Effects of flowering plants on parasitism of lettuce leafminers (Diptera: Agromyzidae)

The efficacy of flowering plant strips in enhancing parasitism of lettuce leafminers (Diptera: Agromyzidae) was evaluated. Field experiments were carried out in 2002 and 2003 at two organic farms located in the province of Bologna, northern Italy. The population density of leafminers and their parasitoids in lettuce plots surrounded by flowering plants was compared with lettuce plots bordered by bare soil. In the 2003 season, parasitism was enhanced by the flowering plants, whereas in 2002 the difference between the parasitism in lettuce with flowering plants and control plots was close to the level of statistical significance. In both years, parasitism by endoparasitoids was higher than that of ectoparasitoids, but flowering plants showed a more pronounced positive effect on parasitism by ectoparasitoids than endoparasitoids. Overall, the increase in parasitism in lettuce surrounded by nectar-providing plants was higher in the first week after transplanting than in later weeks. However, the augmentation of parasitism in lettuce bordered by flowering plants did not result in a significant reduction of the agromyzid pests when compared with fields bordered by bare soil.