- Email: [email protected]
The potential of the predator Teretriosoma nigrescens lewis (Coleoptera: Histeridae) for the control of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) on dried cassava chips and cassava wood
I, pp.91-96, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-474X196 S15.00 -t 0.00
J. srored Prod. Res. Vol. 32, No.
Pergamon
0022474X(95)00030-5
The Potential of the Predator Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) for the Control of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) on Dried Cassava Chips and Cassava Wood J. HELBIGT and F. A. SCHULZ* Humboldt-University of Berlin, Faculty of Agriculture and Horticulture, Department of Phytomedicine and Phytopathology, Lentzeallee 55-57, 14195 Berlin, Germany (Received 11 October 1995)
Abstract-The potential of the predator Teretriosoma nigrescens for the biological control of Prostephanus truncatus infesting dried cassava chips and wood of cassava (Manihot esculenta) was investigated. Trials on population growth were carried out using glass jars under ambient conditions in Togo. On both substrates, the development of the population of the larger grain borer was controlled effectively. After 12 weeks, with a predator to prey relationship of 100:5, the population of P. truncutus was reduced by 64.4% on dried cassava chips and by 76.5% on wood of hf. escukntu. The substrate weight losses were reduced by 32.3 and 55.7% respectively. Copyright 0 1996 Eisevier Science Ltd Key worak: Prostephanus truncatus, Teretriosoma nigrescens, cassava chips, cassava wood,
biological control.
INTRODUCTION
After the accidental introduction of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) into Togo, which probably happened in 1981 (Harnisch and Krall, 1984), the pest spread over the whole country within 10 y (Richter and Biliwa, 1991). Not only has the region of maize cultivation in the south of the country become affected by the pest, but also the northern parts where cassava is an important staple food. Cassava (Manihot escufenta Crantz) is stored as dried broken parts of the root, the so-called cassava chips. The two most important storage pests of cassava chips are the bostrichids Dinoderus spp. and P. truncates (Nyakunga, 1982; Hodges et al., 1985). Hodges et al. (1985) reported losses of 73.6% on fermented and of 52.3% on unfermented dried cassava chips after a 4-month storage period in Tanzania. In glass jar trials, Helbig (1993) found a higher loss potential with P. truncatus on this substrate than with Dinoderus bifoveolatus. The importance of this loss potential is due to the extent of production and storage of dried cassava roots. In Togo 120,000 to 150,000 t of cassava chips are produced annually (Adam, 1988) and a large part of the production is stored. In the “Region des Plateaux” more than half of the farmers store over 50% of their cassava yields as chips, while in the regions “de la Kara” and “Centrale” in excess of 75% of the cassava yield is stored as chips by practically all farmers. *This work is dedicated to Professor Dr F. A. Schulz, who died in March 1995. TAuthor for correspondence. 91
J.
92
Helbigand F. A. Schulz
Storage periods range from several weeks to one year and, normally, there is no treatment with insecticides, despite the fact that the insects are described as the main problem by farmers. The losses resulting from infestation of the cassava chips by P. truncatus are considerable, and as the commodity is an alternative substrate to the maize the matter is of considerable concern. Therefore, the biological control of the pest on cassava chips with the introduced predator Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) would be of great significance. However, for an effective biological control strategy, not only has the pest to be controlled on the stored product, but also places of survival and alternative habitats of the pest have to be taken into consideration in order to obtain a substantial reduction of the pest population. After several years of speculation about a!ternative habitats and the relationship of the larger grain borer to wood, it was shown that P. truncatus could reproduce on wood of M. esculenta, Poincinia regia Bojer (Helbig et al., 1990) and Sterculia tragacantha Lindl. (Helbig, 1993). Recently, the pest was also found on wood of Spondius purpurea L. and Bursera fagaroides (H.B.K.) Engl. in a natural habitat in Mexico by Ramirez-Martinez, De Alba Avila and Ramirez-Zurbia (1994). The effect of the predator would be optimal, if it reduced the population of the pest in these habitats and thus reduced the danger to stored products. The investigation described below is the first carried out on the effect of T. nigrescens on the development of the larger grain borer on cassava chips and wood (sticks).
