Outbreak History (1953-2014) of Spring Defoliators Impacting Oak-Dominated Forests in Virginia, with Emphasis on Gypsy Moth (Lymantria disparL.) and Fall Cankerworm (Alsophila pometariaHarris)

Abstract

Oaks are a major component of forests in the eastern United States, including Virginia. In addition to the economic importance of select oak species for timber production, oaks are an essential component of many forest ecosystems, benefitting multiple species of wildlife and enhancing biodiversity. There is some concern for the future of oak forests because, in many locales, increasing rates of decline and loss among maturing cohorts of oak-dominant canopies are not being sufficiently replaced by oak regeneration. In time, without proper management, oaks may become significantly less abundant as they are gradually replaced by more competitive tree species and non-native invasive plants. Among a variety of spring defoliators, the gypsy moth ( Lymantria dispar L.) and fall cankerworm ( Alsophila pometaria Harris) have been major contributors to oak decline in Virginia due to repeated outbreaks spanning many acres and successive years. Many spring defoliators exhibit eruptive population dynamics and spring defoliation causes more harm to host trees than defoliation that occurs later in the growing season. Annual defoliation data from a variety of spring defoliators were collected and summarized from pest survey records by the Virginia Department of Forestry spanning 1953-2014. While gypsy moth was the most damaging agent during this time span, fall cankerworm has been more damaging from 2005-2014. Fall cankerworm is the most common native defoliator in Virginia, with outbreaks occurring approximately every five years for the last 60 years. While the extent of gypsy moth outbreaks seems to have declined recently, fall cankerworm outbreaks appear to be getting worse in Virginia. Other native spring defoliators that prefer oaks have been in outbreak status, albeit less frequently. Overall, defoliator outbreaks may exacerbate oak decline where they occur, resulting in more rapid loss of oak-dominant canopies and a transition to alternate forest types over time.