Biology of striped stem borer, Chilo suppressalis (Walker), in relation to resistance management for transgenic Bacillus thuringiensis (Bt)-rice

Results of the study showed that SSB [striped stem borer] moth emergence peaked at 1900 to 2100 h. Males and females were ready to mate about three hours after eclosion. The females usually rested on the plants beside the location of pupal eclosion or took a short flight. The males were more active and readily dispersed after full development and hardening of their wings. Some SSB males mated with the calling female nearby before dispersing (as a result of male and female pupae having been placed side-by-side). Almost all males dispersed when the male and female pupae were put further apart (approx. 10 m), indicating that mating relative to movement was influenced by the availability and proximity of mates. This suggests that the effectiveness of field-to-field refuges for resistance management for Bt rice could be undermined by assortative mating where the emergence of adult C. suppressalis naturally occurs side-by-side in the rice field. The occurrence and survival of SSB on five possible alternative host plants, namely: Echinochloa crus-galli (L.). E. colona (L.) Link, Eleusone indica (L.) Gaertn, Ischaemum rugosum Salisb., and Panicum repens (L.) were evaluated. Only a small number of SSB larvae and pupae were recovered from E. crus-galli and E. colona, and none was recovered from the other three weedy host during the field surveys in 1999 and 2000 in Laguna Province [Philippines]. First instar larva survived on E. crus-galli, E. colona, E. indica and I. rogosum after 4 days in cut stem assays. However, SSB showed low survival and prolonged development when tested in whole plant assay of the weedy plants for periods of 14 and 25 days. These results suggest that these hosts are unlikely to support a sufficient SSB number to act as successful refuge for Bt rice. The F2 screen methodology entails sib-mating with isofemale lines to detect any resistance allele that was initially present in the fertilized eggs of field-collected females. The F2 screen experiment usign artificial diet containing 12 ug Cry1Ab toxin per ml showed large variation (34 to 100%) and a lower (83.7%) than anticipated level (95%) of mortality of F2 larvae from 107 isofemale lines. More consistent results were obtained using intact Bt plants. The F2 screen for 151 isofemale lines using IR72 transformed with a Cry1Ab/cry 1ac hybrid toxin gene showed that the frequency of a resistance allele conferring major resistance is 1.6 x 10 sup - 3 with a 96% detection probability. The upper 95% Credibility Intervals of the frequency of such an allele is 4.8 x 10 sup-3. This upper estimate approaches 10 sup-3, which has been suggested as the maximum initial resistance allele frequency that would enable the high dose/refuge resistance management strategy to be successfully applied