Parasitoid fitness and Cry1Ab : does the insecticidal protein Cry1Ab derived from Bacillus thuringiensis affect the beneficial parasitoid Cotesia marginiventris?

Smith, Susan K (2007) Parasitoid fitness and Cry1Ab : does the insecticidal protein Cry1Ab derived from Bacillus thuringiensis affect the beneficial parasitoid Cotesia marginiventris? University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Any novel transgenic crop requires rigorous testing prior to commercialisation using an ecological risk assessment framework. The aim of this study is to help inform this process using the parasitoid Cotesia marginiventris, the host Spodoptera littoralis, purified Cry1Ab insecticidal protein and Bt176 transgenic maize.

Under direct exposure in injecting trials, parasitoid tibia length and sex ratio showed no significant difference between C. marginiventris injected with CrylAb or distilled water. Mean pupation was significantly longer in Cry1Ab infected treatment (9.5 days) when compared to the injected control group (9.1 days). C. marginiventris raised on S. littoralis hosts ingesting the toxin at 100μg g^-1 Cry1Ab showed a significantly extended pupation time (11.1 days) when compared to controls (9.5 days), mean tibia length was also significantly shorter at this concentration. The negative effects on pupation time and tibia length were therefore only measurable at 5-10 times plant expression levels. ELISA analysis confirmed that S. littoralis hosts ingested high amounts of Cry1Ab whilst feeding on both plant tissue and artificial diet treatments. However, whilst concentrations of Cry1Ab were found in S. littoralis larvae, frass and parasitoid cocoons none was detectable in parasitoid adults. Therefore the toxin is not accumulating up the food chain and appears to be secreted by C. marginiventris juveniles prior to adult emergence.

Further laboratory feeding experiments were carried out to establish the host mediated effects of Cry1Ab on C. marginiventris attack rate, host preference, parasitism success, emergence success and tibia length when reared on S. littoralis hosts ingesting 200, 20, 0μg g^-1 Cry1Ab, Bt176 transgenic maize or its near isoline. Host preference of adult parasitoids was also assessed to establish if adults showed any avoidance behaviour towards hosts reared on Bt176 maize, no avoidance behaviour was found. There was no significant difference between attack rate, proportion of attacks resulting in cocoon formation or proportion of adults emerging between the treatments. Tibia length was significantly shorter in the Bt176, 200 and 20μg g^-1 CrylAb treatments when compared to the near isoline and control artificial diet treatments. However, there was a trend towards reduced parasitism and emergence success following exposure to the Cry1Ab toxin. C. marginiventris larvae were exposed to the toxin via S. littoralis hosts at 100μg g^-1 over two generations. Following two generations of exposure to the toxin pupation time was significantly longer (10.4 days) when compared to controls (10.0 days). Neither tibia length, parasitism or emergence success were significantly different following exposure over two generations.