Combination of Bacillus thuringiensis and Habrobracon hebetor for the biological control of Plodia interpunctella

In this paper, we review our work on biological control of Plodia interpunctella (Hubner) (Zhang et al., 1995) in our laboratory over the years. We isolated, screened and evaluated Bacillus thuringiensis strains collected from warehouses against P. interpunctella and determined the lethal concentration for 50% of the population (LC50) of highly toxic isolates and compositions of crystal proteins and the genotypes of these isolates; evaluated the role of host-instar and refuge on the parasitization behavior of Habrobracon hebetor Say (Hymenoptera: Braconidae); investigated the effect of combining B. thuringiensis with H. hebetor for management of P. interpunctella infestation and assessed the influence of B. thuringiensis on H. hebetor during this combination treatment. The results showed that three strains of B. thuringiensis (IMM130, IMM368 and IMM408) were highly toxic to P. interpunctella among 122 B. thuringiensis isolates obtained from 413 field samples (Zhang et al., 2000a); Isolate IMM408 with LC50 1.24 μg/g diet, was most potent(Akinkurolere et al., 2007). It belongs to H7 serotype and contains ~135kDa crystal proteins and cry1Ab9, cry1Ca1, cry1Da1 and cry2 genotypes (Zhang et al., 2000b). It was observed that H. hebetor could parasitize all larval stages of P. interpunctella, but significantly fewer first and second instars were parasitized under choice and no-choice conditions (Akinkurolere et al., 2009a). Parasitized fourth instars were more profitable to H. hebetor irrespective of refuge or choice factors, as significantly more adult parasitoids emerged from host instars. Therefore, H. hebetor females consistently showed high preference for late instars of P. interpunctella when they were offered a choice between early and late host instars. Refuge significantly hindered H. hebetor from locating the early instars, but not the late instars (Akinkurolere et al., 2009a). Bacillus thuringiensis-parasitoid combination treatment significantly evoked more P. interpunctella mortality than either treatments (B. thuringiensis or parasitoid) when used singly (Akinkurolere et al., 2009b). Bacillus thuringiensis or H. hebetor alone caused 42% and 35% P. interpunctella larval mortality, respectively. The B. thuringiensis-parasitoid combination treatment significantly evoked more P. interpunctella mortality (86%) than other single treatments. Progeny development of parasitoid wasp was dependent upon its susceptibility to B. thuringiensis contaminated hosts. H. hebetor was able to successfully complete its development on the hosts although, fewer wasps emerged from B. thuringiensis-parasitoid combined treatment than in none B. thuringiensis treatments (Akinkurolere et al., 2009b). H. hebetor showed positive response to acetone and hexane extracts from frass and larvae of P. interpunctella, and the active compounds are mostly hydrocarbons (unpublished data).Keywords: Bacillus thuringiensis, Habrobracon hebetor, Plodia interpunctella, Pest management, Parasitoid

Resistance of strains of rice weevil, Sitophilus oryzae (Coleoptera: Curculionidae) to pirimiphos methyl

Insecticidal resistance of five strains of Sitophilus oryzae (L.) to pirimiphos methyl (Actellic®) was investigated in the laboratory at 28 ± 2oC and 89 ± 5% relative humidity. Rice infested by S. oryzaewas sampled from Nigerian stores in Akure (Ondo state), Ibadan (Oyo state), Ikotun (Lagos state) and Ilesha (Osun state) and a laboratory culture (in Akure). Twenty adult weevils from each location were exposed to filter papers impregnated with liquid pirimiphos methyl at one of seven concentrations ranging from 0.0 to 5.0%. Mortality rates of the rice weevils were observed at 3, 6 and 24 h after treatment. The mortality of weevils increased with increasing concentration and period of exposure. Irrespective of the strain, 100% mortality was observed in all the treatments at 24 h after exposure to pirimiphos methyl. Weevils sampled from Ibadan were the most resistant, while laboratory strains were the least resistant at 2 and 5% concentrations.Keywords: Strains, insecticide, Pirimiphos methyl, Resistance, Sitophilus oryzae