Effects of Trunk Barriers on Larval Gypsy Moth (Lepidoptera: Lymantriidae) Density in Isolated- and Contiguous-Canopy Oak Trees

Abstract

The impact of sticky trunk barriers on the density of larval gypsy moths, Lymantria dispar (L.), in oak (Quercus spp.) trees with isolated and contiguous canopies was studied. Measurements of frass drop per unit of area and frass yield per larva were used to estimate larval density. The trunk barriers reduced larval density by an average of ≈27%. The effect of the trunk barriers was the same on isolated- and contiguous-canopy trees on the first sample date, when larvae were predominantly fourth instars. On the second sample date, when larvae were predominantly fifth and sixth instars, the effect of trunk barriers on larval density was greater on contiguous-canopy trees. These results indicate that reinfestation of trees from the canopy of adjacent trees is minimal at low to moderate gypsy moth larval densities (up to ≈80 larvae per square meter). Defoliation levels and subsequent egg-mass density were not affected by trunk barriers. While sticky trunk barriers appear to consistently reduce larval density on both isolated- and contiguous-canopy oak trees, they do not provide adequate foliage protection. Frass drop (number of frass pellets per square meter) was the most sensitive measure of treatment effects. The estimation of larval density by incorporating information about frass yield per larva introduced additional variability which made this a less sensitive measure of treatment effects.