Minimizing Shothole Borer Damage in Orchards

Shothole borers are very small beetles that attack trees. Virtually every species of tree has several types of borers which may feed on it. At a recent presentation in Portland, Oregon, Mike Doerr from Washington State University Tree Fruit Research and Extension Center spoke about a particular shothole borer, Scolytus rugulosus, that is becoming a more common problem in Washington cherries. S. rugulosus is present in Ontario and around the Great Lakes and has a host range including cherry, peach, plum, apple, pear, and apricot, as well as many common forest and ornamental trees. The primary infestations have occurred where large piles of orchard wood were stacked as firewood in or immediately beside an orchard. The solution for shothole borer infestations is very simple – remove and destroy wood piles, dead trees, and prunings from the orchard as quickly as possible.

Shothole borers can attack freshly killed or diseased trees, stressed or low vigour trees, fresh prunings and newly planted trees. If present in sufficient numbers, they can also cause damage to, or even kill otherwise healthy, productive trees. Although there are many different species of Scolytidae (the family of bark beetles which includes shothole borers) they are fairly distinct small (1.8 to 2.5 mm) stubby beetles with their head concealed from above by a “hooded” thorax. They look a bit like they have run into a wall and permanently pushed their heads back into their bodies. Larvae infesting wood are small (0.5 to 4.0 mm) white, legless grubs with slightly darker head capsules. Feeding galleries of the larvae can be found under the bark even in winter. Two generations per year of S. rugulosus are likely in Ontario but the precise timing is uncertain – most likely peak flights of adults occur in May to June and again in August to September.

In Washington, as elsewhere where shothole beetles have been reported in orchards, their damage is pronounced on orchard edges near stacked woodpiles. Generally, these little beetles travel only about 10 to 50 m before boring into a host tree, confining their damage primarily to the outer 2 or 3 rows of orchard trees. Although not usually necessary, monitoring for adult beetle emergence and movement is easy with yellow sticky cards. Recognizing the first signs of infestation by scouting orchard borders (especially near stacked wood or pruning piles along orchard edges) is a better way to monitor. Look for sudden wilting of twigs or yellowing of shoots and small entrance holes about 1.5 mm wide at the base of buds and spurs. Watch also for sap leaking from small entrance holes in trunks or branches.

Even severe infestations of shothole borers are easy to clean up with simple orchard sanitation. To avoid problems with shothole borers, burn prunings promptly and don’t store firewood stacks in or beside orchards. If you think you are suffering from shothole borer damage, let me know and I can help you determine whether the damage is from S. rugulosus or some other boring beetle.