Effects of gypsy moth defoliation on the aquatic biota of headwater streams in Shenandoah National Park

Voshell, J; Marshall, Brett

This document examines the short-term effects of gypsy moth defoliation on headwater stream water quality, benthic macroinvertebrate community structure, and benthic macroinvertebrate function at Shenandoah National Park Executive Summary: This field study investigated the short-term effects of riparian defoliation by gypsy moth larvae on three aspects of headwater stream ecology: water quality, benthic macroinvertebrate community structure, and benthic macroinvertebrate function (expressed as secondary production). The experimental design used six streams in Shenandoah National Park as study sites. Three of the streams were extensively defoliated by gypsy moth larvae (defoliated treatment), whereas the riparian canopies of the other three streams were not affected by gypsy moth larvae (reference treatment). Weekly defoliation measurements, which were made with a spherical densiometer, indicated that the canopies of defoliated streams became significantly more open than those of reference streams after gypsy moth larvae hatched. We did not observe significant differences in pH, dissolved oxygen, alkalinity, conductivity, hardness, nutrient concentrations (nitrate, nitrite, ammonia, orthophosphate, and total phosphorus), or seston as a result of defoliation. Ash-free dry mass and chlorophyll a content of periphyton also remained unchanged after defoliation. There was a Significant increase in the amount of detritus (frass and orts) falling into defoliated streams in the spring, which was followed by a significant decrease in the amount of detritus falling into defoliated streams in autumn. Temperature of defoliated streams was slightly elevated for a brief period after defoliation. We observed only slight changes in benthic community structure in defoliated streams. Taxa richness, EPT index (number of sensitive mayfly, stonefly, and caddisfly taxa), Hilsenhoff Biotic Index, and diversity did not change Significantly after defoliation. Likewise, changes in the percent contribution of different functional-feeding groups were not statistically Significant. Bray-Curtis Coefficient and chord distance did not reveal any significant changes in the structure of defoliated and reference streams after defoliation. Only the Index of Biotic Similarity demonstrated a significant difference, indicating that, at most, only slight changes in community structure occurred. We estimated secondary production by the size-frequency method for selected aquatic insects in different functional-feeding groups. Production of Peltoperlidae (stonefly, shredder) and Diplectrona modesta Banks (caddisfly, collector-filterer) were not significantly affected by defoliation. Glossosoma nigrior Banks (caddisfly, scraper) initiated a second generation in two of the defoliated streams because of temperature changes. Secondary production was much higher in streams having bivoltine populations of G. nigrior. Other species with temperature-sensitive life histories may be affected similarly. We concluded that the short-term effects of riparian defoliation by gypsy moth larvae were minor. We arrived at this conclusion by comparing a variety of structural and functional measures commonly used to detect environmental perturbations and finding almost all of them to be statistically not significant. Given that most IPM strategies in the Appalachians rely heavily on spraying with diflubenzuron, which can cause Significant adverse impacts to the ecological integrity of freshwater natural resources, a strategy of no pest management for gypsy moth larvae poses less risk to streams in SNP than aerial spray programs. Spray programs with Bacillus thuringiensis would not pose a threat to streams but are not needed to protect the ecological integrity of streams. This study does not rule out the possiblity of long-term effects that could result from defoliation-induced tree mortality and accompanying forest succession.