Influence of tannic acid and Cry1Ac toxin of Bacillus thuringiensis on larval survival, growth, and development of Helicoverpa armigera

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) causes huge economic losses in cotton production around the world. Tannin, one of the important secondary substances in cotton plants, can increase the δ‐endotoxin activity of Bacillus thuringiensis ssp. kurstaki. The mechanism of interaction between tannin and Bt toxin on H. armigera is unclear. We investigated the interaction between tannic acid and Cry1Ac toxin in H. armigera, and monitored survival, growth, and development during the larval period after treating the larvae with four concentrations of Cry1Ac toxin (0, 2, 8, and 14 μg^?1) alone or in combination with four concentrations of tannic acid (0, 0.5, 1, and 2 mg g^?1). Mortality of larvae treated with both tannic acid and Cry1Ac was higher than the mortality of larvae treated with tannic acid or Cry1Ac alone. Mortality was 47.5 and 51.5% in larvae treated with 14 μg g^?1 Cry1Ac alone or 2 mg g^?1 tannic acid alone, respectively. In contrast, larval mortality was 75% when treated with the mixture of 14 μg g^?1 Cry1Ac and 2 mg g^?1 tannic acid, suggesting that a mixture of the two enhanced the effectiveness of each one alone. The developmental time of larvae treated with the combination of tannic acid and Cry1Ac was significantly longer than when they were treated with Cry1Ac or tannic acid alone. Larval weight, pupal weight, and pupation rate were also significantly reduced in larvae treated with both toxins, compared with the larvae treated with either toxin alone. These results showed that the interactive effect of tannic acid and Cry1Ac on larval growth inhibition is additive, and that tannic acid improves Cry1Ac toxicity to insects. Tannic acid used in combination with B. thuringiensis might potentially reduce overall insecticide use, thus delaying development of insecticide resistance.     

A single linkage group confers dominant resistance to Bacillus thuringiensis δ‐endotoxin Cry1Ac in Helicoverpa armigera

The BKBT strain of Helicoverpa armigera was derived from a susceptible BK77 strain (collected from Bouake, Cote D’Ivoire in 1977) through 30 generations of selection with activated Bacillus thuringiensis δ‐endotoxin Cry1Ac. Unlike recessive inheritance of Cry1Ac resistance in H. armigera from previous reports, resistance to activated Cry1Ac in the BKBT strain is dominant. A backcross approach was used to map dominant resistance to Cry1Ac in the BKBT strain. One hundred and forty‐seven informative amplified fragment length polymorphism (AFLP) DNA markers covered all 31 linkage groups of H. armigera. Five AFLP markers linked to Cry1Ac resistance in the BKBT strain were on the same autosomal linkage group, which is the only linkage group contributing dominant Cry1Ac resistance in the BKBT strain of H. armigera.     

DIVERSITY IN GUT MICROFLORA OF Helicoverpa armigera POPULATIONS FROM DIFFERENT REGIONS IN RELATION TO BIOLOGICAL ACTIVITY OF Bacillus thuringiensis δ‐ENDOTOXIN Cry1Ac

Transgenic crops expressing toxin proteins from Bacillus thuringiensis (Bt) have been deployed on a large scale for management of Helicoverpa armigera. Resistance to Bt toxins has been documented in several papers, and therefore, we examined the role of midgut microflora of H. armigera in its susceptibility to Bt toxins. The susceptibility of H. armigera to Bt toxin Cry1Ac was assessed using Log‐dose‐Probit analysis, and the microbial communities were identified by 16S rRNA sequencing. The H. armigera populations from nine locations harbored diverse microbial communities, and had some unique bacteria, suggesting a wide geographical variation in microbial community in the midgut of the pod borer larvae. Phylotypes belonging to 32 genera were identified in the H. armigera midgut in field populations from nine locations. Bacteria belonging to Enterobacteriaceae (Order Bacillales) were present in all the populations, and these may be the common members of the H. armigera larval midgut microflora. Presence and/or absence of certain species were linked to H. armigera susceptibility to Bt toxins, but there were no clear trends across locations. Variation in susceptibility of F₁ neonates of H. armigera from different locations to the Bt toxin Cry1Ac was found to be 3.4‐fold. These findings support the idea that insect migut microflora may influence the biological activity of Bt toxins.