development of the maize weevil,

Sitophilus zeumuis Motsclmlsky, were studied under artificial infestation at 25 f 2’C and 70 f 5%

r.h. in the laboratory. Three improved West African maize varieties, Aheleehi, EV8725-SR, Pop63-SR and the local variety, Volta Local, stored shelled and as cobs without husks (= unshelled) were used. Dissections of infested grain were used to monitor insect development at regular intervals. Maize variety and storage form had a significant effect (PC 0.85) on the mean duration of most developmental stages. Across varieties, the mean total duration of larval stages (first to fourth) was 25.2 d on unshelled grain vs 22.8 d on shelled grain. By day 30,76% of third instar larvae in shelled kernels had tunnelled to the germ and fed on the radicle and scutellum portions compared to 44% of the larvae in unshelled grain. ‘J’hepre-emerged adult spent 5.3 d longer in unshelled than ln shelled maize. The mean weights of the developing insect (across varieties) were not significantly influenced by storage form, except for the first and third instar larvae, which gave higher mean weights on shelled than unshelled grain. Both maize variety and storage form influenced the site of weevil emergence from the kernel.

Key words-Maize, Sitophilus zeumais, storage form, maize varieties, insect development.

INTRODUCTION

recent years post-harvest losses to storage insect pests such as the maize weevil, Sitophilus zeamais Motschulsky, have been recognized as an increasingly important constraint in Africa (Markham et al., 1994). Farmers in West Africa often store their maize unshelled with the husk intact. An understanding of the biology of the maize weevil in relation to storage form will assist in the development of improved management practices for the control of this pest. Research on the development of the maize weevil on shelled kernels has been carried out by several authors (Schoonhoven et al., 1974; Tipping et al., 1986; Urrelo and Wright, 1989a, b), and more recently Kossou et al. (1992, 1993) have studied weevil development on unshelled maize. The Median Developmental Period (MDP) (Dobie, 1974) of the maize weevil has been found to be significantly longer on maize stored as cobs (with and without husks) compared to on shelled maize (Kossou et al., 1991, 1993; Vowotor et al., 1994). Kossou et al. (1992) suggested that this longer weevil development time on maize stored as cobs was due to the less nutritious endosperm diet of first instar larvae, which

In

*Author for correspondence, England. SPR31 Ix

at: IITA, L. W. Lambourn

& Co., 26 Dingwall

29

Road,

Carolyn

House,

Croydon

CR9 3EE,

30

K. A. Vowotor

et al.

hatch from eggs laid on the kernel’s crown, as well as to the difficulty experienced by the newly developed adult in emerging from a kernel because of obstruction from adjacent kernels. It was not clear, however, which of these factors was the more important. Experiments were therefore conducted to examine the effect of maize variety and storage form on the duration of the immature stages of S. zeamais.

MATERIALS AND METHODS Experiments were conducted at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. Four West African maize varieties, Abeleehi, EV8725-SR, Pop63-SR and the local variety, Volta Local, selected in order to combine a number of basic contrasting characteristics of commonly grown types, were used for the study. Characteristics of the varieties are summarized in Table 1. Varieties were grown simultaneously in the same field at IITA and after harvest, cobs were artificially dried at 35°C for 2 wk, and then placed in large transparent polythene bags. To kill any insects resulting from field infestations, cobs were deep frozen within the bags until required for the experiments, when they were removed for moisture equilibration for 3 wk. Kernel moisture content was determined using a Dickey-John moisture tester after equilibration of grain at 25+_2”C and 70 f 5% r.h. in the laboratory.