METHODS The effect of T. nigrescens was tested in trials lasting 8 and 12 weeks. Glass jars were kept under natural climatic conditions in Togo but protected against rainfall. The insects used were taken from laboratory cultures. These cultures were maintained under natural conditions of temperature and relative humidity. Each experiment was repeated four times. In the experiment in which cassava chips were used as a feeding substrate, 50 pairs of adult P. truncatus were put on 500 g chips for the 8-week trial and on 700 g for the l%-week trial. Trials were set up in 2 1 glass jars which had a sheet of cellulose paper on the bottom to allow the beetles to walk around, and were closed with a wire gauze top. Cassava chips were examined to ensure they were free from pest attack and then kept frozen. Before use the chips were allowed to equilibrate to trial conditions. Seven days after introducing the pairs of P. truncatus, adult T. nigrescens (sex unknown) were added, using two different predator to prey relationships: “5: 100” and “10: 100” (T.n.: P.t.). The conditions during the trial were 2241°C and 3599% r.h. The cassava wood used consisted of sticks which had been debarked, dried in the sun and cut to a length of 6-8 cm. For the 8-week trial, 300 g of previously conditioned sticks were put in a 1.5 1 glass jar, and for the 1Zweek trial, 400 g were put in 2 1 jars. Twenty-five pairs of adult P. truncatus were put in each jar and after 7 days, five adult T. nigrescens (sex unknown) were added. Climatic conditions during the trial were 21-38°C and 45-99% r.h. At the end of the experiments the jars were fumigated with phosphine to kill the insects. To determine weight losses, the substrates were broken up and freed from dust. Then the contents of the jars were sieved with sieves of 3.15 mm mesh width. The pieces which remained on the 3.15 mm sieve were weighed and the weight difference from the initial input was classified as loss. Statistical analysis was undertaken by means of the t-test or the Tukey-HSD Test. Data were transformed as follows: insect populations: x’ = log,,(x); percentage data: x’ = arcsin &, with p = x/100 (Sachs, 1978; Brosius, 1988; Lauter and Pincus, 1989). The means which are significantly different are marked by different letters in the tables. The evaluation of the experiments in which cassava sticks were used included a classification of the damage, which is as follows: 1= 2= 3= 4= 5=
without damage-no holes; low damage-some holes (l-5); moderate damage-many holes (6-20), wood structure stable; heavy damage-very many holes (> 20), wood structure partly destroyed; very heavy damage-wood structure completely destroyed.
Teretriosoma nigrescens as a biological control of Prostephanus truncatus
93
Table 1. Development of populations of Prostephanus truncatus (P.t.) and Teretriosoma nigrescens (T.n.) after 8 and 12 weeks on dried cassava chips (means of four replicates) Final number of adults Initial number of adults
P. truncatus
T. nigrescens
P.t.: TX.
8 weeks
12 weeks
8 weeks
1oo:o 100: 5 100: 10
2349” 1117b 472’
21918 780b 238c
0 39” 113b
12 weeks 0 105” 151b
Table 2. Weight losses and dust production after 8 and 12 weeks due to Prostephanus truncatus (P.t.) on cassava chips without and with the predator Teretriosoma nigrescens (Tn.) (means of four replicates) Initial number of adults
Losses (%)
P.t.: T.n.
8 weeks
loo:o 100: 5 100: 10
92.3” 67.4b 54.1’
Dust (g)
12 weeks 8 weeks 373.8” 93.1a 269.4b 63.0b 230.1b 41.5’
12 weeks 533.6a 374.4b 253.7’
RESULTS
Suppression of P. truncatus
by T. nigrescens on cassava chips
Development of populations of the larger grain borer was strongly inhibited by T. nigrescens (Table 1). In the experiment with the initial ratio of 5: 100, the population of P. truncatus was reduced by 52% after 8 weeks and by 64% after 12 weeks. The higher number of predators (10 : 100) resulted in a further significant reduction in the number of pest individuals. Multiplication of the predator on a pest population living on cassava chips occurred and the predator to prey ratio of 10 : 100 yielded more T. nigrescens Fl individuals than the ratio 5 : 100. Suppression of the pest population due to the predator had a clear influence on losses (Table 2). Highest losses were found in the control, amounting to 92-93%. When five T. nigrescens were added, losses were reduced significantly by 27% after 8 weeks and by 32% after 12 weeks. Adding 10 predators again resulted in a significantly higher loss reduction. The larger grain borer produced a lot of dust by boring into the cassava chips (Table 2). In the control, 75% of the chips were transformed to dust after the 8- and 12-week periods. In comparison, only 30-33% of maize grains were transformed to dust when repeating the experiment on this commodity (see Helbig, 1993). The smooth structure of cassava chips and the lack of a Table 3. Development of populations of Prostephanus truncatus (P.t.) and Teretriosoma nigrescens (T.n.) after 8 and 12 weeks on wood sticks of Manihot esculenta (means of four replicates) Final number of adults Initial number of adults P.t. : T.n.
50:o 50:5
P. truncatus 8 weeks
9700 227b
T. nigrescens
12 weeks 8 weeks 0 1509” 355b 40
12 weeks 0 49
Table 4. Impact of Teretiosoma nigrescens (T.n.) on weight losses and dust production due to Prostephanus truncatus (P.t.) on wood of Manihot esculenta after 8 and 12 weeks (means of four replicates) Initial number of adults P.t. : T.n.
