FOLIAGE DAMAGE DOES NOT AFFECT WITHIN-SEASON TRANSMISSION OF AN INSECT VIRUS

Gypsy moth defoliation of oak trees has been shown to lead to increase d tannin levels, which, in turn, lead to reduced gypsy moth growth and fecundity. In laboratory experiments, increased tannin levels can int erfere with the transmission of a virus that is consumed by larvae on oak foliage, and high mortality rates of larvae in the field are somet imes associated with low levels of defoliation. These latter results h ave led to the suggestion that gypsy moth defoliation may cause reduce d mortality attributable to the virus by elevating oak tannin levels. In a series of field experiments, we directly tested the hypothesis th at gypsy moth defoliation of oaks leads to reduced virus transmission rates. In each of three study years, in oak forests with almost no nat urally occurring gypsy moths or virus, we measured virus transmission rates in gypsy moths feeding on oaks, with and without experimental de foliation. By carefully synchronizing our experiments with the phenolo gy of natural gypsy moth populations, we mimicked natural virus transm ission processes during that part of the gypsy moth life cycle when vi rus transmission occurs. In our experiments, there was no effect of gy psy moth defoliation on tannin levels; consequently, virus transmissio n in both the held and the lab was unaffected by defoliation. Although we did observe increased tannin levels on more severely defoliated oa k trees in one of two naturally defoliated oak stands late in the seas on, virus transmission had virtually ceased by that time. Our results suggest that gypsy moth defoliation does not affect tannin levels earl y enough in the larval season to have a measurable effect on the inter action between the gypsy moth and its nuclear polyhedrosis virus.