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The use of Cry toxins from Bacillus thuringiensis (Bt) is a breakthrough in the cultivation of transgenic Bt plants because of its high specificity and safety for the environment. However, a serious threat to the sustainability of this technology is the potential for insect populations to develop resistance to Bt toxins. It is important to understand the pathogen-insect interactions to extend the usefulness of products based on B. thuringiensis. Recent studies reported evidence of a tolerance mechanism associated with the immune response. Because of the importance of Alabama argillacea (Hubner, 1818) as a cotton pest, this research assessed its immunological alterations (cellular and humoral) when challenged with formulations of B. thuringiensis var. aizawai and B. thuringiensis var. kurstaki. The results suggest that the fourth instar larvae of A. argillacea do not have the potential to develop natural immune tolerance to the formulations based on B. thuringiensis. Dipel^® led to a quantitative variation in all cell types, while XenTari^® changed prohemocytes, plasmatocytes, granulocytes and oenocytoids. In insects treated with Dipel^® there was no increase in the level of nitric oxide. These differences in response to treatments can be attributed to differences in the composition of the insecticides tested. The results indicate that the insecticide Dipel^® caused significant changes in cellular and humoral immune system of fourth instar larvae of A. argillacea, while XenTari^® caused only changes in the cellular immune system, furthermore Dipel^® was faster to cause the cellular changes mentioned.

Abstract

Growth-regulating insecticides are used for several cotton pests when immature, and although not registered for Anthonomus grandis, their use causes a satisfactory population reduction in adults. However, it is not known which mechanisms are involved in this reduction. Thus, we studied which tissue and cellular changes may be involved in this process. Adults were fed with flower buds treated with lufenuron (4 ml commercial product /l) for 24 h. Then, the histology of the gonads, oxidative stress and apoptosis were evaluated after periods of 24 h and 48 h. Our study revealed that lufenuron activates the oxidative stress pathway in A. grandis, causing significant changes at the cellular level. These changes may have been caused by stimulation of the production of oxygen ions, free radicals and hydrogen peroxides, resulting in an increase in lipid peroxidation 48 h after treatment. These effects were confirmed by the presence of histopathologies in the gonads of this pest such as disorganization of the follicular cell epithelium, reduction of the yolk, disappearance of the germinal vesicle, reduction of sperm bundles and cysts. Such alterations injured the gonads and impaired tissue homeostasis. It is concluded that together these factors lead to a desirable population reduction when it comes to managing the pest in the field, because the aim is to keep it below the control level. In addition to this desired effect, it is worth noting that lufenuron is safer when compared to other commonly used products, another feature that makes its use quite interesting.