Weevil development

Tests were conducted in a controlled environment room kept at 25 i 2°C and 70 f 5% r.h. Ten cobs or the equivalent amount in shelled grain (ca. 5000 kernels) for each variety and storage form per replication were placed in cotton cloth bags (30 x 35 cm). Eight hundred 2-wk-old unsexed adults were placed per bag for a 4 d oviposition period after which the adults were removed. Experiments were replicated 4 times. A high weevil density was used to reduce the chances of kernels escaping oviposition. Using the modified stained egg-plug technique described by Pedersen (1979) (cited by Horber, 1989), both shelled grains and grains originating from cobs were stained with acid-fuchsin to locate egg-plugs for dissection. A destructive sampling method, which involved dissecting infested kernels at 3 d intervals, was adopted. Dissections of kernels were carried out in a Petri dish using a razor blade (for eggs, first and second instar larvae) or a scalpel, under an illuminated stereo microscope (magnification 6.440). Only kernels with one egg-plug were dissected because it is known that where multiple oviposition occurs, there is possible internecine competition, with the survivor eliminating the other stages present (Birch, 1945; cited by Howe, 1952), and this competition within a kernel has been reported to slow down the development of the surviving larvae. Since the rate of weevil development in an infested kernel with missing adjacent kernels on a cob may be influenced by changes in the microclimatic conditions created by such gaps, the infested kernels from cobs were selected where there were no gaps before staining and dissection. Dissections of kernels started 2 d after the termination of the egg-laying period (i.e. on day 6 after the initiation of oviposition) and continued every third day thereafter. Ten infested kernels of each replicate from each variety and storage form were dissected on each occasion. Immature stages were carefully removed from the kernels with a fine camel hair brush and placed individually into plastic cages measuring 1.5 x 1.5 x 1.5 cm. Where dissection could not be completed on all kernels in a day, the undissected kernels were placed in labelled Petri dishes and deep-frozen, to arrest further development, for dissection the next day (Sharifi and Mills, 1971a). After dissection, each developing insect removed was weighed using a Mettler’ H54 analytical balance (precision +O.OOl mg). The stages of larval development were determined using the range of head-capsule widths described by Sharifi and Mills (1971b) and the duration of stages was expressed in days, as measured from the beginning of the oviposition period until the time of each dissection. With the aid of a calibrated oculometer fitted onto a stereo microscope, the morphometry (length and breadth) of 10 emerged F1 weevils per replicate was determined.

Hard dent Hard flint Soft dent Flourv

Variety*

Abeleehi EV8725-SR Pop 63-SR Volta Local

307.02a 314.lOa 261.37b 237.37~

1000 kernel wt (g)

1.27a 1.27a I .37a 1.30a

Kernel density (g/cm’) 0.73b 0.74a 0.74a 0.74a

Bulk density (kg/m’) 0.07b 0.09a 0.07b O&k

Pericarp 0.02la 0.013b 0.009bc 0.006c

Tipcap 1.15a 1.13a 1.02a 0.98a

Endosperm

Germ 0.18ab 0.21a 0.20a 0.16b

0.08b 0.09a 0.07c 0.06d

Germ side O.lOb O.lla 0.09b 0.07c

Side opposite germ 11.4a 10.4b 10.4b 9.4c

Length 8.5a 8.5a 7.9b 7.6c

Breadth

Kernel size (mm)

10.8 19.7 25.1 32.3 37.4 42.5 53.3 10.0 17.7 24.4 29.0 33.4 40.8 46.5

10.0 18.2 23.6 30.5 33.2 39.0 44.2

EV8725-SR

10.2 19.9 25.1 32.7 36.7 42.3 53.7

Abeleehi

9.3 16.0 21.4 27.9 31.4 37.5 43.1

10.8 19.8 24.0 32.3 34.5 40.7 51.4

Pop63-SR

Variety

9.6 15.6 22.0 27.7 31.9 37.9 43.6

9.5 18.9 23.1 30.8 33.4 39.2 50.4

Volta Local

9.7 16.9 22.9 28.8 32.5 38.8 44.5

10.3 19.6 24.3 32.0 35.5 41.2 52.2

Mean

0.3 0.7 1.2 0.8 1.2 1.3 1.2

LSD (5%) between varieties

0.2 0.5 0.8 0.5 0.8 1.0 0.9

LSD (5%) between storage forms

‘Time points for completion of the developmental stages (in days) are means of 4 replicates determined by dissecting 10 kernels per replicate of each treatment (i.e. variety and storage form) at 3 d intervals. Development time was recorded from the initiation of the oviposition period.

Ull4b&d EgJZ First instar larva Second instar larva Third instar larva Fourth instar larva Pupa Pre-emerged adult sln?lied Egg First instar larva Second instar larva Third instar larva Fourth instar larva Pupa Pre-emerged adult

Storage form and developmental stage

Table 2. Cumulative develoomental time@ (in days) for S. zeamais in kernels of four maize varieties stored shelled or unshelled

‘Means followed by the same letter within a column are not significantly different from each other at P~0.05by Duncan’s multiple range test. *All varieties white. ‘Wet basis.

