This is the final part of a series on summer tree insects. This article examines two-lined chestnut borer, and viburnum leaf beetle
As spring transitions into summer, temperatures gradually rise, and plants enter the next phase of their development. This period coincides with the appearance of numerous insects, many of which infest vulnerable trees and shrubs. When infestations occur, they can be detrimental to plant health. The following discusses some of the insects that commonly infest plants in summer, and how they impact their hosts.
Two-lined Chestnut Borer (Agrilus bilineatus)
Two-lined chestnut borer is a beetle that is native to North America. It is a member of the jewel beetle family Buprestidae. Two-lined chestnut borer was once considered the principal pest of American chestnut. It is now considered a major nuisance of oak trees. The insect primarily infests stressed trees. The persistent feeding of the insect disrupts the host’s vascular system. This causes the host to decline. Severe infestations often culminate in plant mortality.
American chestnut was once the primary host for two-lined chestnut borer. The insect now commonly infests oak trees. Some of the more frequently attacked species include white oak, scarlet oak, northern pin oak, bur oak, chestnut oak, northern red oak, post oak, black oak, and live oak. The adult beetles are attracted to oaks that are declining, or suffering from environmental stress. Urban oaks that undergo stress due to trunk or root injuries, soil compaction, and changes in soil depth are also vulnerable to infestation. Oaks that have been defoliated by insects such as gypsy moth, elm spanworm, fall cankerworm, and forest tent caterpillar are often infested.
Symptoms of Infestation
By mid-July, infested foliage turns brown, and wilts prematurely. The wilted foliage will often remain attached to the tree for several weeks or months before being shed. Infested branches generally die back. When this transpires, the affected branches will likely be devoid of growth the following year.
Severe infestations generally cause the host to decline. In forested settings, or densely wooded residential sites, outbreaks may occur, resulting in pockets of dead and declining trees. Trees may succumb to heavy infestation within the first year. Trees that survive initial infestations generally fail within two to three years. Infested trees are more susceptible to diseases, particularly Armillaria root rot. Armillaria root rot works in conjunction with two-lined chesnut borer to cripple and disfigure host trees.
- Maintain tree vigor through sound cultural practices. Ensure trees are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base of susceptible trees to improve soil quality, moderate soil temperature, and retain soil moisture. Fertilize trees to promote healthy growth.
- Prune and dispose of heavily infested branches in late summer, when they can be easily detected. Pruning cuts should be administered below the last wilted leaves on each branch.
- Avoid mechanical injuries to vulnerable trees.
- Larval parasites provide some natural control. One larval parasite, the chalcid wasp Phasgonophora villosus, is highly effective at limiting two-lined chestnut borer populations.
- The downy and hairy woodpeckers Picoides pubescens and Picoides villosus assist in reducing insect populations. They feed primarily on the overwintering larvae.
- Trees with abundant insect populations should be culled to prevent further infestations. Remove infested trees during late summer or early fall, when the larvae are still developing.
- Temporary chemical treatments can be adopted to promote recovery in trees that are undergoing environmental, cultural, or biological stress. These include trunk injections, trunk and foliar sprays, and soil treatments.
- Pesticide treatments may be employed to protect shade and ornamental trees from infestation.The first application should be performed in early spring, one to two weeks before the adults emerge. To ensure success, thoroughly drench the tree’s branches and trunk. Two subsequent applications should be adminstered at two week intervals.
- Infested logs can be treated with pesticides. A single application should be performed on the bark in early spring, one to three weeks prior to the adults’ emergence.
Viburnum Leaf Beetle (Pyrrhalta viburniI)
Viburnum leaf beetle is an insect that feeds on the foliage of viburnum plants. The beetle is native to Europe and Asia. It was first identified in North America in 1947 in Ontario, Canada. The larval and adult stages of the insect are voracious feeders. Dense populations can rapidly defoliate entire plants. Repeated defoliations may result in plant mortality.
Viburnum leaf beetle infests viburnum plants. The most susceptible varieties are American cranberrybush, arrowwood, European cranberrybush, Chinese, Taiwanese, Rafinesque, Sargent, and Wright viburnum, mapleleaf, nannyberry, rusty blackhaw, smooth witherod, and wayfaringtree.
Symptoms of Infestation
Branches or twigs that are infested with eggs are most apparent in winter, when viburnums have shed their leaves. The larvae and adults tend to concentrate their feeding on the lower leaf surface, between the midrib and the larger veins. The persistent feeding causes the skeletonization of the leaves. Severe infestations can rapidly defoliate plants. Weakened plants may eventually succumb to infestation.
- When planting, select varieties that exhibit an increased resistance to viburnum leaf beetle. Some of the most notable examples include burkwood, carlcephalum, David, dawn, doublefile, Japanese, Judd, Koreanspice, lantanaphyllum, leatherleaf, Siebold, and tea viburnum.
- From late winter to early spring, inspect the twigs and branches of viburnums for egg sites. In late spring, monitor the lower leaf surface for the presence of larvae.
- Prune out and dispose of twigs and branches that are infested with eggs.
- Several natural predators feed on the larvae, including lady beetle adults and larvae, lacewing larvae, and spined soldier bug nymphs. Lady beetle adults and spined soldier bug adults also prey on the insect.
- An adhesive barrier may be placed around the base of susceptible viburnums to prevent the larvae from entering the soil to pupate. Avoid placing sticky materials directly on the bark, as some may have phytotoxic effects.
- Several insecticides are registered for use in controlling viburnum leaf beetle. Applications should be performed in late spring, when the larvae first hatch.
- Horticultural oil can be sprayed on to egg laying sites prior to leaf expansion. The oil will smother the eggs, and prevent them from hatching.
Photo courtesy of Hectonichus CC-by-3.0