Leafroller parasitism across an orchard landscape in central Washington and effect of neighboring rose habitats on parasitism

Abstract

Parasitism of sentinel leafroller larvae of Pandemis pyrusana and Choristoneura rosaceana (Tortricidae) was measured in apple, pear and cherry orchards within a 680ha landscape mosaic in south-central Washington in 1999–2000. Average seasonal rates of parasitism of sentinel larvae were 12% in spring and 29.6% in summer corresponding to the overwintering and summer larval cohorts of pest leafroller generations. Tachinidae accounted for 72% of spring and 79% of summer parasitism compared to 9.2% and 7.6% by the exotic parasitoid, Colpoclypeus florus (Eulophidae), 14% and 0.9% by Apanteles polychrosidis and 3.9% and 12.2% by Oncophanes americanus (Braconidae), respectively. Parasitism rates of sentinel larvae in autumn increased to 81%, of which 55% was by C. florus and 44% was by tachinids. A significant decline in parasitism with increasing distance from the Yakima River was evident for C. florus (P<0.0001), O. americanus (summer only, P=0.011) and A. polychrosidis (P=0.018). No statistically significant association of parasitism with pesticide use was detected. Sentinels larvae placed in two orchards adjacent to patches of the Wood’s Rose, Rosa woodsii (Rosaceae), that harbored the strawberry leafroller, Ancylis comptana (Tortricidae), showed parasitism rates by C. florus approaching 100% in spring at one site and in summer at a second site. These studies show that proximity to riparian habitats is associated with higher parasitism of leafrollers for three parasitoids. The presence of the non-pest leafroller, A. comptana, infesting R. woodsii may represent a valuable overwintering host for C. florus leading to increased parasitism of pest leafrollers by C. florus in commercial fruit orchards. Experimental creation of rose habitats to test this concept is indicated.