INFLUENCE OF FORM AND QUANTITY OF SELENIUM ON THE DEVELOPMENT AND SURVIVAL OF AN INSECT HERBIVORE

Even plants classified as 'nonaccumulators' can sequester concentratio ns of sodium selenate, sodium selenite, selenocystine and selenomethio nine that can strongly influence insect development and survival. Thes e forms of selenium (Se), tested in diet-incorporation bioassays, prov ed toxic to larvae of a generalist insect herbivore at relatively low levels. Sodium selenite was the most toxic form tested against Spodopt era exigua (Hubner), with an LC50 of 9.14 mu g g(-1) wet wt (21.11 mu g g(-1) dry wt). Selenocystine was intermediate with an LC50 of 15.21 mu g g(-1) wet wt. The least toxic forms, sodium selenate and selenome thionine, had LC(50)s below 50 mu g g(-1) dry wt, the upper level for tissues of plants classified as nonaccumulators. Ingestion of some for ms of Se also affected growth and development. Increasing concentratio ns of sodium selenate and sodium selenite decreased pupal weight and a dded significantly to the time needed for development to the pupal and adult stages. The time required to complete the larval stage increase d by over 25% and the time from egg to adult emergence was extended by 22% to nearly 30%. Selenocystine and selenomethionine did not signifi cantly increase developmental times, even at concentrations that kille d 90% or more of the test populations. Analyses of relative growth rat e, relative growth index, and an analysis of covariance technique for measuring growth indicated that the form of Se affected growth rates, growth inhibition responses of the larvae, and toxicological effects. Thus, quantity and the form of Se accumulating in plants grown on Se-c ontaminated sites are likely to influence the population dynamics of i nsect herbivores. The implications of these results for the ecology of contaminated sites are discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.