50:o 50:5
Losses (%) 8 weeks
33.4” 16.5b
Dust (g)
12 weeks 8 weeks 69.3” 53.7” 32.7b 23.8b
12 weeks 171.98 74.3s
J. Helbig and F. A. Schulz
Fig. 1. Frequency of five categories of damage to wood of Manihot esculenta due to Prostephanus truncatus after 8 weeks without and with the predator Teretriosoma nigrescens (T.n.) (means of four replicates; categories: 1 = without damage, 2 = low damage, 3 = moderate damage, 4 = heavy damage, 5 = very heavy damage).
protective covering both favour the boring activity of P. truncatus. This result gives an indication of the very considerable losses which could result from infestation by this pest. Suppression of P. truncatus
/JJJT. nigrescens on wood of M. esculenta
The predator T. nigrescens clearly affected the population development of the larger grain borer on the wood of M. esculenta (Table 3). After both 8- and 1Zweek periods the population of the pest was reduced by 76.5% as compared to the control. T. nigrescens reproduced successfully in this trial, the five predators giving rise to means of 35 and 44 Fl individuals in the 8- and 12-week periods, respectively (Table 3). Losses and dust production were significantly reduced in the trials with the predator (Table 4). After 8 weeks, losses were reduced by 51% and dust by 52% and in the longer lasting 1Zweek trial 56% and 57%, respectively. In addition to weight loss, the type of damage inflicted by P. ttuncatw on the sticks of M. esculentu was classified. Five categories were used, dependent on the number of holes and the condition of the wood (see methods). The damage frequency is depicted in Figs 1 and 2. After 8 weeks (Fig. 1) there was a relatively regular distribution over all categories in the control. There were 20% of the sticks in categories 4 and 5, which indicates that much wood was nearly destroyed
2
4
3 Catqofy CM4hout
5
of dame T.n. Baith
Tn.
Fig. 2. Frequency of five categories of damage to wood of Manihot esculenta due to Prostephanus truncatus after 12 weeks without and with the predator Tererriosoma nigrescens (T.n.) (means of four replicates; see Fig. 1 for categories).
Teretriosoma
nigrescens as a biological control of Prostephanus
truncatus
95
at this early stage of the trial. In comparison to this, the distribution was clearly concentrated on category 2 (low damage) and complete destruction of sticks, category 5, was rare when T. nigrescens was present. By prolonging the trial duration to 12 weeks (Fig. 2), there was a shift of the distribution to the higher categories in the control, Sixty per cent of the wood pieces were completely destroyed in the absence of T. nigrescens. In contrast, when T. nigrescens was present distribution of damage remained nearly constant. In comparison to the trial of 8 weeks duration, category 1 was observed more seldomly, whereas categories 3 and 4 occurred more frequently. The rapid progression of the damage in the control is in contrast to the gradual increase when the predator was present.
DISCUSSION
Development of a population of T. nigrescens and its effectiveness in suppressing the larger grain borer when the latter was breeding on dried cassava chips or wood (sticks) of M. esculenta was similar to that observed in studies on maize, carried out by Rees (1987), Boye (1988) Helbig et al., (1992) and Helbig (1993). In two trials of 14 weeks duration, Rees (1987) found that the number of adult P. truncatus resulting from populations developing on maize was reduced by 89.5 and 77.7%. According to BGye (1988), the reduction of the population of the larger grain borer after 14 weeks was 70% on maize cobs and 85.1% on shelled maize as compared to the control. In the present study, after 12 weeks the pest population was reduced by 64.4% (predator to prey ratio 5 : 100) and 89.1% (ratio 10: 100) on cassava chips and 76.5% on sticks of M. esculenta (ratio 5 : 50). As T. nigrescens was able to reproduce on the larger grain borer feeding on cassava chips and cassava wood, it can be assumed that in the case of migration of the borer on to these other feeding substrates, the predator will follow and help to control this pest as it does on maize. Biological control of the larger grain borer by T. nigrescens is of importance for the storage of dried cassava chips as well as for maize. Annual production is considerable and the product is commonly stored for periods in excess of several weeks, generally without insecticide treatment. The larger grain borer thus poses a severe threat, and its biological control may significantly contribute to loss reduction. The results of the investigation on the relationship between P. truncatus and T. nigrescens on wood of M. esculenta have to be seen in relation to the population of larger grain borer which lives outside the stores in the field. They demonstrate the possibility of controlling P. truncatus on cassava sticks which on some farms are stored behind the houses as firewood and often become infested by the larger grain borer. In these habitats, which are a potential source of infestation of the stores, T. nigrescens could play an important role in controlling the larger grain borer. When considering the use of T. nigrescens as a biological control agent one needs to take account of the possibility of an increased use of insecticides. T. nigrescens is more susceptible to the usual insecticides used in stores than the larger grain borer (Golob et al., 1990a). Hence, a treatment with insecticides would kill the predator too. Normally this is not of great importance because the predator is attracted by the aggregation pheromone of the larger grain borer (Bbye et al., 1992) which could not build up a population when effective prophylactic insecticides are applied. Therefore, T. nigrescens would not enter such a treated store. Attraction would be restricted to those stores where insecticides ineffective against the larger grain borer had been used. Because of the rapid development of resistance in populations of the larger grain borer (Golob et al., 1990b; Haubruge, 1990) the potential danger to T. nigrescens is relevant. If there was significant resistance, it would be necessary to decide whether the use of insecticides would have to be stopped totally to facilitate continued biological control with the predator. This would mean that damage by other pests has to be tolerated. The decision to use one or the other control strategy would have to be made for each case because it would depend on the impact of each pest species on the losses in the particular store.
