Three metabolites from an entomopathogenic bacterium, Xenorhabdus nematophila, inhibit larval development of Spodoptera exigua (Lepidoptera: Noctuidae) by inhibiting a digestive enzyme, phospholipase A2

An entomopathogenic bacterium, Xenorhabdus nematophila, has been known to induce significant immunosuppression of target insects by inhibiting immune-associated phospholipase A(2) (PLA(2)), which subsequently shuts down biosynthesis of eicosanoids that are critical in immune mediation in insects. Some metabolites originated from the bacterial culture broth have been identified and include benzylideneacetone, proline-tyrosine and acetylated phenylalanine-glycine-valine, which are known to inhibit enzyme activity of PLA(2) extracted from hemocyte and fat body. This study tested their effects on digestive PLA(2) of the beet armyworm, Spodoptera exigua. Young larvae fed different concentrations of the three metabolites resulted in significant adverse effects on larval development even at doses below 100 mu g/mL. In particular, they induced significant reduction in digestive efficiency of ingested food. All three metabolites significantly inhibited catalytic activity of digestive PLA(2) extracted from midgut lumen of the fifth instar larvae at a low micromolar range. These results suggest that the inhibitory activities of the three bacterial metabolites on digestive PLA(2) of S. exigua midgut may explain some of their oral toxic effects.