The role of two eulophid parasitoids in populations of the leafminer, Phyllonorycter mespilella (Lepidoptera:Gracillariidae) in British Columbia

Abstract

In 1991, orchards in the Naramata region of the Okanagan valley in British Columbia had significantly larger populations of the leafminer, Phyllonorycter mespilella, than those of the Osoyoos/Oliver region, 50 km south, where parasitism had been shown to keep the miner below treatment levels. We questioned if different roles of the parasitoid species caused the discrepancy. Leaves were collected and leafminers and parasitoids assessed from overwintering populations and also weekly from May through October (1992) in apple orchards representative of the areas. Pnigalio flavipes and Sympiesis marylandensis were the major parasitoid species overwintering in 52.3 and 46.7% respectively of the P. mespilella mines. The percentages of the two species did not diITer significantly between the orchards screened in both areas and did not account for the differences in the numbers of overwintering or summer generation mines. P. flavipes was the dominant parasitoid species in both regions through the three summer generations. S. marylandensis was only found at low levels in three of the eight orchards until the second and third generations. Parasitoid-induced-mortality in 1992 did not have a consistent significant impact on intraseasonal leafminer increase. Five of the orchards studied had leafminer populations above treatment thresholds.

Key words: Treefruit; biological control; Hymenoptera; leafminer