European shot-hole borer
The pear blight beetle (also known as European shot-hole borer), Anisandrus (= Xyleborus) dispar, lives up to one of its names leaving trees with small diameter holes resembling tiny shot holes. An investigation of borer activity in Oregon nurseries indicated that this species of ambrosia beetle is active in various sites in the Willamette Valley. We've identified this borer in damage from container and field production sites. Click the hyperlink below to see a very informative website regarding this beetle.
The Shothole Borer: An Ambrosia Beetle of Concern for Chestnut Orcharding in the Pacific Northwest
Identification aid is available at the Anisandrus link at the Bark Beetles of the United States website.
The beetle has a large host range. Hosts include: Acer, Aesculus, Alnus, Betula, Castanea, Celtis, Crataegus, Corylus, Cydonia, Fagus, Fraxinus, Juglans, Liriodendron, Magnolia, Malus, Platanus, Populus, Prunus, Punica, Pyrus, Quercus, Salix, Styrax,Tilia, Ulmus, and Vitis (Additional hosts can be found in the Plantwise Pear Blight Beetle webpage). There are also reports from Pinus, Cedrus, and Tsuga.
The European shot hole borer overwinters as adults. It is not uncommon to find the rear end of the beetle protruding from the hole it bore. After wintering inside a suitable host, the female X. dispar beetle takes advantage of the first warm day in late winter or spring to seek another host, preferably a tree exhibiting signs of stress. The much smaller male is flightless, staying in the tree in which it was born. The female’s initial attraction is to ethanol emitted by stressed trees. Once the female shothole borer finds a good host, she sends out an aggregation pheromone which attracts beetles of the same species. This volatile chemical is responsible for secondary attacks on trees, with hordes of beetles gathering on that tree or others nearby. Thus shothole borers often leave trees riddled with entrance holes. It is thought that X. dispar has two flights per year in Oregon, in early spring (our trap catches show a peak in late March/early April) with activity into May and June.
Check for current damage in trees which might have experienced stress. Sap weeping from the small diameter wounds is one symptom readily seen. Fresh sawdust on the trunk indicates recent entry or emergence of the adults. These beetles often appear in large numbers even on the same tree.
Beetle flights occur when temperatures reach 50 degrees F, particularly on a bright, sunny day. We have found it extremely helpful for growers with a history of shothole borers to monitor flights of the borers. The Lindgren funnel trap with an ethanol lure (Contech) was used successfully in our research to obtain flight data for several common borers. Reding and colleagues have found using two ethanol lures increased the trap catch of two shothole borers, X. crassiusculus and X. germanus. We do not know if this would be the case with shothole borers more commonly found in Western Oregon.
Improve growing conditions to reduce stressed trees. Drought, waterlogged soils, and freeze damage can all lead to increased risk of borer damage. In container nurseries, keeping the plant media of flood-intolerant trees below 50% moisture levels greatly reduces or eliminates ambrosia beetle attacks (Ranger et al. 2016). In field nurseries, susceptible trees may be rogued and burned or treated with long residual protective insecticide applications during beetle flight periods to reduce risk of damage [See PNW Insect Management Handbook recommendations].
(Disclaimer: listing or not listing of specific resources is not an endorsement or lack of endorsement).
Ranger, C., P. Shultz, M. Redding, S. Frank, and D. Palmquist. 2016. Flood Stress as a Technique to Assess Preventative Insecticide and Fungicide Treatments for Protecting Trees Against Ambrosia Beetles. Journal of IPM. 2016. 7(40). 11 p.http://ecoipm.org/wp-content/uploads/2014/08/Ranger_etal_2016.pdf (February 2017)
Reding, M., Shultz, P.B., Ranger, C.M., and J.B. Oliver. 2011. Optimizing ethanol-baited traps for monitoring damaging ambrosia beetles (Coleoptera: Curculionidae, Scolytinae) in ornamental nurseries. J. Econ. Entomol. 104(6):2017-24. <8 June 2017>
Schuh, J. and D.C. Mote. 1948. Insect Pests of Nursery and Ornamental Trees and Shrubs in Oregon.
H. Omroa Bhagwandin, Jr. The Shothole Borer: An Ambrosia Beetle of Concern for Chestnut Orcharding in the Pacific Northwest. Western Chestnut Growers Assn.
Original: 7 May 2011
Updated 8 June 2017.
Author: R.L. Rosetta, Extension Nursery Integrated Pest Management, Department of Horticulture, Oregon State University/NWREC.
First instar larva, pupa, male (lower left) and female (upper right) pear blight beetle