REFERENCES Adam F. (1988) Prkntation du Togo. In Proceedings of the Regional African Workshop on the Containment of Larger Grain Borer, Report III, pp. 73-88. Arusha, Tanzania. SPR 32/1-G
and Control
J. Helbig and F. A. Schulz
96
B6ye J. (1988) Autiikologische Untersuchungen xum Verhalten des GroBen Kombohrers Prostephttnus truncutus (Horn) (Coleoptera, Bostrichidae). Ph.D. thesis, University of Kiel, 195 pp. Biiye J., Laborius G. A. and Schulz F. A. (1992) Response of Teretriosoma nigrescens Lewis (Col., Histeridae) to the pheromone of Prostephanus truncatus (Horn) (Col., Bostrichidae). Anzeiger j7ir SchSdlingskunde, PJlanzenschutz und Umweltschutz 65, 153-157.
Brosius G. (1988) SPSS/PC +, Basics und Graphics. Einjihrung und Prakttsche Beispiele. McGraw-Hill, Hamburg. Golob P., Broadhead P. and Wright M. (199Oa) Susceptibility of Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) to insecticide. In 5th International Working Conference on Stored-Product Protection (Edited by Fleurat-Lessard F. and Ducom P.), pp. 12591263. Bordeaux, France. Golob P., Broadhead P. and Wright M. (199Ob) Development of resistance to insecticides by populations of Prostephunus truncatus (Horn) (Coleoptera; Bostrichidae). In 5th International Working Conference on Stored-Product Protection (Edited by Fleurat-Lessard F. and Ducom P.), pp. 999-1006. Bordeaux, France. Ham&h R. and Krall S. (1984) Outbreaks and new records. Togo. Further distribution of the larger grain borer in Africa. FAO Plant Protection Bulletin 32, 113-114. Haubruge E. (1990) Toxicite et resistance de Prostephunus truncutus (Horn) (Col., Bostrichidae) a la permtthrine. In 5th International Working Conference on Stored-Product Protection (Edited by Fleurat-Lessard F. and Ducom P.), pp. 1009-1017. Bordeaux, France. Helbig J. (1993) Untersuchungen zu Gkologie und Biologie von Prostephunus truncntus (Horn) (Coleoptera, Bostrichidae) in Togo unter besonderer Beriicksichtigung der Wechselbexlehung mit dem Pradator Teretriosoma nigrescens Lewis (Coleoptera, Histeridae). Ph.D. thesis, Technical University of Berlin. Helbig J., Detmers H. B., Laborius G.-A. and Schulz F. A. (1990) Investigations on the capability of Prostephanus truncutus (Horn) (Coleoptera: Bostrichidae) to develop on different types of wood. In 5th Internationul Working Conference Stored-Productus Protection (Edited by Fleurat-Lessard F. and Ducom P.), pp. 89-97. Bordeaux, France. Helbig J., Laborius G.-A. and Schulz F. A. (1992) Recherches sur l’influence de Teretriosoma nigrescens Lewis (Co].: Histeridae) sur le developpement des populations de Prostephttnus truncutus (Horn) (Col.: Bostrichidae) dans des conditions semi-pratiques du Togo. In implementation of and Further Research on Biological Control of the Larger Grain Borer. Proceedings of FAOIGTZ Coordination Meeting (Edited by Biiye J., Wright M. and Laborius G.-A.), pp. 11I-122. Lome, Togo. Hodges R. J., Meik J. and Denton H. (1985) Infestation of dried cassava (Munihot esculenta Crantz) by Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae). Journal of Stored Products Research 21, 73-77. L&tter H. and Pincus R. (1989) Mathematisch-Statistische Datenanalyse. Oldenbourg, Mfmchen-Wein. Nyakunga Y. B. (1982) The biology of Prostephanus truncatus (Horn) (Coleoptera, Bostrichidae) on cassava. M.Sc. Thesis, University of Reading, 64 pp. Ramirez-Martinez M., De Alba-Avila A. and Ramirez-Zurbia R. (1994) Discovery of the larger grain borer in a tropical deciduous forest in Mexico. Journal of Applied Entomology 118, 354-360. Rees D. P. (1987) Laboratory studies on predation by Teretriosoma nigrescens (Lewis) (Col.: Histeridae) on Prostephanus truncatus f,Hom) (Col.: Bostrichidae) infesting maize cobs in the presence of other maize pests. Journal of Stored Products Research 23, 191-195.
Richter J. and Biliwa A. (1991) Landesweite Erhebung mittels Pheromonfallen zur Verbreitung von Prostephanus truncatus (Horn) (Col.: Bostrichidae) in Togo. Anzeiger fir SchSdlingskunde, Pfanzenschutz und Umweltschutz 64, 89-92. Sachs L. (1978) Angewandte Stattstik. Statbtische Methoden und ihre Anwendungen. Springer, Berlin.
