Thuringiensin toxicity to Tetranychus urticae Koch and Panonychus ulmi (Koch) (Tetranychidae) and effects on cuticle development of immature stages of T. urticae

Thuringiensin is a novel pesticide that has been evaluated against a range of insects and spider mites. It is generally more effective against immature stages than adults of spider mites, but there is considerable variation in the published data on this active ingredient. Current reviews have suggested that differences in bioassay methods, formulations, and species may be responsible for these variations. Therefore a more systematic and intensive investigation was carried out, where the mode of action of thuringiensin on cuticle synthesis inhibition was specially taken into account. The direct and residual toxicity of thuringiensin to different stages of T. urticae and P. ulmi and the influence on T. urticae reproduction and population development were evaluated in laboratory bioassays. Direct toxicity was higher than residual toxicity to T. urticae for all life stages. Immature stages of T. urticae and P. ulmi were more susceptible to thuringiensin than adults. P. ulmi was more susceptible to thuringiensin than T. urticae. Fecundity of T. urticae was significantly reduced when females were exposed to residues for 2 d. Complete suppression of T. urticae population development was achieved when the F1 generation was exposed to thuringiensin residues; however, the level of suppression was concentration dependent. The effects of temperature, host plant, active ingredient mobility, surfactant, residue age and larval age on the response T. urticae to thuringiensin were also evaluated in laboratory bioassays. T. urticae larvae were significantly more susceptible to thuringiensin at 13°C than those at 28°C. A significantly different response of T. urticae larvae to thuringiensin residues on peach and apple leaves was found. Older T. urticae larvae were significantly more susceptible than younger larvae when exposed to thuringiensin residues. No systemic or trans laminar effects of thuringiensin were detected; leaf side and surfactant did not affect the efficacy of thuringiensin. The effect of thuringiensin on the ultrastructure of T. urticae cuticle was investigated using an electron microscope. Thuringiensin was found to affect epicuticle synthesis in larvae, protonymphsand deutonymphs. The disruptive effects of thuringiensin on larval cuticle formation was observed from 12 h onwards, indicating that the effect of the time of exposure of mites to thuringiensin is related to the stage of cuticle development. The results suggest that the effects of thuringiensin occur in a relatively short time. Therefore the high toxicity of thuringiensin to immature spider mites stages and the sublethal effects on females suggest that thuringiensin may successfully control field populations and the effect of host plant and temperature may enhance the effectiveness of thuringiensin. The use of thuringiensin in spider mites control programmes is discussed. Practical suggestions on the development of bioassays for active ingredients of this type are also discussed.