In the Australian cotton industry, toxins produced by the soil bacterium Bacillus thuringiensis (Bt toxins) are utilized to control lepidopteran pests: Helicoverpa armigera (cotton bollworm) and H. punctigera. Bt toxins kill insects by forming pores in the insect midgut, which leads to sepsis. The extensive use of Bt toxins including in the form of transgenic crops, has put a strong selection pressure on the pest insects in the field, leading to the development of resistant strains. Understanding the resistance mechanism is essential for planning the resistance management strategy to prolong the effectiveness of the Bt toxins.

Previous studies have demonstrated that larvae of cotton bollworm can develop a low-level tolerance to Bt toxins after being exposed to a sub-lethal dose (Rahman et al. 2004; Rahman et al. 2011). This induced tolerance is associated with increased immune activity in the midgut and hemolymph. The induced tolerance and the increase in the immune activity can be transferred to the next generation mainly via maternal effect, and the level of tolerance can increase over generations of exposure. In addition, highly-selected Cry1Ac-resistant strain also exhibits the same feature as low dose selected inducible tolerance H. armigara (Akhurst et al. 2003; Ma et al. 2005). The development of Cry1Ac tolerance is a threat to the use of Bt technology. Even though many studies have reported the increased immune response against Bt toxins, the role of the immune system in facilitating inducible tolerance against Bt toxins is unclear. Understanding the mechanism of inducible tolerance will help strengthening the established resistant management strategy.

The effect of the maternal experience on the offspring’s immune system (trans-generational immune priming; TGIP) has been demonstrated in several studies. Although there is speculation on the mechanism of TGIP such as the insertion of immune substances into eggs, and changes in the DNA methylation state of the offspring’s genome, the genes and metabolic pathways involved in the transmission are still undefined.

Given that immune components could be maternally transmitted via eggs, together with the importance of egg parasitoids to integrated cotton pest management, it is important to also understand whether there is any negative effect of Bt tolerance/exposure on H. armigera eggs with regard to parasitisation. A study done on egg parasitism by Trichogramma brassicae has also demonstrated that the parasitism success is greatly reduced in eggs from H. armigera survived from GM Bt maize (Steinbrecher 2004). Thus, the primary aim of this study was to investigate the TGIP mechanism of inducible Bt tolerance. The study also investigated the effect of inducible tolerance on key metrics of egg parasitism by the parasitoid wasp, Trichogramma pretiosum.

In this study, we compared the gene expression profiles of eggs from susceptible, Cry1Ac-tolerant and Cry1Ac-resistant H. armigera. We also investigated the parasitism success of Trichogramma wasp on eggs of susceptible and Cry1Ac-tolerant insects by measuring the number of eggs being successfully parasitized, and the number of progeny produced.