J. srored Prod. Res. Vol. 32, No.
Pergamon
0022474X(95)00030-5
The Potential of the Predator Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) for the Control of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) on Dried Cassava Chips and Cassava Wood J. HELBIGT and F. A. SCHULZ* Humboldt-University of Berlin, Faculty of Agriculture and Horticulture, Department of Phytomedicine and Phytopathology, Lentzeallee 55-57, 14195 Berlin, Germany (Received 11 October 1995)
Abstract-The potential of the predator Teretriosoma nigrescens for the biological control of Prostephanus truncatus infesting dried cassava chips and wood of cassava (Manihot esculenta) was investigated. Trials on population growth were carried out using glass jars under ambient conditions in Togo. On both substrates, the development of the population of the larger grain borer was controlled effectively. After 12 weeks, with a predator to prey relationship of 100:5, the population of P. truncutus was reduced by 64.4% on dried cassava chips and by 76.5% on wood of hf. escukntu. The substrate weight losses were reduced by 32.3 and 55.7% respectively. Copyright 0 1996 Eisevier Science Ltd Key worak: Prostephanus truncatus, Teretriosoma nigrescens, cassava chips, cassava wood,
biological control.
INTRODUCTION
After the accidental introduction of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) into Togo, which probably happened in 1981 (Harnisch and Krall, 1984), the pest spread over the whole country within 10 y (Richter and Biliwa, 1991). Not only has the region of maize cultivation in the south of the country become affected by the pest, but also the northern parts where cassava is an important staple food. Cassava (Manihot escufenta Crantz) is stored as dried broken parts of the root, the so-called cassava chips. The two most important storage pests of cassava chips are the bostrichids Dinoderus spp. and P. truncates (Nyakunga, 1982; Hodges et al., 1985). Hodges et al. (1985) reported losses of 73.6% on fermented and of 52.3% on unfermented dried cassava chips after a 4-month storage period in Tanzania. In glass jar trials, Helbig (1993) found a higher loss potential with P. truncatus on this substrate than with Dinoderus bifoveolatus. The importance of this loss potential is due to the extent of production and storage of dried cassava roots. In Togo 120,000 to 150,000 t of cassava chips are produced annually (Adam, 1988) and a large part of the production is stored. In the “Region des Plateaux” more than half of the farmers store over 50% of their cassava yields as chips, while in the regions “de la Kara” and “Centrale” in excess of 75% of the cassava yield is stored as chips by practically all farmers. *This work is dedicated to Professor Dr F. A. Schulz, who died in March 1995. TAuthor for correspondence. 91
J.
92
Helbigand F. A. Schulz
Storage periods range from several weeks to one year and, normally, there is no treatment with insecticides, despite the fact that the insects are described as the main problem by farmers. The losses resulting from infestation of the cassava chips by P. truncatus are considerable, and as the commodity is an alternative substrate to the maize the matter is of considerable concern. Therefore, the biological control of the pest on cassava chips with the introduced predator Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) would be of great significance. However, for an effective biological control strategy, not only has the pest to be controlled on the stored product, but also places of survival and alternative habitats of the pest have to be taken into consideration in order to obtain a substantial reduction of the pest population. After several years of speculation about a!ternative habitats and the relationship of the larger grain borer to wood, it was shown that P. truncatus could reproduce on wood of M. esculenta, Poincinia regia Bojer (Helbig et al., 1990) and Sterculia tragacantha Lindl. (Helbig, 1993). Recently, the pest was also found on wood of Spondius purpurea L. and Bursera fagaroides (H.B.K.) Engl. in a natural habitat in Mexico by Ramirez-Martinez, De Alba Avila and Ramirez-Zurbia (1994). The effect of the predator would be optimal, if it reduced the population of the pest in these habitats and thus reduced the danger to stored products. The investigation described below is the first carried out on the effect of T. nigrescens on the development of the larger grain borer on cassava chips and wood (sticks).