Endosperm type and texture

Pericarp thickness (mm)

of the varieties used in the study1

Weight of kernel components’ (9)

Table 1. Characteristics

3.lb 4.0a 3.lb 3.7a

Depth

6.8a 7.2a 5.9c 5.4d

Kernel volume (cm?

32

K.

A. Vowotoret al.

Data analyses

Duration and weights of the different stages of development, and of F, adults, were analysed as a completely randomized design. Least significant differences (LSDs) were used to separate means where significant F values were observed. Data on the distribution of weevil emergence sites on kernels were analysed using Chi-square. RESULTS AND DISCUSSION Egg incubation

Small but significant differences were observed in mean egg incubation periods between maize varieties across the storage forms (F= 9.21, P~0.01). The egg incubation period was longest on the flint variety EV8725-SR, and shortest on “Volta Local”, a floury variety. Across varieties, significant differences were found between storage forms (F= 26.33, P < O.Ol), with mean egg incubation periods of 10.3 d on unshelled and 9.7 d on shelled maize (Table 2). There was also a significant (F=9.64, PcO.01) interaction between maize variety and storage form. Sharifi and Mills (197 1a), using radiographic techniques, found that S. zeamais eggs laid in wheat kernels at 13.4% moisture content hatched between 4 and 9 d (mean 6.5 d). Using wheat as a substrate, Howe (1952) reported a median weevil egg incubation time of 6.1 d at 25°C and 70% r.h. and found the egg incubation period to increase with decreasing temperatures. The incubation periods we observed were longer than those reported by other authors, but our figures were calculated from the beginning of the oviposition rather than the mid-oviposition period, thus overestimating the duration of the stage. We believe differences may also be associated with the microclimatic conditions related to differences in endosperm texture between wheat and maize kernels and the low moisture content (relative to the other studies) of the kernels during the period of incubation. The moisture contents of the varieties during egg incubation were 12.3,11.9,11.2 and 11.1% for Abeleehi, Pop63-SR, Volta Local and EV8725-SR, respectively. Larval stages First instar. Significant differences (P < 0.01) were observed in the mean duration of the first instar stage between varieties (F= 10.87) and storage forms (F= 57.78) and their interaction (F= 3.09, P~0.05). Across storage forms, the mean duration of the first instar was longest on the hard dent variety Abeleehi (19.0) and shortest on the floury Volta Local (17.2). First instar larvae did not tunnel in any particular direction while feeding within the endosperm tissue. This behaviour of the larva was also reported by Urrelo and Wright (198913)in their experiments on shelled maize kernels. These authors found the duration of first instar larvae to be longer when first instar larvae were artificially placed in the endosperm than in the germ or endosperm-germ interface. On unshelled kernels, weevils oviposit primarily on the kernel’s crown, while on shelled kernels other areas of the kernel such as the tip cap are preferred (Kossou et al., 1992; Vowotor, 1992). Schoonhoven et al. (1974) reported that removing the maize embryo from the weevil diet increased the developmental period. As discussed by Kossou et al. (1992), when eggs are laid on the crown, first instar larvae can only feed in the endosperm, while larvae originating from eggs laid nearer to the tip cap have access to both the embryo and the endosperm. The developmental time of the first instar larva was probably prolonged by additional feeding on the crown tissue until larvae gradually advanced into the endosperm tissue. First instar larvae from shelled maize were significantly heavier (F= 23.30, PcO.01) than larvae from unshelled maize (Table 3). Neither the effects of variety nor the interaction between variety and storage form on larval weight were significant. The weight of first instar larva was probably influenced by the time of assessments and the site where the larva had fed within the kernel, which would have been affected by the storage form (Kossou et al., 1992, 1993) and the quality of food the larva consumed. Second instar. Second instar larval durations were significantly affected (F= 7.70, P~0.01) by variety. Mean larval durations were 24.3,24.8,22.7 and 22.5 d for Abeleehi, EV8725-SR, Pop63-SR and Volta Local, respectively.

