The effect of urbanization on egg parasitism levels in the fall cankerworm
March 4, 2015
By Derek M. Johnson, Assistant Professor, Biology, VCU and Abby Nelson, Master’s Student, Biology, VCU
Insect outbreaks are the primary cause of large-scale natural disturbance in North American forests. Defoliation by insects has been shown to decrease tree growth, increase tree mortality, and have significant effects on forest ecosystem processes. Global changes due to human activities have been proposed to result in more frequent insect outbreaks. The fall cankerworm (Alsophila pometaria) has recently increased in the frequency and scale of outbreaks in eastern Virginia. The recent increase in fall cankerworm outbreaks in eastern Virginia from 2012-14, and the 10-20 years of damage this moth has done to trees in Charlotte and Raleigh, North Carolina, begs the question of whether urbanization plays a role in these outbreaks.
Anyone who has spent time under Richmond’s urban and suburban trees in the spring of the last three years has encountered high densities of inchworms, the larval stage of fall cankerworms, on their cars, homes, clothing, and just about anywhere else imaginable. The fall cankerworm feeds on a broad range of deciduous tree species and has one generation per year. The adults emerge from the ground in late fall or winter to mate. Females are flightless, thus must climb the trunks of trees in order to oviposit their eggs in the forest canopy. The eggs overwinter and hatch in the spring. A suite of parasitic wasp species attack fall cankerworm eggs and may play an important role in suppressing populations below outbreak levels. Parasitoids have been shown to be particularly susceptible to pollutants, and forest fragmentation may restrict the movement of these tiny wasps; thus, we hypothesize that urban effects reduce either the abundance or species richness of fall cankerworm parasitoids, and this indirectly causes longer, more frequent, and more severe outbreaks in urban areas compared to rural areas around Richmond.
In particular, we are studying the effects of human population density and forest fragmentation on the population dynamics of fall cankerworms and their parasitoids. We have banded trees in 15 sites (including the VCU Rice Rivers Center) in and around Richmond with a sticky substance called Tanglefoot® to trap and count females as they try to climb the trunk. The sites represent a range of human population densities and levels of forest cover. This study will provide important information towards understanding why fall cankerworms are becoming more of a forest pest in recent years. More broadly, this study will provide information on how urbanization may affect pest population dynamics in a rapidly changing world.