Feeding deterrence of Spodoptera exigua (Lepidoptera: Noctuidae) larvae by low concentrations of linear furanocoumarins

We conducted choice tests and behavioral observations of Spodoptera exigua (Hubner) larvae to study the effect of linear furanocoumarins (bergapten, xanthotoxin, and psoralen) individually or in combination at concentrations commonly found in Apium graveolens L. and UV-A light on larval preference and feeding behavior. Linear furanocoumarins had a negative and concentration-dependent effect on diet preference by S. exigua larvae. Behavioral observations showed that preference of control diet is the result of feeding deterrence by these compounds. The larvae leave the furanocoumarin-containing diet more frequently by becoming more mobile after ingestion of the treated diet, as measured by time spent questing (in which larvae walk or lift their thoraxes, and move from side to side in a searching motion) and number of questing occurrences. The UV-A effect was not significant. We conclude that linear furanocoumarins (at the concentrations found in A, graveolens) can play an important role as behavioral modifiers rather than as toxic defenses against larvae of the generalist herbivore S. exigua.