Gypsy Moth Defoliation in Virginia - 2008

Click here to see a larger version of the map.Defoliation by the gypsy moth increased significantly this year to 112,343 acres from 73,408 acres last season. Although there were a few new areas of defoliation this year, most areas affected were a continuation of last year’s hot spots, albeit expanded considerably in most instances. The worst hit area was in Augusta and Rockingham counties, which saw a combined defoliation estimate of more than 42,000 acres. Much of this area included remote National Forest land with very few major roads. Defoliated areas also expanded in Giles, Craig, Montgomery, Roanoke, Allegheny and Loudoun counties as well as within Shenandoah National Park. Frederick and Shenandoah counties were the only areas that saw less defoliation than last year, presumably due to highly effective aerial spray applications and an even greater preponderance of cool, wet weather during spring. Approximately ¾ of the defoliated area (more than 82,000 acres) occurred on Federal lands.

This year’s total was the highest since 2001, when more than 400,000 acres of defoliation occurred across Virginia. Except for 2001, this was the highest total defoliation in the Commonwealth since 1995. The emergence of the gypsy moth fungus, Entomophaga maimaiga, in 1996, along with wet weather, provoked a major crash in gypsy moth populations. This occurred after six successive years of state-wide defoliation levels that were between 450,000-850,000 acres annually.

There is no question that E. maimaiga has continued to play a very significant role in moderating gypsy moth populations ever since its arrival in the U.S. However, successive years of dry, spring weather limit the effectiveness of this fungus and can facilitate the resurgence of gypsy moth populations. This occurred leading up to the outbreak in 2001, and dry spring weather predominated during the 2005-07 seasons, leading up to current outbreak levels.

Gypsy moth caterpillars hatch from eggs during late April into May and feed throughout most of May and early June. The best time for cool wet weather and the gypsy fungus to have a maximum effect is during early to mid-May when early-instar caterpillars are feeding. This year, unusually cool, wet weather persisted over a three-week period during May, just at the right time. Unfortunately, the gypsy moth fungus does not typically kill infected caterpillars until they have almost completely matured and completed most of their feeding. While many of these caterpillars will ultimately succumb and die due to the fungus, this typically won’t affect defoliation levels until the following year. In a number of areas, evidence of diseased caterpillars was everywhere. In other areas, there was little apparent disease and loads of new egg masses, portending additional problems with defoliation next year. Another wet May in 2009 may push overall gypsy moth populations into a decline.