Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2003
Publication Date: 4/1/2004
Citation: Webb, R.E., Bair, M.W., White, G.B., Thorpe, K.W. 2004. Suppression of gypsy moth (lepidoptera: lymantriidae) populations by the entomopathogenic fungus entomophaga maimaiga as affected by gypsy moth larval population density and supplemental watering. Journal of Entomological Science 39(2): 223-234. Interpretive Summary: Gypsy moths are serious defoliators of forest, park and residential shade trees. While effective registered pesticides exist, many would prefer to rely, if possible, on an effective natural control agent such as the fungus Entomphaga maimaiga. Outbreaks (epizootics) of the gypsy moth-adapted entomopathogenic fungus leave resident fungal resting spores in the soil at the base of trees. Resting spore-induced infection can occur throughout the period that larvae are present. However, the fungus epizootic generally peaks late in the season. Resting spore emergence is dependent on natural rainfall, which is unpredictable. The study assessed the potential for inducing an earlier epizootic of E. maimaiga by inducing the early activation of resting spores by supplemental watering. Assuming a dry-to-normal level of precipitation during the gypsy moth field season, fungal kill may occur at low rates until late in the season, providing little impact on gypsy moth populations. Egg mass population reductions were significantly greater in watered versus non-watered plots, indicating that supplemental watering had a beneficial impact. Supplemental watering is an inexpensive, common sense strategy that may especially reward the land manager in dry years. This technique would most likely be useful for individual homeowners, community of homeowners, or similar small-scale management.
Technical Abstract: We assessed the potential for inducing an earlier epizootic of the fungus Entomophaga maimaiga by inducing early sporulation of resting spores by spraying water around the base of trees. Potential test sites along Skyline Drive in the Shenandoah National Park were characterized during the year 2000. Eight plots with appropriate gypsy moth populations, and with measured (naturally-deposited) E. maimaiga resting-spore loads around the base of trees, were selected for a supplemental watering study conducted in 2001. Four plots received weekly supplemental watering around the base of trees to supplement natural rainfall, and 4 received only natural rainfall. Live larvae were sampled and assessed for mortality factors on 1 June and on 15 June. Larvae were sampled from the ground and from the canopy. More larvae died due to the fungus in watered plots than in control plots for both sample dates, and this was true at both sample heights. However, treatment effects were non-significance. Height effects were significant for the second date with implications for sampling protocols for this pathogen. Analysis of pre-versus post-season egg mass counts indicated that a significant difference (P=0.0433) in egg mass population change occurred between treated and control plots. Egg mass populations in the watered plots declined an average of 78%, while egg mass populations in control plots declined only 4%.