Gypsy Moth Defoliation in Virginia - 2009

Click here to see a larger version of the map.Wet spring weather led to a significant decline in gypsy moth populations due to the effectiveness of Entomophaga maimaiga under these conditions. Statewide defoliation levels had been steadily rising since 2005, with each successive year producing another dry spring. Last year’s total defoliation was more than 112,000 acres of mostly heavy defoliation. This year, we were down to 29,000 acres of defoliation, most of which was classified as light (Figure 2).

It’s worth noting that, while Entomophaga maimaiga can be very effective at killing larvae, this usually does not occur until larvae are almost mature and have finished feeding. Therefore, the fungus typically does not stop defoliation from occurring the same year it becomes active. However, by preventing those larvae from becoming adults, mating and laying eggs, few egg masses will be around for next year, when the real impact of the fungus will be seen.

That said, with so many egg masses going into this year, it was surprising to see such a dramatic drop in defoliation acreage and intensity so quickly. In fact, defoliation initially seemed like it was going to be pretty bad. By mid-June, I was starting to get a number of calls from foresters reporting anywhere from 70 to 90 percent defoliation levels in some locations. All indications were that, despite the rain, we wouldn’t see the benefits of it until next year. What happened, however, was it just kept on raining, through June and beyond. Normally, I begin to survey for gypsy moth in mid-June, when maximum damage from the air becomes fully apparent, and finish around mid-July. During this time frame, foliage does not change much from the air. By August however, refoliation of many trees begins to occur (at least among those still healthy enough), and brown areas start to appear light green again.

This year, because of the incessant rain, green up began as early as late June. By the time I flew most areas, including those that initially appeared to be heavily defoliated, they were greening up again. From the air, it was quite often difficult to make out whether there was even anything going on; when defoliation was present, it often could only be classified as ‘light’. To all appearances, even trees that were heavily defoliated this year were able to recover much faster than they normally would have. Although I hate to make predictions, I feel confident that if we see even a normal amount of spring rainfall next year, gypsy moth defoliation could go down to near zero for the first time in five years. Despite this positive trend, the damage has been done in a number of locations that have seen severe defoliation for the last three to four years. The most prominent example is the Poor/Bent Mountain area just southwest of Roanoke. This mountain, once covered in chestnut oak, has been devastated. Widespread oak mortality is also present within the George Washington and Jefferson National Forest in northwest Giles County, as well as northwest Augusta County and southwest Rockingham County.