Effects of emerald ash borer on communities of bark-dwelling and wood-boring arthropods associated with ash trees
Invasive insects represent a growing threat to North American forests. Among these insects, emerald ash borer (EAB), Agrilus planipennis, has been the most costly to date and has the potential to functionally extirpate the ash (Fraxinus) genus from the continent. Wider ecological effects caused by the loss of ash trees have already been observed, and many arthropods that feed exclusively on ash are potentially at risk of secondary extinction. However, we still know relatively little about the effects of EAB on arthropods also inhabiting ash trees. Our objective was to quantify the community composition of bark-dwelling and wood-boring arthropods in ash trees at EAB-infested sites over multiple years, thereby recording any changes associated with increased EAB density. Toward that end, we sampled a total of 205 ash trees from 35 sites in eight Maryland counties from 2011-2014. After being felled, trees were cut into sections approximately 1 m in length and stored in rearing barrels. Collection cups on the ends of barrels were checked daily and any arthropods found were removed and identified to the level of order or below; any EAB that emerged were excluded from analyses.
We collected a total of 2,031 individual arthropods, representing 13 orders, 63 families, and 42 genera. Twenty-nine arthropods were identified to species. Coleoptera and Hymenoptera were the most abundant orders, and the most common families in those orders were Cerambycidae and Braconidae, respectively. The red-headed ash borer (Neoclytus acuminatus) was by far the most abundant arthropod identified to species. Community composition shifted with the duration since infestation by EAB, with both arthropod richness and diversity generally being negatively associated with years since EAB infestation. Notably, however, numbers of many parasitoids (both introduced non-native biological control agents, and native species) were positively associated with years since EAB infestation, suggesting that some of these species may be numerically responding to EAB. Many of the species we found are not obligate feeders on ash, and consequently these findings provide further evidence of the potential broader effects of the EAB invasion on the arthropod communities within North American forests.