Development of the maize weevil

33

Table 3. Weight (mg) of S. zeamais at different developmental stages as influenced by maize variety and storage form’ Storage form and developmental stage unsheued First instar Second instar Third instar Fourth instar Popa Pre-emerged adult Adult (C&24h) Shelkd First instar Second instar Third instar Fourth instar Pupa Pre-emerged adult Adult (O-24 h)

Variety Abeleehi

EVg725-SR

Pop63-SR

Volta Local

Mean

LSD (5%) between varieties

0.38 0.87 2.81 4.63 4.62 3.15 3.13

0.35 0.82 2.83 4.48 4.61 3.26 3.18

0.36 0.86 3.09 4.53 4.73 3.12 3.09

0.38 0.86 2.74 4.69 4.64 3.09 2.99

0.37 0.86 2.87 4.58 4.65 3.15 3.10

0.03 0.12 0.18 0.21 0.15 0.19 0.17

0.37 0.85 2.88 4.52 4.62 3.32 3.15

0.41 0.83 2.99 4.64 4.64 3.22 3.08

0.38 0.84 3.05 4.54 4.67 3.07 3.16

0.39 0.80 3.09 4.65 4.68 3.10 2.99

0.40 0.83 3.00 4.59 4.65 3.18 3.09

LSD (5%) between storage forms 0.01 0.06 0.08 0.1 I 0.07 0.10 0.17

‘Weights (rag) ofdevelopmental stages are means of4 replicates determined by dissecting IO kernels per replicate of each treatment (i.e. variety and storage form), at 3 d intervals.

Across varieties, second instar larval durations were significantly different (F= 13.1, P < 0.01) between storage forms (24.3 and 22.9 d for shelled and unshelled kernels, respectively). The interaction of variety and storage form was not significant. Larvae in both shelled and unshelled maize kernels tunnelled first towards the radicle end of the germ tissue before moving to the plumule end, irrespective of where eggs were laid or first instar larvae developed. Reasons for the larval behaviour at this stage of development were not investigated, but there may be an association with avoidance of toxic substances at the plumule end of the germ tissue and/or a response to nutritional factors in the radicle end which may be critical for further development of the larvae. Additional studies are required to clarify this. No significant differences were observed between weights of second instar larvae among the different maize varieties or storage forms (Table 3); the mean weight per larva was 0.85 mg. Third instar. The time for completion of the third instar was significantly longer (F= 157.0, P~0.01) on unshelled than on shelled kernels (Table 2). Differences in larval duration between the maize varieties and the interaction between maize variety and storage form were also significant (F= 157.0 and 3.0, P < 0.05, respectively). Most third instar larvae widened the tunnels considerably by biting the walls and packed loosened material into the tunnel. Observations during the dissection of kernels revealed that larvae used powerful body contractions to push tunnel contents from the anterior to the posterior end of the tunnel. When not feeding, a larva would remain motionless for about 2-4 s embedded within the tunnel. By day 30 from the beginning of oviposition, 75.6% of the third instar larvae in shelled kernels had tunnelled to the germ, and fed on the radicle and scutellum portions. In unshelled kernels, only 43.8% of the larvae had reached and fed on these tissues by that date. The mean weight per individual third instar larva in unshelled kernels was significantly lower than that observed in shelled kernels (F= 12.65, P~0.01) (Table 3). The mean weight of larvae developing in kernels of Pop63-SR was significantly greater (F= 6.95, P < 0.01) than in the other three varieties. Fourth instar. By day 33,98% of fourth instar larvae in shelled kernels and 92% on unshelled ones had tunnelled to and were found feeding on the germ tissue. The duration of this stage was significantly longer (P< 0.05) on the varieties Abeleehi and EV872.5-SR than on the softer ones Pop63-SR and Volta Local (F= 36.06, P < 0.01). Differences in larval duration between the storage forms were not significant at the P > 0.05. No significant differences were detected in larval mean weight among varieties or storage forms. Pre-pupa and pupal stage

The duration of the pre-pupal stage of S. zeamais and S. oryzae in wheat kernels has been reported to be 1 d (Sharifi and Mills, 1971a, b). Since dissections were carried out at 3 d intervals, it was not

