Effects of Entomopathogenic Hyphomycetes against the Larger Grain Borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), and Its Predator, Teretriosoma nigrescens Lewis (Coleoptera: Histeridae)

Journal of Invertebrate Pathology 77, 75–77 (2001) doi:10.1006/jipa.2000.4986, available online at http://www.idealibrary.com on

NOTE Effects of Entomopathogenic Hyphomycetes against the Larger Grain Borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), and Its Predator, Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) The larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), is a major pest of stored maize and cassava that was accidentally introduced into Africa (Harnisch and Krall, 1984). In the context of a classical biological control program, the predator Teretriosoma nigrescens Lewis (Coleoptera: Histeridae) has been released in several African countries (Compton and Ofosu, 1994; Giles et al., 1995; Borgemeister et al., 1997). At present it is not certain whether T. nigrescens will provide a complete solution to the larger grain borer problem in Africa. Therefore, other control measures that could be used in combination with T. nigrescens are required. Strains of the entomopathogenic fungi Beauveria bassiana (Balsamo) Vuillemin and Metarhizium anisopliae (Metschnikoff) Sorokin have been qualitatively researched as microbials against P. truncatus (Schulz and Laborius, 1987; Burde, 1988). Our objective was to quantify adulticidal effects of strains of B. bassiana and M. anisopliae against P. truncatus and its predator T. nigrescens. Our experiments were conducted at the Benin station of the International Institute of Tropical Agriculture (IITA) at Cotonou, Republic of Benin (West Africa) with four strains obtained from various Institutes (Table 1). B. bassiana conidia were produced on potato dextrose agar (Merck/BDH) at 22°C. M. anisopliae conidia were cultured on Sabouraud dextrose agar (4% glucose) (Merck/BDH) at 28°C. The percentage of germinated conidia was calculated three times using 300 conidia per strain, and their viability ranged from 94 to 97%. P. truncatus and T. nigrescens were collected near Cotonou (Benin). P. truncatus were reared on 500 g maize grains in 800-ml glass jars covered with brass gauze at 30 ⫾ 1°C and 75 ⫾ 5% R.H. under a L12:D12 h photoperiod. In each jar the maize grains were infested with 300 unsexed adult P. truncatus. T. nigrescens were reared in the same abiotic conditions as P. truncatus. In each jar, 30 T. nigrescens adults were put on 500 g maize grains and 300 P. truncatus adults as prey. The four strains were assayed at 10 9 conidia ml ⫺1 in aqueous solution (with Tween 80 at 0.05%) and a con-

trol (Tween 80 at 0.05% v/v of distilled water) on adult P. truncatus and T. nigrescens. For each species, adults were placed individually in metal tubes (1 cm diameter and 2.5 cm high) (C. Lorek, pers. com.). The bottom of the tubes was covered with a 2-mm-thick layer of ground maize grains (cultivar DMR) glued with methyl cellulose. Fifty microliters of the fungal suspension or the control solution was evenly pipetted over the grains laid in each tube. Each tube was treated separately 1 h before the insects were introduced, to allow the aqueous suspension on the grains to dry. The tubes were then placed in a box, divided into compartments (48 tubes per box), which was covered with a metal plate to prevent the insects from escaping but allowing air circulation. The experiment was conducted in a laboratory at 28°C and 55% RH. Insect mortality was recorded every second day for 14 days. Individuals that stayed immobile following probing with a fine camel brush were considered dead. The treatments were carried out on 24, 7- to 14-dayold adults (12 males and 12 females) per strain for each of the four replicates. The experiments were conducted as randomized complete block designs, with four blocks, each being performed on a different day. Statistical analysis of mortality rates for the adult P. truncatus and T. nigrescens was done using an analysis of variance, on days 6 and 14 following treatments. In conjunction with significance of treatment effects with ANOVA, SNK tests for mean separation were performed with SuperANOVA (Abacus Concepts, 1989). To evaluate the infection by entomopathogenic fungi, dead individuals were incubated in petri dishes to verify colonization of B. bassiana or M. anisopliae. A filter paper, wetted with distilled water, was placed in a petri dish and cadavers rinsed with distilled water were placed on the paper. The petri dishes were sealed with parafilm. Colonization of cadavers was observed over a period of 4 days at 28°C. For all assays, ⬎90% of the dead P. truncatus and T. nigrescens treated with B. bassiana or M. anisopliae produced conidia following incubation, except for strain IMI 327910, which yielded 81.1% colonization in the bioassay with adult T. nigrescens at a concentra75