METHODS The effect of T. nigrescens was tested in trials lasting 8 and 12 weeks. Glass jars were kept under natural climatic conditions in Togo but protected against rainfall. The insects used were taken from laboratory cultures. These cultures were maintained under natural conditions of temperature and relative humidity. Each experiment was repeated four times. In the experiment in which cassava chips were used as a feeding substrate, 50 pairs of adult P. truncatus were put on 500 g chips for the 8-week trial and on 700 g for the l%-week trial. Trials were set up in 2 1 glass jars which had a sheet of cellulose paper on the bottom to allow the beetles to walk around, and were closed with a wire gauze top. Cassava chips were examined to ensure they were free from pest attack and then kept frozen. Before use the chips were allowed to equilibrate to trial conditions. Seven days after introducing the pairs of P. truncatus, adult T. nigrescens (sex unknown) were added, using two different predator to prey relationships: “5: 100” and “10: 100” (T.n.: P.t.). The conditions during the trial were 2241°C and 3599% r.h. The cassava wood used consisted of sticks which had been debarked, dried in the sun and cut to a length of 6-8 cm. For the 8-week trial, 300 g of previously conditioned sticks were put in a 1.5 1 glass jar, and for the 1Zweek trial, 400 g were put in 2 1 jars. Twenty-five pairs of adult P. truncatus were put in each jar and after 7 days, five adult T. nigrescens (sex unknown) were added. Climatic conditions during the trial were 21-38°C and 45-99% r.h. At the end of the experiments the jars were fumigated with phosphine to kill the insects. To determine weight losses, the substrates were broken up and freed from dust. Then the contents of the jars were sieved with sieves of 3.15 mm mesh width. The pieces which remained on the 3.15 mm sieve were weighed and the weight difference from the initial input was classified as loss. Statistical analysis was undertaken by means of the t-test or the Tukey-HSD Test. Data were transformed as follows: insect populations: x’ = log,,(x); percentage data: x’ = arcsin &, with p = x/100 (Sachs, 1978; Brosius, 1988; Lauter and Pincus, 1989). The means which are significantly different are marked by different letters in the tables. The evaluation of the experiments in which cassava sticks were used included a classification of the damage, which is as follows: 1= 2= 3= 4= 5=
without damage-no holes; low damage-some holes (l-5); moderate damage-many holes (6-20), wood structure stable; heavy damage-very many holes (> 20), wood structure partly destroyed; very heavy damage-wood structure completely destroyed.
Teretriosoma nigrescens as a biological control of Prostephanus truncatus
93
Table 1. Development of populations of Prostephanus truncatus (P.t.) and Teretriosoma nigrescens (T.n.) after 8 and 12 weeks on dried cassava chips (means of four replicates) Final number of adults Initial number of adults
P. truncatus
T. nigrescens
P.t.: TX.
8 weeks
12 weeks
8 weeks
1oo:o 100: 5 100: 10
2349” 1117b 472’
21918 780b 238c
0 39” 113b
12 weeks 0 105” 151b
Table 2. Weight losses and dust production after 8 and 12 weeks due to Prostephanus truncatus (P.t.) on cassava chips without and with the predator Teretriosoma nigrescens (Tn.) (means of four replicates) Initial number of adults
Losses (%)
P.t.: T.n.
8 weeks
loo:o 100: 5 100: 10
92.3” 67.4b 54.1’
Dust (g)
12 weeks 8 weeks 373.8” 93.1a 269.4b 63.0b 230.1b 41.5’
12 weeks 533.6a 374.4b 253.7’
RESULTS
Suppression of P. truncatus
by T. nigrescens on cassava chips
Development of populations of the larger grain borer was strongly inhibited by T. nigrescens (Table 1). In the experiment with the initial ratio of 5: 100, the population of P. truncatus was reduced by 52% after 8 weeks and by 64% after 12 weeks. The higher number of predators (10 : 100) resulted in a further significant reduction in the number of pest individuals. Multiplication of the predator on a pest population living on cassava chips occurred and the predator to prey ratio of 10 : 100 yielded more T. nigrescens Fl individuals than the ratio 5 : 100. Suppression of the pest population due to the predator had a clear influence on losses (Table 2). Highest losses were found in the control, amounting to 92-93%. When five T. nigrescens were added, losses were reduced significantly by 27% after 8 weeks and by 32% after 12 weeks. Adding 10 predators again resulted in a significantly higher loss reduction. The larger grain borer produced a lot of dust by boring into the cassava chips (Table 2). In the control, 75% of the chips were transformed to dust after the 8- and 12-week periods. In comparison, only 30-33% of maize grains were transformed to dust when repeating the experiment on this commodity (see Helbig, 1993). The smooth structure of cassava chips and the lack of a Table 3. Development of populations of Prostephanus truncatus (P.t.) and Teretriosoma nigrescens (T.n.) after 8 and 12 weeks on wood sticks of Manihot esculenta (means of four replicates) Final number of adults Initial number of adults P.t. : T.n.
50:o 50:5
P. truncatus 8 weeks
9700 227b
T. nigrescens
12 weeks 8 weeks 0 1509” 355b 40
12 weeks 0 49
Table 4. Impact of Teretiosoma nigrescens (T.n.) on weight losses and dust production due to Prostephanus truncatus (P.t.) on wood of Manihot esculenta after 8 and 12 weeks (means of four replicates) Initial number of adults P.t. : T.n.
