European shothole borer, pear-blight beetle (Anisandrus dispar)
Lesser shothole borer, fruit-tree pinhole borer (Xyleborinus saxesenii)
Shothole borer (Scolytus rugulosus)
Pest description and crop damage Any of this complex of small beetle species from the beetle subfamily Scolytinae are sometimes referred to as "shothole borer" because of the characteristic damage caused by entrance or emergence of adult beetles from the woody host, leaving many small holes resembling a shotgun pattern. Shothole borer (Scolytus rugulosus) feeds directly on the wood and each gallery contains many tunnels and emergence holes. The adult female beetle bores into the tree and excavates along the grain of the wood where roughly 50 eggs are deposited along gallery walls. Each larva mines its own tunnel out from the wall of the gallery, feeding perpendicular to the wood grain and replacing the empty space in the tunnel with sawdust excrement (frass). The many larval tunnels fan out from the egg gallery, each one expanding in diameter as the larvae inside mature. The larvae pupate at the ends of the mines leaving the adults to finish the tunnel by chewing their way to the outside.
The two other "shothole borer" species recovered from hazelnuts (Anisandrus dispar and Xyleborinus saxesenii) belong to an ecological guild known as ambrosia beetles. Ambrosia beetles have an obligate association with fungi, which they introduce to their tunnels and cultivate as their only food source. The larvae live together in the galleries feeding on the fungus rather than boring through the wood. The tunnels are maintained by the adult females and all debris generated by the residents is ejected through the entrance hole. The fungi introduced by ambrosia beetles can cause wilting and die-back of branches as the vascular tissue becomes restricted as the fungi colonize the tree. Opportunistic wood decay fungi may be introduced into the wood as a result of attack by any of the borers affecting hazelnuts. The adults can be difficult to identify, but the galleries are easily distinguished. The clean, uniform-size tunnels in the galleries of ambrosia beetles can be easily distinguished from the numerous expanding frass-filled tunnels radiating out from the central egg gallery in S. rugulosus.
Shothole borers were introduced to North America and have been found in the PNW since the early 1900s. They are pests of forest trees, ornamental shade trees, and shrubs as well as nut trees. Borers are primarily a problem on injured or stressed plants, but healthy trees growing adjacent to blocks of neglected trees also may be attacked. Hazelnut orchards adjacent to woodlands are also at risk. The adult shothole borer is a brown or black beetle about 0.08 inch long. The larvae are white, legless, and about 0.17 inch long.
Biology and life history Shothole borer larvae overwinter beneath the bark of infested trees where they pupate. Adults emerge in spring or early summer, mate, and fly to susceptible trees to feed at the base of leaves or small twigs. They then tunnel into the tree, excavating galleries parallel to the wood grain where they lay their eggs. The eggs hatch and the larvae feed by tunneling at right angles to the main burrow, causing a characteristic pattern of damage. The burrows are filled with frass and increase in diameter as the larvae mature. After 6 to 8 weeks, the larvae pupate at the ends of the galleries, then emerge as adults starting in March. This activity creates many small, round exit holes that produce a "shothole" effect. There are two generations per year. The holes that these species leave in their hosts are always small and round in contrast with the flattened D-shaped exit holes left by Pacific flatheaded borer.
Scouting and thresholds Examine branches in late spring for holes 0.08 inch in diameter, oozing sap and sawdust. Beetles are especially attracted to unhealthy trees, particularly trees where roots may be suffering from lack of oxygen because soil is saturated with water. Entrance holes often appear as wet stains on hazelnut trunks.
The best management tool is keeping trees healthy through proper pruning, adequate watering, and fertilizing. Healthy trees repel the beetles by plugging galleries with sap and resins. Remove and destroy infested wood on the tree or nearby piles of infested green wood, especially cherry. Once the bark on cut wood dries and sloughs off, it is no longer a host for the beetles. Whitewash trunks of young trees to prevent sunburn and reduce potential hazard of attack from shothole borer. Tanglefoot or other sticky substances applied to trunks and branches may trap some beetles. Yellow sticky cards are effective for monitoring flights of adult beetles (March-September). Commercial or homemade lures that release ethanol are also very attractive to adult beetles. Intensive mass trapping of adult beetles in orchards could help reduce damage.
Management-chemical control: HOME USE
- azadirachtin (neem oil)-Some formulations are OMRI-listed for organic use.
- plant-derived essential oils-Some formulations are OMRI-listed for organic use.