New Pests of the Understory

In a time of world trade and global movement of people and products, hitchhiking insects are becoming more and more common. In the past 20 years, almost 60 exotic insect species have established in Washington state. Some of these hitchhikers can become serious agricultural and forestry pests. The risk continues to grow as global markets continue to expand.

A 2010 study led by Julieann Aukema, a forest ecologist with the National Center for Ecological Analysis and Synthesis in Santa Barbara, California, estimated that there is 32 percent risk that a wood boring insect more damaging than the emerald ash borer will be introduced into the United States in the next ten years. In addition to exotic insects that can cause significant economic impacts to agriculture and natural resources, there are a number of species affecting the natural and cultural ecosystems. The following are a few examples of newly introduced insects that are, or likely will, impact the forest understory and those that rely on it.

Viburnum leaf beetle

Viburnum leaf beetle adult.
Viburnum leaf beetle adult. Photo: T. Murray/ Washington State University
Viburnum leaf beetle larvae.
Viburnum leaf beetle larvae. Photo: T. Murray/Washington State University

The viburnum leaf beetle (VLB), Pyrrhalta viburni, was first discovered in Washington state in Whatcom County in 2004. Since then, it has spread down to King County. Recent collections of VLB have been made in Spokane. VLB overwinters in its egg state in the stems of last year’s new viburnum growth. Larvae hatch when the first leaves unfold in spring. Damage caused by feeding larvae is very distinctive and won’t be confused with any other feeding damage on viburnums. After feeding, larvae migrate to the soil to pupate for a few weeks. Adults emerge and continue to feed on foliage causing additional damage. Adult beetles feed, mate and lay eggs until first frost. Viburnum plants are not able to tolerate multiple defoliation events over consecutive years. The native Viburnum edule, high bush cranberry, is susceptible to attack. Many wildlife species rely on high bush cranberry for a reliable food source. To learn more about the viburnum leaf beetle in Washington state.

Lily leaf beetle

Lily leaf beetle adult.
Lily leaf beetle adult. Photo: E. LaGasa/Washington State Dept. of Agriculture

The lily leaf beetle (LLB), Lilioceris lilii, was discovered in Washington state just outside of Seattle in Bellevue during the spring of 2012. Thus far, LLB has only been found in Bellevue, Seattle and Issaquah. Adult beetles are very conspicuous as scarlet red beetles. Adults overwinter in protected areas and move to feed, mate and lay eggs on emerging true lilies (Lilium spp.) and fritillaries (Fritallaria spp.) in the spring. Eggs are laid in irregular rows on the underside of the lily leaves. Once eggs hatch, beetle larvae feed on the lily foliate and developing flower buds. Larvae cover themselves in excrement and other debris as a defensive tactic and superficially resemble slugs. Two key native species in the Pacific Northwest that are likely susceptible are the tiger lily, Lilium columbianum, and the chocolate lily, Fritillaria lanceolate. Learn more about the lily leaf beetle in Washington state.

Azalea lace bug

Azalea lace bug adult
Azalea lace bug adult. Photo: T. Shanan/Oregon Dept. of Agriculture

The azalea lace bug, Stephanitis pyrioides, was first discovered in Seattle, King County, in 2008. The following year, it was identified in Oregon. Lace bug nymphs emerge from eggs in the spring. Having a piercing-sucking mouthpart, the nymphs feed by removing the liquids from plant leaves creating a stippled or bronzed burn on the leaf surface. Distinctive tar spots appear on the undersides of leaves as evidence of their presence. Adult lace bugs are quite attractive with a clear, lacy appearance. In the Pacific Northwest there will be multiple generations per year. Azalea lace bugs are causing significant damage and mortality to landscaped azaleas and rhododendrons in both the Seattle and Portland areas. What is most concerning about this newly introduced insect is the degree of damage it can cause and the expanded host ranges documented in the Pacific Northwest. Jim LaBonte from Oregon Department of Agriculture has found damage on huckleberry and salal in addition to other native plant species. Learn more about the azalea lace bug in the Pacific Northwest.

Spotted winged drosophila

Spotted wing drosophila.
Spotted wing drosophila. Photo: E. LaGasa/ Washington State Dept. of Agriculture

The spotted winged drosophila (SWD), Drosophila suzukii, is a significant new pest to many small fruits and has had a major impact on blueberry, raspberry and cherry production in regions of the Pacific Northwest. SWD was first discovered in 2009 in Seattle, just shortly after its detection in California the previous year. Since then, SWD has spread across the continent. SWD adults overwinter in protected areas. When berries and other food resources become available in spring, SWD adults lay eggs into ripening fruit using an ovipositor—an appendage—with a saw-like edge. The ability to egg-lay in under-ripe fruits has made this fruit fly a serious pest. Being a fruit fly, SWD has a high reproductive capacity and fast generation time. Populations can build rapidly. Larvae feed on the flesh of fruit and quickly cause the fruit to rot. Larvae pupate outside the fruit and emerge as adults to repeat the process.

