How do phytochemical and morphological lychee plant traits affect the lychee erinose mite Aceria lichii (Acari: Eriophyidae)?

The lychee erinose mite (LEM), Aceria litchii, is a serious pest of lychee plants. This tiny mite induces the formation of hypertrophic trichomes (erinea) on leaves, flowers, fruit, and other plant structures, hampering plant growth and yield. How LEM induces physical and chemical changes in lychee plants and how plants respond to LEM infestation is unknown. We investigated changes in lychee volatile emissions induced by LEM and their role in plant-LEM interaction. We used GC-MS to characterize the volatile profile of infested and uninfested lychee plant parts (baby shoots, flushes, flowers, young and old leaves). Because erinea color changes with the progression of LEM infestation, we characterized the volatile profile of infested leaves from all types of erinea, i.e. light white, white, amber and dark brown color. Dual choice bioassays were used to investigate which plant parts and which volatile compounds (oils) were the most attractive to LEM. We found qualitative and quantitative differences between the volatile profiles of uninfested and infested plant parts, with remarkable changes observed with increasing levels of LEM infestation. The most abundant volatiles were nonanal, decanal, zingiberene, β-caryophyllene and ar-curcumene, varying greatly in their concentrations among the four types of erinea. The bioassays revealed that uninfested flushes are the most attractive plant part to LEM. LEM attraction to nonanal, decanal, zingiberene, β-caryophyllene and ar-curcumene was influenced mostly by compound concentration; sometimes showing repellence with high concentrations. Finally, we discuss possible applications of our findings in agricultural settings.