50:o 50:5
Losses (%) 8 weeks
33.4” 16.5b
Dust (g)
12 weeks 8 weeks 69.3” 53.7” 32.7b 23.8b
12 weeks 171.98 74.3s
J. Helbig and F. A. Schulz
Fig. 1. Frequency of five categories of damage to wood of Manihot esculenta due to Prostephanus truncatus after 8 weeks without and with the predator Teretriosoma nigrescens (T.n.) (means of four replicates; categories: 1 = without damage, 2 = low damage, 3 = moderate damage, 4 = heavy damage, 5 = very heavy damage).
protective covering both favour the boring activity of P. truncatus. This result gives an indication of the very considerable losses which could result from infestation by this pest. Suppression of P. truncatus
/JJJT. nigrescens on wood of M. esculenta
The predator T. nigrescens clearly affected the population development of the larger grain borer on the wood of M. esculenta (Table 3). After both 8- and 1Zweek periods the population of the pest was reduced by 76.5% as compared to the control. T. nigrescens reproduced successfully in this trial, the five predators giving rise to means of 35 and 44 Fl individuals in the 8- and 12-week periods, respectively (Table 3). Losses and dust production were significantly reduced in the trials with the predator (Table 4). After 8 weeks, losses were reduced by 51% and dust by 52% and in the longer lasting 1Zweek trial 56% and 57%, respectively. In addition to weight loss, the type of damage inflicted by P. ttuncatw on the sticks of M. esculentu was classified. Five categories were used, dependent on the number of holes and the condition of the wood (see methods). The damage frequency is depicted in Figs 1 and 2. After 8 weeks (Fig. 1) there was a relatively regular distribution over all categories in the control. There were 20% of the sticks in categories 4 and 5, which indicates that much wood was nearly destroyed
2
4
3 Catqofy CM4hout
5
of dame T.n. Baith
Tn.
Fig. 2. Frequency of five categories of damage to wood of Manihot esculenta due to Prostephanus truncatus after 12 weeks without and with the predator Tererriosoma nigrescens (T.n.) (means of four replicates; see Fig. 1 for categories).
Teretriosoma
nigrescens as a biological control of Prostephanus
truncatus
95
at this early stage of the trial. In comparison to this, the distribution was clearly concentrated on category 2 (low damage) and complete destruction of sticks, category 5, was rare when T. nigrescens was present. By prolonging the trial duration to 12 weeks (Fig. 2), there was a shift of the distribution to the higher categories in the control, Sixty per cent of the wood pieces were completely destroyed in the absence of T. nigrescens. In contrast, when T. nigrescens was present distribution of damage remained nearly constant. In comparison to the trial of 8 weeks duration, category 1 was observed more seldomly, whereas categories 3 and 4 occurred more frequently. The rapid progression of the damage in the control is in contrast to the gradual increase when the predator was present.
DISCUSSION
Development of a population of T. nigrescens and its effectiveness in suppressing the larger grain borer when the latter was breeding on dried cassava chips or wood (sticks) of M. esculenta was similar to that observed in studies on maize, carried out by Rees (1987), Boye (1988) Helbig et al., (1992) and Helbig (1993). In two trials of 14 weeks duration, Rees (1987) found that the number of adult P. truncatus resulting from populations developing on maize was reduced by 89.5 and 77.7%. According to BGye (1988), the reduction of the population of the larger grain borer after 14 weeks was 70% on maize cobs and 85.1% on shelled maize as compared to the control. In the present study, after 12 weeks the pest population was reduced by 64.4% (predator to prey ratio 5 : 100) and 89.1% (ratio 10: 100) on cassava chips and 76.5% on sticks of M. esculenta (ratio 5 : 50). As T. nigrescens was able to reproduce on the larger grain borer feeding on cassava chips and cassava wood, it can be assumed that in the case of migration of the borer on to these other feeding substrates, the predator will follow and help to control this pest as it does on maize. Biological control of the larger grain borer by T. nigrescens is of importance for the storage of dried cassava chips as well as for maize. Annual production is considerable and the product is commonly stored for periods in excess of several weeks, generally without insecticide treatment. The larger grain borer thus poses a severe threat, and its biological control may significantly contribute to loss reduction. The results of the investigation on the relationship between P. truncatus and T. nigrescens on wood of M. esculenta have to be seen in relation to the population of larger grain borer which lives outside the stores in the field. They demonstrate the possibility of controlling P. truncatus on cassava sticks which on some farms are stored behind the houses as firewood and often become infested by the larger grain borer. In these habitats, which are a potential source of infestation of the stores, T. nigrescens could play an important role in controlling the larger grain borer. When considering the use of T. nigrescens as a biological control agent one needs to take account of the possibility of an increased use of insecticides. T. nigrescens is more susceptible to the usual insecticides used in stores than the larger grain borer (Golob et al., 1990a). Hence, a treatment with insecticides would kill the predator too. Normally this is not of great importance because the predator is attracted by the aggregation pheromone of the larger grain borer (Bbye et al., 1992) which could not build up a population when effective prophylactic insecticides are applied. Therefore, T. nigrescens would not enter such a treated store. Attraction would be restricted to those stores where insecticides ineffective against the larger grain borer had been used. Because of the rapid development of resistance in populations of the larger grain borer (Golob et al., 1990b; Haubruge, 1990) the potential danger to T. nigrescens is relevant. If there was significant resistance, it would be necessary to decide whether the use of insecticides would have to be stopped totally to facilitate continued biological control with the predator. This would mean that damage by other pests has to be tolerated. The decision to use one or the other control strategy would have to be made for each case because it would depend on the impact of each pest species on the losses in the particular store.
