Developmental Responses of a Terrestrial Insect Detritivore, Megaselia Scalaris (Loew) to Four Selenium Species

Megaselia scalaris (Loew) (Diptera: Phoridae) is an important and ubiquitous terrestrial detritivore that consumes both animal and plant material. Because both plants and animals convert selenium pollutants into various forms, the relative toxicities of ecologically relevant concentrations of sodium selenate, sodium selenite, seleno-L-methionine, and Se-(methyl) selenocysteine hydrochloride to larvae were assessed in diet bioassays. In addition, ovipositional preferences of adults and developmental effects on the eggs and larvae were measured. With chronic exposure selenocysteine was the most toxic of the selenium species to the larvae (LC50: 83 microg/g wet weight), followed by seleno-L-methionine (LC50: 130 microg/g), selenate (LC50: 258 microg/g), and selenite (LC50: 392 microg/g). Ovipositing females did not discriminate between the highest treatment concentrations of any of the pollutants as compared to the controls, indicating a lack of avoidance behavior. Larval development time was significantly increased with exposure to selenate at 100 microg/g wet weight and above, selenite at 300 microg/g and above, and at 50 microg/g and 25 microg/g and above for seleno-L-methionine and selenocysteine respectively. Pupal development was not affected by any of the selenium treatments. Significant differences between male and female adult eclosion times were observed, with females eclosing later than males as selenium concentrations increased. Significant decreases in larval survival relative to controls occurred at the lowest treatment tested (100 microg/g) for both selenate and selenite and at 100 microg/g for seleno-L-methionine, and 50 microg/g for selenocysteine. The population level implications of lack of avoidance of contaminated food, and the effects of increased development times, reduced survivorship, and non-synchronized male and female emergence are discussed.