K. A. Vowotor et al.

34

possible to determine accurately the duration of this stage of development. The pre-pupal and pupal stages were therefore combined. Mean pupal duration was significantly affected (P < 0.01) by maize variety (F=9.33) or storage form (F=27.50). Mean pupal weights were not influenced by these factors. By day 42, 72% of pupae dissected had constructed pupal chambers across both endosperm and germ tissues, 23% entirely within the germ and 5% within the endosperm tissue. A careful examination of the larvae that made pupal chambers in the endosperm tissue showed 75% to have previously tunnelled through the germ tissue, and fed on it. In general, before pupation, the maize weevil larva feeds both on the endosperm and germ tissues, obtaining nutrients and water for development as reported earlier by Urrelo and Wright (1989b). Schoonhoven et al. (1974) reported an increase in the number of S. zeamais progeny and weight as well as a shortening of the duration of development when weevils were fed on ground maize to which additional germ tissue was added. A similar observation has been made for Sitotroga cerealella by Mills (1965). Pre-emerged

adult stage

Storage form had a significant effect on the duration of this stage. The pre-emerged adult spent 5.3 d longer in unshelled than in shelled maize (Table 2). The major contribution to the relatively longer developmental period of S. zeamais in kernels from cobs compared to those from the shelled grain treatment occurred in this stage of development. There were also significant differences in the mean duration of this stage among the maize varieties. Weight differences of pre-emerged adult weevils between unshelled and shelled maize were not significant. The body colour of pre-emerged adults was uniformly light brown or tan at the early stages of their development. Body structures were well-formed, with the rostrum in the adult elongated and more anteriorly placed than in pupae. Three pairs of weevils which had not emerged from their kernels in cobs of Volta Local but had chewed out cavities from adjacent kernels were found mating on day 45, suggesting that their development had been completed earlier. Taking into consideration that mating in S. zeamais usually does not occur before weevils are 3-d-old (Walgenbach and Burkholder, 1987), this suggests that the weevils found mating were fully developed by day 42, but were unable to emerge. Prior to this study, there has been no report of mating occurring before “emergence”. Site of adult emergence

Weevils in unshelled kernels (on cobs) had difficulty in emerging, probably due to obstruction of the first emergence hole by adjacent kernels, resulting in aborted emergence tunnels, as described by Kossou et al. (1992). Careful dissections revealed two types of new emergence holes resulting from aborted emergence: an exit hole made through the crown or side of the kernel and another in which weevils had to widen the original exit hole by chewing tissues around it, including some from adjacent kernels obstructing the emergence. In the latter case, weevils emerged through the widened gap created between adjacent kernels. Urrelo and Wright (1989b) working on shelled maize reported that the choice of weevil emergence site depended on the hardness of the endosperm. Results from this study, suggest that both storage form (x2= 9.64,4 df, PcO.01) and maize variety (x2 = 69.55, 12 df, PcO.01) affect site of emergence. Similar findings were reported by Kossou et al. (1992). Adult size and weight

There were no significant differences in the weight of adults (Table 3), and this would seem to confirm that, whereas the factors studied had an effect on developmental rate, this was not reflected in the final weight achieved. Neither were there significant differences in weevil sex ratio or body length among the different maize varieties or storage forms, although there was a significant effect of maize variety (F= 2.97, P < 0.05) and storage form (F= 6.74, P< 0.05) on body width (Table 4). A significant difference (F= 121.72, PC 0.01) in body length was found between the sexes, with female weevils generally longer than males (mean length 4.35 vs 4.05 mm), irrespective of maize variety or storage form. Female weevils were also significantly wider than males (mean width 1.09 vs 1.05 mm)

Development of the maize weevil Table 4. Effect of maize variety and storage form on the morphometry

35 of newly emerged adult S.

zeamais’

Length (mm)b Variety

Width (mm)b

Male

Female

Male

Female

Abeleehi Unshelled Shelled

4.11 f0.04 4.05 f0.12

4.34 + 0.05 4.37 * 0.03

1.04 * 0.02 1.05 * 0.03

1.09 * 0.03 1.08 f 0.03

EV8725-SR Unshelled Shelled

3.9950.05 4.04 + 0.07

4.33 f 0.08 4.29 k 0.08

1.05 + 0.02 1.03+0.03

1.14+0.02 1.06 + 0.02

PopGSR Unshelled Shelled

4.00 k 0.06 4.14*0.07

4.31+0.04 4.44kO.02

1.12*0.02 1.06+0.13

1.15kO.02 1.07 + 0.02

4.04 * 0.05 4.07 + 0.05

4.39kO.02 4.42 + 0.03

1.04 + 0.02 1.04 * 0.02

1.08 f 0.02 1.06 + 0.03

VoltaLocal Unshelled Shelled LSD (5%) between varieties

0.08

0.03

LSD (5%) between storage forms

0.06

0.02

LSD (5%) between sexes

0.06

0.02

’ Lengths and widths + SE are means from 4 replicates of emerged F, adults (O-24 h) determined 10 individuals per replicate for each treatment (i.e. variety and storage form).

