Feeding preferences of Spodoptera exigua in response to form and concentration of selenium

Minimal information is available on the impact of various organic and inorg anic forms of the ecologically and agriculturally important pollutant, sele nium (Se), on insect herbivores. We conducted bioassays with artificial die t to examine the feeding responses of a generalist herbivore, Spodoptera ex igua (Hubner) (Lepidoptera: Noctuidae), to various forms and concentrations of Se. Two different-aged cohorts of larvae were examined in choice tests with control diets vs. test diets incorporating lethal concentrations (LC10 , LC30, LC50, and LC70) of sodium selenate, sodium selenite, seleno-DL-cyst ine, and seleno-DL-methionine, Tests initiated with neonates showed larvae significantly preferred control diet over diet with sodium selenate, sodium selenite, or selenocystine, but at most concentrations showed no preferenc e between selenomethionine and control diet. Choice tests initiated with th ird instars demonstrated a preference for control diet over sodium selenate treatments, and sodium selenite treatments. In contrast, no significant re sponses were found in tests initiated with third instars offered the choice between selenocystine or selenomethionine and untreated controls. Addition ally, comparisons of consumption demonstrated that inorganic selenium compo unds were antifeedants whereas the organic selenium compounds tested have l ittle antifeedant activity. The toxicity of all of the tested forms of sele nium, in combination with the lack of antifeedant activity of some compound s, has the potential to affect both the distribution and diversity of terre strial herbivores in both agricultural and natural systems. (C) 1999 Wiley- Liss, Inc.