Effects of maggot density on the growth, survival, and asymmetry of the snail-killing fly, Sepedon fuscipennis (Diptera: Sciomyzidae)

Anecdotal observations show that drought conditions in the Midwestern United States caused a decline in populations of snail-killing flies during the 1980s. Sepedon fuscipennis is a shallow water or semi-aquatic predator of pulmonate snails, and several maggots can attack a single snail suggesting that if food is scarce that negative population effects will occur. In a lab experiment, we placed 1, 2, or 5 maggots in a rearing dishes (n=5 per treatment) and offered them physid snails. Snails were replaced when they were consumed or abandoned. Time from first feeding to pupariation and adult emergence was recorded, as was maggot mortality. Several characters of the puparia and adults were measured for asymmetry. The groups with five maggots had a development time that was 7% longer than the experimental groups with one maggot. Higher maggot density also led to an increased mortality rate than the lower maggot densities. Statistical analysis of the characters measured for asymmetry indicated an increase in asymmetry in puparia and adults, suggesting that intraspecific competition leads to non-lethal developmental defects within Sepedon fuscipennis.