from

across all varieties and storage forms (F= 11.95, P < 0.0 1). Okelana and Osuji (1984) found females to be longer than male weevils emerging from shelled maize maintained at 50 and 70% r.h., at 30°C. Implications for storage

The present results help to explain the factors responsible for the longer weevil developmental period observed on maize stored as cobs and confirm earlier findings by Kossou et al. (1992, 1993). Maize farmers in Africa often store maize unshelled with the husks intact. This cultural practice appears to have some important practical implications. Farmers normally dry their maize in open cribs or barns during the periods preceding consumption or sale of the produce, under conditions conducive for the development of the maize weevil. By storing unshelled maize the developmental period of the maize weevil is prolonged, reducing the chances of build up of destructive populations of the insect. This cultural practice results in no extra cost and is thus easily accessible to resource-poor farmers. We believe the results of our studies provide useful information for the development of a pest management programme for this insect. Acknowledgements-We

are

grateful

to

W. G. Meikle and C. Borgemeister for their useful comments during the preparation

of this paper.

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Mills R. B. (1965) Early germ feeding and larval development of the Angoumois grain moth. J. &on. Ent. 58, 220-223. Okelana F. A and Osuji F. N. C. (1984) Influence of relative humidity at 30°C on the oviposition, development and mortality of Sitophilus zeamais Motsch. (Coleoptera: Curculionidae) in maize kernels. J. Stored Prod. Res. 21, 13-19. Pedersen J. R. (1979) Selection of oviposition sites on wheat kernels by Sitophilus spp: effect of moisture, temperature and kernel size. Ph.D. Dissertation, Kansas State University, Manhattan, KS. Schoonhoven A. V., Mills R. B. and Horber E. (1974) Development of Sitophilus zeamais Motsch. in maize kernels and pellets made from maize kernel fractions. J. Stored Prod. Res. 10, 73-80. Sharifi S. and Mills R. B. (197la) Developmental activities and behavior of the rice weevil inside wheat kernels. J. Econ. Ent. 64, 1114-1118.

Sharifi S. and Mills R. B. (197lb) Radiographic studies of Sitophilus zeamais Motschulsky in wheat kernels. J. Stored Prod. Res. 7, 195206. Tipping P. W., Rodriguez G. C., Poneleit G. C. and Legg D. L. (1986) Feeding activity of Sitophilus zeamais (Coleoptera: Curculionidae) on resistant and susceptible corn genotypes. Enoir. Em. 16, 654-658. Urrelo R. and Wright V. F. (1989a) Oviposition performance of Sitophilus zeamats Motsch. (Coleoptera: Curculionidae) on resistant and susceptible maize accessions. J. Kansas Em. Sot. 62,23-31. Urrelo R. and Wright V. F. (1989b) Development and behaviour of immature stages of the maize weevil (Coleoptera: Curculionidae) within kernels of resistant and susceptible maize. Ann. Em. Sot. Am. 82, 712-716. Vowotor K. A. (1992) Effect of maize variety and storage form on oviposition and development of the maize weevil Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae). M.Phil. Thesis, University of Ghana, Legon. Vowotor K. A., Bosque-Perez N. A. and Ayertey J. N. (1994) Effect of maize variety and storage form on oviposition and development of the maize weevil, Sitophilus zeamais. In Proc. 6th Int. Wkg Conf on Stored-product Protection, Canberra, Australia. CAB International, Cambridge, U.K. In press. Walgenbach C. A. and Burkholder W. E. (1987) Mating behaviour of the maize weevil Sitophilus zeamais (Coleoptera: Curculionidae). Ann. Ent. Sot. Am. SO, 578-583.