Cadherin fragments of Lepidopteran and Coleopteran species do not enhance toxicity of Cry1Ca and Vip3Aa proteins to Spodoptera exigua (Hübner) (Lepidoptera:Noctuidae)

Abstract

ABSTRACT Bacillus thuringiensis Berliner 1915 (Bt) is an entomopathogenic bacterium used to control insect pest worldwide. During its life cycle, Bt produces different insecticidal proteins, among which Vegetative Insecticidal Proteins (Vip) and Crystal proteins (Cry) are the most commercially used. Although large number of Bt toxins have been reported, only few of them (such as Cry1Ca and Vip3Aa) showed toxicity to the cosmopolitan pest Spodoptera exigua (Hübner) (Lepidoptera:Noctuidae). Diverse strategies have been developed to enhance the toxicity of Bt toxins and to delay the onset of resistance, including the addition of synergistic agents. Several synergistic compounds are based on Cry receptors. Especially cadherin receptor is one of the most widely studied and characterised across-taxa. In this regard, the insect cadherin fragments can enhance the toxicity of Cry insecticidal toxins, although less is known about the interaction between cadherin fragments and Vip toxins. The purpose of this work is to study the synergistic effect on toxicity of two different cadherin fragments (from Tenebrio molitor (Linnaeus) (Coleoptera: Tenebrionidae) and S. exigua) with two Bt toxins (Cry1Ca and Vip3Aa) on the lepidopteran pests S. exigua and Grapholita molesta (Busck) (Lepidoptera:Tortricidae). In our methodology, single dose response assays at different Bt protein:cadherin fragment ratios were performed. Upon potential synergistic results, the selected mixture was accurately analysed by dose–response bioassays to verify toxicity enhancement. However, no robust synergistic effect was observed after testing all the combinations of insect species-toxin-cadherin fragment, confirming that synergistic effects of cadherin fragments may not be generalised for all combinations.