REFERENCES Adam F. (1988) Prkntation du Togo. In Proceedings of the Regional African Workshop on the Containment of Larger Grain Borer, Report III, pp. 73-88. Arusha, Tanzania. SPR 32/1-G
and Control
J. Helbig and F. A. Schulz
96
B6ye J. (1988) Autiikologische Untersuchungen xum Verhalten des GroBen Kombohrers Prostephttnus truncutus (Horn) (Coleoptera, Bostrichidae). Ph.D. thesis, University of Kiel, 195 pp. Biiye J., Laborius G. A. and Schulz F. A. (1992) Response of Teretriosoma nigrescens Lewis (Col., Histeridae) to the pheromone of Prostephanus truncatus (Horn) (Col., Bostrichidae). Anzeiger j7ir SchSdlingskunde, PJlanzenschutz und Umweltschutz 65, 153-157.
Brosius G. (1988) SPSS/PC +, Basics und Graphics. Einjihrung und Prakttsche Beispiele. McGraw-Hill, Hamburg. Golob P., Broadhead P. and Wright M. (199Oa) Susceptibility of Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) to insecticide. In 5th International Working Conference on Stored-Product Protection (Edited by Fleurat-Lessard F. and Ducom P.), pp. 12591263. Bordeaux, France. Golob P., Broadhead P. and Wright M. (199Ob) Development of resistance to insecticides by populations of Prostephunus truncatus (Horn) (Coleoptera; Bostrichidae). In 5th International Working Conference on Stored-Product Protection (Edited by Fleurat-Lessard F. and Ducom P.), pp. 999-1006. Bordeaux, France. Ham&h R. and Krall S. (1984) Outbreaks and new records. Togo. Further distribution of the larger grain borer in Africa. FAO Plant Protection Bulletin 32, 113-114. Haubruge E. (1990) Toxicite et resistance de Prostephunus truncutus (Horn) (Col., Bostrichidae) a la permtthrine. In 5th International Working Conference on Stored-Product Protection (Edited by Fleurat-Lessard F. and Ducom P.), pp. 1009-1017. Bordeaux, France. Helbig J. (1993) Untersuchungen zu Gkologie und Biologie von Prostephunus truncntus (Horn) (Coleoptera, Bostrichidae) in Togo unter besonderer Beriicksichtigung der Wechselbexlehung mit dem Pradator Teretriosoma nigrescens Lewis (Coleoptera, Histeridae). Ph.D. thesis, Technical University of Berlin. Helbig J., Detmers H. B., Laborius G.-A. and Schulz F. A. (1990) Investigations on the capability of Prostephanus truncutus (Horn) (Coleoptera: Bostrichidae) to develop on different types of wood. In 5th Internationul Working Conference Stored-Productus Protection (Edited by Fleurat-Lessard F. and Ducom P.), pp. 89-97. Bordeaux, France. Helbig J., Laborius G.-A. and Schulz F. A. (1992) Recherches sur l’influence de Teretriosoma nigrescens Lewis (Co].: Histeridae) sur le developpement des populations de Prostephttnus truncutus (Horn) (Col.: Bostrichidae) dans des conditions semi-pratiques du Togo. In implementation of and Further Research on Biological Control of the Larger Grain Borer. Proceedings of FAOIGTZ Coordination Meeting (Edited by Biiye J., Wright M. and Laborius G.-A.), pp. 11I-122. Lome, Togo. Hodges R. J., Meik J. and Denton H. (1985) Infestation of dried cassava (Munihot esculenta Crantz) by Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae). Journal of Stored Products Research 21, 73-77. L&tter H. and Pincus R. (1989) Mathematisch-Statistische Datenanalyse. Oldenbourg, Mfmchen-Wein. Nyakunga Y. B. (1982) The biology of Prostephanus truncatus (Horn) (Coleoptera, Bostrichidae) on cassava. M.Sc. Thesis, University of Reading, 64 pp. Ramirez-Martinez M., De Alba-Avila A. and Ramirez-Zurbia R. (1994) Discovery of the larger grain borer in a tropical deciduous forest in Mexico. Journal of Applied Entomology 118, 354-360. Rees D. P. (1987) Laboratory studies on predation by Teretriosoma nigrescens (Lewis) (Col.: Histeridae) on Prostephanus truncatus f,Hom) (Col.: Bostrichidae) infesting maize cobs in the presence of other maize pests. Journal of Stored Products Research 23, 191-195.
Richter J. and Biliwa A. (1991) Landesweite Erhebung mittels Pheromonfallen zur Verbreitung von Prostephanus truncatus (Horn) (Col.: Bostrichidae) in Togo. Anzeiger fir SchSdlingskunde, Pfanzenschutz und Umweltschutz 64, 89-92. Sachs L. (1978) Angewandte Stattstik. Statbtische Methoden und ihre Anwendungen. Springer, Berlin.