Effect of selenium-treated alfalfa on development, survival, feeding, and oviposition preferences of Spodoptera exigua (Lepidoptera:Noctuidae)

We examined the effect of irrigating alfalfa (Medicago sativa L.) with selenium-contaminated water on the oviposition response, larval feeding preference, development and survival of the beet armyworm, Spodoptera exigua Hubner, a generalist herbivore. Alfalfa was grown in sand cultures under three levels of sodium selenate irrigation: (1) control with no Se added; (2) a low rate of 0.0066 g sodium selenate/60 liters water; (3) and a high rate of 0.20 g sodium selenate/60 liters water. The low concentration treatment resulted in 2.88 +/- 0.52 mug Se/g plant dry weight and did not affect percent survival to adult eclosion compared with the control at 1.26 +/- 0.11 mug Se/g dry weight. The high rate generated 305.81 +/- 52.14 mug Se/g dry weight of alfalfa and significantly fewer insects survived compared with insects fed control alfalfa at 1.11 +/- 0.12 mug Se/g dry weight. High Se levels, but not low levels, decreased the relative growth index for larvae. In two-choice bioassays (treated/control) neonate larvae did not discriminate between control and Se-treated plants at high or low levels. Fourth instars did not discriminate between plants with low Se levels and control plants, but preferred to consume plants with high, usually lethal concentrations of Se. Females preferred ovipositing on plants with low Se concentrations over control plants, but did not discriminate between plants with high Se levels and untreated controls. This indicates that although females and late instars may be able to differentiate between Se-treated and control alfalfa they do not avoid plants containing high concentrations of Se. Thus, alfalfa with high Se-treatment levels is resistant to S. exigua, and may serve as a population sink, where females oviposit and few offspring survive to reproduce.