COMPOSITION, SEASONAL ABUNDANCE, AND PARASITISM OF LIRIOMYZA (DIPTERA, AGROMYZIDAE) SPECIES ON LETTUCE IN ARIZONA

Liriomyza trifolii (Burgess) and L. sativae Blanchard were the leafmin er species found in commercial and experimental plantings of lettuce i n the Yuma Valley, AZ. L. trifolii was the dominant species observed i n commercial lettuce fields in fall surveys, although populations did not reach damaging levels. In experimental plots, L. sativae was the p rincipal species observed. The sharp contrast in Liriomyza species com position between lettuce grown in commercial and experimental plots wa s likely a reflection of pesticide-use patterns in commercial lettuce fields. Seasonal abundance and parasitism of leafminers in experimenta l plantings of lettuce from 1990 to 1992 varied with temperature and p lanting date. Adults, active larvae, and mines with dead or parasitize d larvae were most abundant during the fall growing season. Edible lea ves and heads were not mined at harvest. Populations in winter and spr ing plantings occurred at relatively low densities, likely in response to cooler temperatures and parasitism. The parasitoid complex associa ted with Liriomyza in fall lettuce primarily consisted of Opius spp. a nd Chrysocharis parksi Crawford, whereas Diglyphus spp. occurred at lo wer densities. Natural suppression of leafminer populations by parasit oids and other mortality factors appeared to be important for maintain ing Litiomyza populations below damaging levels.