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NOTE

TABLE 1 Mean Cumulative Mortality (%) ⫾ SE of Adult P. truncatus and T. nigrescens, 6 and 14 Days after Treatment with Entomopathogenic Fungi at Concentrations of 10 9 Conidia ml ⫺1 P. truncatus Strain (country of origin) Control M. anisopliae KAM 3 (Cameroon) 2 B. bassiana IMI 330194 (Kenya) 3 IMI 327910 (Togo) 3 I94-907 (Uganda) 4 1 2 3 4

Day 6

T. nigrescens Day 14

Day 6

Day 14

16.7 ⫾ 7.0a

2.1 ⫾ 2.1a

3.1 ⫾ 3.1a

77.1 ⫾ 12.0b

90.6 ⫾ 3.6b

20.8 ⫾ 6.6b

41.7 ⫾ 9.2b

83.3 ⫾ 8.8b 85.4 ⫾ 6.9b 85.4 ⫾ 8.4b

92.7 ⫾ 4.6b 95.8 ⫾ 4.2b 95.8 ⫾ 2.9b

26.0 ⫾ 2.6b 14.6 ⫾ 5.5ab 13.5 ⫾ 3.1ab

56.2 ⫾ 3.6b 47.9 ⫾ 6.5b 43.8 ⫾ 4.3b

10.4 ⫾ 6.5a

1

Within a column, numbers followed by the same letters are not significantly different (SNK test, P ⫽ 0.05). BBA, Federal Biological Research Centre for Agriculture and Forestry, Darmstadt, Germany. International Mycological Institute (Egham, Surrey, England). International Institute for Biological Control, Imperial College, Ascot, England.

tion of 10 9 conidia ml ⫺1. No colonization was observed in the control groups. As the mortality rates for male and female adult P. truncatus did not differ on day 6 (F ⫽ 0.94; df ⫽ 1,38; P ⫽ 0.76) and on day 14 (F ⫽ 1.0 ⫺36; df ⫽ 1,38; P ⫽ 1.0), the sexes were pooled for further statistical analyses. There were no statistical block effects. On day 6, significantly higher mortality rates were recorded for all fungal treatments (range: 77.1– 85.4%) applied to adult P. truncatus compared to the control group (i.e., 10.4%) (F ⫽ 13.1; df ⫽ 4,15; P ⫽ 0.0001). On day 14, mortality in the fungal treatments ranged from 90.6 to 95.8% and was 16.7% in the control (F ⫽ 54.6; df ⫽ 4,15; P ⫽ 0.0001) (Table 1). On both days 6 and 14, SNK tests detected significant (␣ ⫽ 0.05) differences between any fungal strains and the control, but no significant differences among the strains. Fourteen days after treatment, ⬎40% of the adult T. nigrescens had died in the B. bassiana and M. anisopliae treatments (Table 1). There were no statistical block effects. Mortality rates of T. nigrescens for each of the treatment differed significantly (␣ ⫽ 0.05) from those of the control group on day 6 (F ⫽ 4.28; df ⫽ 4,15; P ⫽ 0.016) and on day 14 (F ⫽ 11.8; df ⫽ 4,15; P ⫽ 0.0002). On day 6 SNK tests detected significant (␣ ⫽ 0.05) differences between the control and the strains KAM 3 and IMI 330194. On day 14 SNK tests detected significant (␣ ⫽ 0.05) differences between all fungal strains (range: 41.7–56.2%) and the control (⫽3.1%). At a concentration of 10 9 conidia ml ⫺1 adult mortality rates were nearly twice as high for P. truncatus (90 – 95%) as for its predator (42–56%). Preliminary experiments were also conducted to determine whether our strains could exert larval or ovicidal mortality. Applications of formulations of strains KAM 3, IMI 327910, and I94-907 containing 10 9 conidia ml ⫺1 resulted in 100% mortality of P. truncatus