In 2013, SWD was found infested huckleberries at high elevations in the Indian Heaven Wilderness Area of the Gifford Pinchot National Forest. Almost 50 percent of the huckleberries picked turned to be infested by SWD. Since 2013, SWD has consistently been collected from infested huckleberries in high elevations (5100 feet) in remote areas. SWD was able to disperse successfully in nooks and crannies of the Mount Adams and Mount Hood forests very rapidly. SWD has likely done so in other forests where huckleberries are common.

The economic impact to agriculture and natural resources of new pests is the focus for research and investments; there are few resources available to understand the impact on natural and cultural systems. The significance of these new pest introductions into natural areas has yet to be fully realized. To put it in perspective however, humans have harvested huckleberries from the Indian Heaven Wilderness Area for almost 10,000 years without experiencing wormy, rotten berries until now.

By Todd A. Murray, Director, WSU Agricultural and Natural Resources Extension Program Unit

Spring Brings Out Pine Engraver Bark Beetles

Pine engraver frass
Pine engraver frass seen on trunk of infested treed. Photo: Brytten Steed, USDA Forest Service,

As temperatures begin to warm up, the pine engraver bark beetle will once again rear its ugly head. The pine engraver is a very small (1/8 to 3/16 inches long) bark beetle that attacks small diameter pine trees (2-8 inches DBH*) or the tops of large pine trees.

Upon emergence in the spring, the pine engraver typically infests fresh slash, wind throw, or snow-damaged trees. The adults mate and build “galleries” under the bark and the female begins laying eggs. Within 4-14 days, the eggs will hatch into larvae and feed on the phloem (living, inner-most layer of bark). After 10-20 days, the larvae will pupate and emerge as adults 10 days later. Overall, it takes about 40-55 days for the pine engraver to complete development from egg to adult. The new generation of adults produced will begin fresh attacks following emergence. If slash is available, this will be preferentially infested. If no slash is available, the beetles will begin attacking nearby live, standing trees. By mid- to late-August another generation will be completed in this material. The generation that emerges in August typically seeks out places to hibernate for the winter, but sometimes makes a feeding attack prior to hibernation, where trees are attacked and the phloem eaten, but no brood is produced.

Signs of pine engraver infestation include reddish-colored frass (refuse and excrement) on the outside of the bark, Y or H-shaped galleries under the bark and/or woodpecker damage on the surface of the bole. The pine engraver is often associated with the western pine beetle and the red turpentine beetle.

Typical y-shaped pine engraver gallery.
Typical y-shaped pine engraver gallery. Photo: USDA Forest Service Region 6 Pacific Northwest Archive, USDA Forest Service,

The pine engraver can outbreak, and outbreaks can include hundreds of trees. Outbreaks typically occur as a result of drought, overcrowding, and/or the creation of slash, windthrow, or snow damage. Thinning dense pine stands can help mitigate the potential of a pine engraver outbreak by increasing the availability of water, sunlight, and nutrients for the residual trees. This enhances their vigor and allows for more defensive capabilities, such as increased resin flow.

The timing of slash creation is crucial. It is best not to create slash from January through July. Slash created during this time period does not have enough time to dry out prior to the several flights that the pine engraver may carry out each year. Slash created in fall or early winter will usually dry by spring and will be unsuitable habitat. If slash must be created during high risk months, there are several disposal options that may help reduce the risk of infestations that lead to outbreaks.

Pine engraver-infested trees with slash pile circled in red

Following the creation of slash, the best way to mitigate outbreak potential is by burning slash piles. This of course cannot always be accomplished due to dry, fire-weather conditions. Another option is called the “green chain” where you continuously make slash throughout the entire pine engraver flight period so that the beetles continue to infest the slash (which they prefer) rather than the standing, live trees. At the end of the season these piles can be burned. A third option would be “pile high and deep”, in which case you would create a very large slash pile (10-20 feet in width, length, and height). This method can be effective because the first generation of pine engraver emerging from the outer portion of the slash pile will likely continue moving deeper into the pile where branches are still moist and viable. Again, at the end of the season these piles can be burned. Although these methods have been shown to be effective in some cases, please remember, bark beetles can be unpredictable and may still choose to attack your live, standing trees.

pine engraver-infested trees
A stand of pine engraver-infested trees with a slash pile of diseased wood circled in red.

Some other methods of mitigation include completely removing slash from the site. Just be sure not to move it somewhere that may cause damage to someone else’s property. You may also debark larger slash, but this method is hard work and very time consuming if you have a lot of slash. Debarking is best when only one or two trees have been felled. Another alternative is to chip slash, but remember, even though the beetles cannot live and breed in the chips, they will be attracted to the area by the volatiles (chemicals trees give off that can attract bark beetles) that are emitted from the chips and the beetles may start attacking standing trees in the area around the chips. This is also true of the “lop and scatter” method. You can lop all slash greater than three inches in diameter into small pieces and scatter these pieces in a sunny area to dry out the phloem rendering the slash unsuitable for pine engraver infestation, but again, the volatiles emitted may attract beetles to the area and they may then attack your standing trees since the lopped up slash is not viable.

Finally, limit pruning activities to October through December if possible, as pruning wounds also release volatiles that may attract pine engraver. And never stack freshly cut wood next to live, standing trees during high-risk months.

*DBH = Diameter at breast height or diameter of the tree’s bole at 4.5 feet.

Read more in the US Forest Service’s Management Guide for Pine Engraver