Parasitism by Trichogramma spp. (Hymenoptera: Trichogrammatidae) in Relation to Nantucket Pine Tip Moth (Lepidoptera: Tortricidae) Egg Density and Location

Abstract

Patterns of parasitism by Trichogramma spp. relative to density and location of Nantucket pine tip moth, Rhyacionia frustrana (Comstock), eggs were examined during 1994 in a 13-ha, 2-yr-old Pinus taeda L. plantation in northeastern Georgia. Egg densities were adjusted by temporarily enclosing trees in fine mesh screen cages and introducing different numbers of tip moths. After exposing eggs to naturally occurring parasitoids, all shoots were removed, and eggs were counted and evaluated for parasitism. Tip moth egg densities ranged from 203 to 2,579 per tree for the spring generation, and from 1 to 89 per tree for the summer generation. Tip moths deposited significantly greater numbers of eggs on needles than shoots on high-versus low-egg-density trees. Overall parasitism rates were 37.2 and 43.3% for spring- and summer-generation eggs, respectively. Parasitism was density independent at the whole-tree level in both generations, but spring-generation parasitism was strongly inversely density dependent in the top whorl. Numbers of eggs per needle fascicle ranged from 1 to 15 for spring-generation trees. Actual numbers of parasitized eggs per discovered fascicle (one on which parasitism occurred) increased with egg density, but the percentage of parasitized eggs decreased. Egg density had no effect on fascicle discovery. These factors produced an inversely density-dependent relationship between egg parasitism and density at the fascicle level. Eggs located on shoots suffered significantly greater parasitism rates than those on needles. Implications of these results are discussed.