larvae on day 4, whereas control mortality was 0%. The strains had no effect on eggs of P. truncatus. The differential effect on the strains of the pest and the predator and larvicidal effects (preliminary results) showed that mycoinsecticides have potential against P. truncatus. The next logical step should be to find, for both larvae and adults, a dose that would kill a large proportion of P. truncatus while having minimal impact on the predator T. nigrescens. Unpublished results (C. Bourassa et al.) obtained under simulated field conditions indicated that further research should also be done to improve technologies such as spore production, storage, formulation, and spraying, so that mycoinsecticides could be effective under real agronomic conditions. Key Words: Prostephanus truncatus; Teretriosoma nigrescens; Beauveria bassiana; Metarhizium anisopliae; mycopesticides; maize; microbial control. This work was financed by a Canadian International Development Agency (CIDA) fellowship awarded to C. Bourassa under the program “Research grant for Canadian citizens.” Additional financial and material support was provided by the International Institute of Tropical Agriculture (IITA biological control project). We thank The´otime Adouhoun, Richard Oussou, Pascal Degbey, and Gabriel Heviefo for their technical assistance. We also thank Gisbert Zimmermann, Jean-Charles Coˆte´, and Sylvia Todorova for commenting on an early version of the manuscript. This is contribution No. 335/2000.09.03R of the Horticultural and Development Research Center, Agriculture and Agri-Food Canada at Saint-Jean-surRichelieu. REFERENCES Abacus Concepts 1989. SuperANOVA, Accessible General Linear Modeling, Berkeley, CA. Borgemeister, C., Djossou, F., Adda, C., Schneider, H., Djomamou, B., Degbey, P., Azoma, B., and Markham, R. H. 1997. Establishment, spread and impact of Teretriosoma nigrescens (Coleoptera: Histeridae), an exotic predator of the larger grain borer (Coleoptera: Bostrichidae) in southwestern Benin. Environ. Entomol. 26, 1405–1415.

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NOTE Burde, S. 1988. Mikrobielle Antagonisten von Prostephanus truncatus (Horn) (Coleoptera, Bostrichidae)—Grundlagen fu¨r eine Biotherapie im tropischen Vorratsschutz [Microbial antagonists of Prostephanus truncatus (Horn) (Col., Bostrichidae)—Basis for a biological treatment in tropical store protection]. Deutsche Gesellschaft fu¨r Technische Zusammenarbeit (GTZ), Eschborn, Germany. Compton, J. A. F., and Ofosu, A. 1994. Biological control of the larger grain borer with Teretriosoma nigrescens. pp. 1– 8. In “Sixth Quarterly Report: Ghana Larger Grain Borer Project (Research Programme-Volta Region)” (J. A. F. Compton, Ed.). Ghana Ministry of Food and Agriculture, Accra, Ghana, and U.K. Overseas Development Administration, London. Giles, P., Hill, G., Nang’ayo, F., Farrell, G., Stabrawa, A., and Wekesa, P. 1995. Entomological and socio-economic investigations for the development of integrated pest management strategies against Prostephanus truncatus. Kenya Agriculture Research Institute, Nairobi, Kenya, and Natural Resources Institute, Chatham, Kent, U.K. Harnisch, R., and Krall, S. 1984. Togo: Further distribution of the larger grain borer in Africa. FAO Plant Prot. Bull. 32, 113– 114. Schulz, G., and Laborius, G. A. 1987. Strategy for bio-integrated control of Prostephanus truncatus (Horn): (Col., Bostrichidae) In “Proceedings of the 4th International Working Conference on Stored-Product Protection, September 1986,” pp. 497–503. Tel Aviv, Israel.

C. Bourassa* C. Vincent† C. J. Lomer‡ C. Borgemeister‡ ,§ Y. Mauffette* *Biology Department Universite´ du Que´bec a` Montre´al P.O. Box 8888 Montreal, Quebec, Canada H3C 3P8 †Horticultural Research and Development Centre Agriculture and Agri-Food Canada 430 Gouin Boulevard Saint-Jean-sur-Richelieu, Quebec, Canada J3B 3E6 ‡Plant Health Management Division International Institute of Tropical Agriculture P.O. Box 08-0932 Cotonou, Benin §Institute of Plant Diseases and Plant Protection University of Hanover Herrenha¨user Str. 2 30419 Hanover, Germany Received June 1, 1999; accepted October 20, 2000; published online December 19, 2000