Boring beetles, a new old problem in Ontario peaches?

Over the last couple of years, peach growers in the Niagara region have reported tree decline and loss as a result of infestation by tiny wood boring beetles. The type of injury caused by these beetles is very different from infestation by lesser and greater peach tree borers, which are the larvae of day flying clearwing moths, and the strategies used to monitor and manage them are different.

Solving the problem starts with understanding which beetle species are attacking trees - and there may be several of them, compounding the issue.

Peach bark beetle (Phloeotribus liminaris), a "shot hole borer" that attacks Prunus species (peach, cherry and plum), is present in most fruit-growing states and provinces in eastern North America. This pest is not a newcomer. Some of the first reports of peach bark beetle in Ontario date back to the late 1900s, when the insect emerged as a pest of stone fruit in the Niagara region (25th Annual Report of the Entomological Society of Ontario, Ontario Department of Agriculture, 1894, p.80):

"The peach orchards in the Niagara district have, during the past two years, suffered severely from the Peach Bark-beetle … it is hoped that before long a practical remedy will be discovered. It has usually been stated that this insect attacks only injured or declining trees, this, however is certainly not the case, for it was found in perfectly healthy and thrifty two-year-old peach trees; although much more abundantly on older trees with rough bark … The remedies (washing the trunks with kerosene emulsion, linseed oil, and whitewash containing Paris green) … have been rather conflicting, but there is every reason to think that before long, a sure means of prevention will be found."

Presumably the problem resolved itself, possibly as a result of the use of broad spectrum insecticides / trunk sprays; however, it seems borers are re-emerging as a threat to stone fruit production in Niagara.

As the quote above suggests, shot hole borers (several species, not just the peach bark beetle) usually attack stressed fruit trees. The peach bark beetle is a tiny brown beetle (1.5-2 mm long) with fine yellowish hairs arising from punctures, visible under magnification (Figure 1). The adult beetle drills holes in the bark and wood of twigs, branches, and trunks of trees. Females create small tunnels under the bark, usually running across to the grain of the wood (rather than parallel to it). The developing larvae leave the main tunnel and form galleries that radiate out from it. If you peel back the bark from trees infested by shothole borers, you can see the galleries underneath. Some people describe the galleries as being "centipede-like". The larvae feed on the sapwood, filling the galleries with frass, and if present in large numbers, they can eventually girdle a tree. The most obvious sign of shot hole borers are the small holes and the gum that flows from the entry wounds (Figure 2). Not all entry wounds result in a gallery! Healthy trees secrete a gummy resin that will either kill the insect or hamper its efforts.

Figure 1. Peach bark beetle

Figure 1. Peach bark beetle, Phloetribus liminaris. Photo credit: James D. Young, USDA APHIS PPQ, Bugwood.org

Figure 2. Damage caused by shot hole borers in peach

Figure 2. Damage caused by shot hole borers in peach. Photo credit: Hannah Fraser, OMAFRA

The injury is different from that caused by ambrosia beetles, a related group of similar-looking borers we investigated in Ontario apple orchards in 2016 as part of a preliminary study. To the untrained eye, shot hole borers and ambrosia beetle look very similar. Both are small beetles, brown to black in colour, and about 2-4 mm in length. Ambrosia beetles bore directly through the bark and into the wood, usually at a lenticel. The females excavate fairly large three dimensional galleries within the wood, and in the process will push out sawdust-like particles from the entry hole that may protrude from like a toothpick (Figure 3). Female ambrosia beetles carry a fungus that spreads within the gallery; it is on this that the developing larval brood feed (not on the wood). Staining of the wood by the fungus may be present. If the tree is healthy, it will often recover from light infestations. Trees affected by ambrosia beetles may exhibit sudden wilting and death, or delayed emergence in spring. Affected limbs may break easily.

Figure 3. Granulate ambrosia beetle symptom.

Figure 3. Granulate ambrosia beetle symptom. Photo credit: Laura Lazarus, North Carolina Division of Forest Resources, Bugwood.org

Ambrosia beetles will also attack stone fruit and other landscape trees. As with shot hole borers, there are several species, including the recently identified granulate (Asian) ambrosia species (Xylosandrus crassiusculus), a new arrival to Ontario. One of the more common species captured in our apple survey was the black stem borer (X. germanus), another introduced species, but which has been present in Ontario for over 20 years.
We don't have enough information about local flight activity patterns for all these insects. They can be active in late winter or fairly early in the spring, as they move to new host trees, and there are often two broods (generations) per year. It is possible to monitor for them using simple ethanol-baited traps (Figure 4) which mimic the volatiles (odors) coming off stressed trees, keeping in mind these traps are generic and will attract ambrosia and shot hole borer beetles from nearby infested sources - and they are hard to tell apart. Monitoring is important to show when the beetles are migrating to new host trees, and potentially when to protect trees from infestations (except we do not have any registered control products to target incoming females).

Figure 4. Ethanol-baited home-made "juice container" trap.

Figure 4. Ethanol-baited home-made "juice container" trap. The red liquid in the bottom of the container is a preservative to help prevent the decay of trapped beetles, but it can be replaced with water and a drop of unscented dish soap. Photo credit: Hannah Fraser, OMAFRA

In other jurisdictions, trunk sprays coinciding with flight periods of overwintered "new colonizing" beetles are used to help reduce infestations; however, there are no insecticides registered for these beetles in tree fruit orchards in Canada. Keeping trees health is key to preventing and / or minimizing the effect of infestation. Pruning out infested branches and dead / dying trees promptly, and burning or shredding these to eliminate breeding / emerging sites can help to reduce the problem. Studies using mass trapping or perimeter trapping of ambrosia beetles (ethanol baited traps, freshly cut hardwood logs along borders) showed promise in reducing injury in tree nurseries. More research is required to develop more effective pest management for these beetles in orchards.

Are you seeing injury in tree fruit? We're interested in collecting samples. Please contact Hannah Fraser hannah.fraser@ontario.ca.


For more information:
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E-mail: ag.info.omafra@ontario.ca