Impact of insecticide use on the leafminer fly Liriomyza huidobrensis and their hymenopteran parasitoids and predators in potato crop on the central Peruvian coast

In vegetable and potato cropping system of the Peruvian coast the leafminer fly Liriomyza huidobrensis (Diptera: Agromyzidae) is the most destructive pest for potato and is typically controlled using broad-spectrum insecticides. The objective of our study was to compare leafminer fly infestation and main natural enemies of potato fields that differed in insecticide spray regimen. The study was conducted in the Cañete valley, in the central coast of Peru, during the 2007 winter cropping season. Three insecticide regimes were evaluated in experimental and farmer potato fields: selective insecticides (abamectin and ciromazine), broad-spectrum insecticides (conventional), and non-insecticides (control). Selective insecticides (2.2 larvae/leaf) had significantly lower densities of larval infestation than control (8.9 larva/leaf) and conventional (5.7 larva/leaf) treatments. Parasitism in selective insecticides (65%) and control (77.2%) treatments did not differ from one another, but both had significantly higher parasitism than conventional fields applied with broad-spectrum insecticides (20.5%). Proportion of leafminer: parasitoid adults reared from potato leaves was 0.3, 0.6 and 4.1 for control, selective insecticides and conventional fields treatments, respectively. Main parasitoids Halticoptera arduine (Walter) (Hymenoptera: Pteromalidae) and Diglypus websteri (Crawford) (Hymenoptera: Eulophidae) registered significantly higher densities in control than in selective insecticide and conventional fields treatments. No significant differences were found between both insecticide regimes. Total ground-dwelling predator densities and densities of ground beetles (Coleoptera: Carabidae) and earwigs (Dermaptera: Labiduridae) were lowest in conventional fields, whereas densities of rove beetles (Coleoptera: Staphylinidae) did not differ between selective insecticide and conventional fields. Results show that broad-spectrum insecticides used on a calendar basis by conventional potato farmers were not effective against leafminer fly and were detrimental for parasitoids and predators associated with potato agroecosystem. Use of selective insecticide within conventional potato production will help to retain low leafminer fly density and effective natural enemy conservation.