Glucosylation prevents plant defense activation in phloem-feeding insects

The metabolic adaptations by which phloem-feeding insects counteract plant defense compounds are poorly known. Two-component plant defenses, such as glucosinolates, consist of a glucosylated protoxin that is activated by a glycoside hydrolase upon plant damage. Phloem-feeding herbivores are not generally believed to be negatively impacted by two-component defenses due to their slender piercing-sucking mouthparts, which minimize plant damage. However, here we document that glucosinolates are indeed activated during feeding by the whitefly Bemisia tabaci. This phloem feeder was also found to detoxify the majority of the glucosinolates it ingests by the stereoselective addition of glucose moieties, which prevents hydrolytic activation of these defense compounds. Glucosylation of glucosinolates in B. tabaci was accomplished via a transglucosidation mechanism, and two glycoside hydrolase family 13 (GH13) enzymes were shown to catalyze these reactions. This detoxification reaction was also found in a range of other phloem-feeding herbivores.

Pyrethroid insecticide resistance in British populations of the cabbage whitefly, Aleyrodes proletella

BACKGROUND

The cabbage whitefly, Aleyrodes proletella L., is emerging as a significant pest of field brassica crops in certain regions of the United Kingdom. In order to investigate the contribution of pesticide resistance to this phenomenon, A. proletella populations were sampled from five different areas in England in 2008 and 2009. Adult residual leaf-dip bioassays were carried out using pyrethroid and neonicotinoid insecticides.

RESULTS

Significant resistance to pyrethroids was found in multiple samples collected from two areas. No evidence of cross-resistance to neonicotinoids was found in a subset of the pyrethroid-resistant populations. While the patterns of resistance to different pyrethroids were broadly correlated, the magnitude of resistance factors differed substantially. Survival of strains at a putative diagnostic concentration of lambda-cyhalothrin was found to provide a guide to their LC50. Significant differences in LC50 were found when different brassica crops were used in the bioassay, although the resistance patterns between strains were maintained.

CONCLUSION

Reduced susceptibility to multiple pyrethroid insecticides exists in populations of A. proletella in the United Kingdom, corresponding to recent major outbreaks. The mechanism(s) of resistance are yet to be determined, but molecular structural differences in pyrethroids probably influence the magnitude of cross-resistance within this group of insecticides.