Response of Last Instar Helicoverpa armígera Larvae to Bt Toxin Ingestion: Changes in the Development and in the CYP6AE14, CYP6B2 and CYP9A12 Gene Expression

Abstract

Bt crops are able to produce Cry proteins, which were originally present in Bacillus thuringiensis bacteria. Although Bt maize is very efficient against corn borers, Spanish crops are also attacked by the earworm H. armigera, which is less susceptible to Bt maize. Many mechanisms could be involved in this low susceptibility to the toxin, including the insect's metabolic resistance to toxins due to cytochrome P450 monooxygenases. This paper examines the response of last instar H. armigera larvae to feeding on a diet with Bt and non-Bt maize leaves in larval development and in the gene expression of three P450 cytochromes: CYP6AE14, CYP6B2 and CYP9A12. Larvae fed on sublethal amounts of the Bt toxin showed reduced food ingestion and reduced growth and weight, preventing most of them from achieving the critical weight and pupating; additionally, after feeding for one day on the Bt diet the larvae showed a slight increase in juvenile hormone II in the hemolymp. Larvae fed on the non-Bt diet showed the highest CYP6AE14, CYP6B2 and CYP9A12 expression one day after feeding on the non-Bt diet, and just two days later the expression decreased abruptly, a finding probably related to the developmental programme of the last instar. Moreover, although the response of P450 genes to plant allelochemicals and xenobiotics has been related in general to overexpression in the resistant insect, or induction of the genes when feeding takes place, the expression of the three genes studied was suppressed in the larvae feeding on the Bt toxin. The unexpected inhibitory effect of the Cry1Ab toxin in the P450 genes of H. armigera larvae should be thoroughly studied to determine whether this response is somehow related to the low susceptibility of the species to the Bt toxin.

Global identification of microRNAs associated with chlorantraniliprole resistance in diamondback moth Plutella xylostella (L.)

“…A set of miRNAs were found to target several families of important genes that are often involved in insecticide resistance, such as cytochrome P45025262728, esterase29, GSTs30, ABC transporter family protein3132, cuticle protein33, glutamate-gated chloride channel3435 and superoxide dismutase (SOD)36. The target genes of some selected miRNAs are listed in Table 5.…”

Sublethal effects of chlorantraniliprole on juvenile hormone levels and mRNA expression of JHAMT and FPPS genes in the rice stem borer, Chilo suppressalis

“…The findings that sublethal concentrations of insecticides including chlorantraniliprole may disturb insect hormone balance are somewhat intriguing. While chlorantraniliprole targets ryanodine receptor, an intracellular calcium (Ca 2+ ) release channel, and calcium (Ca 2+ ) is a key second messenger that plays important physiological roles in various cells, it seems unlikely that the specific mode of action of chlorantraniliprole contributes to its effect on hormone biosynthesis since similar effects were also observed in other insecticides with different modes of action . Alternatively, the potential interrelation of xenobiotic responses and hormonal regulation of insect development might provide insights into the underlying mechanisms.…”

“…It has been suggested that insecticides are likely to interact with secondary sites of action, which may not lead to the death of the insect pests but may produce sublethal effects . Recently, several lines of evidence have indicated that sublethal concentrations of insecticides may affect fitness‐related traits by disturbing insect hormone balance; while the underlying mechanisms remain poorly studied, it has been suggested that sublethal concentrations of insecticides may compromise homeostasis of insect pests and change the gene expression in components of hormone biosynthesis pathways …”

Feeding behaviour and survival of Bacillus thuringiensis‐resistant and Bacillus thuringiensis‐susceptible larvae of Helicoverpa armigera (Lepidoptera: Noctuidae) exposed to a diet with Bacillus thuringiensis toxin

“…Muñoz et al () showed that Bt‐susceptible H. armigera larvae which were fed sub‐lethal amounts of Bt toxin reduced food ingestion, growth and weight, which did not allow the majority of them to achieve a critical weight to pupate. Food intake increased when larvae were moved from Bt diet to non‐Bt diet.…”