This is the second part of a series on summer tree insects. This article examines grass bagworm, balsalm wooly adelgid, and black turpentine 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.
Grass Bagworm (Eurukuttarus confederata)
Grass bagworm is a species of bagworm that infests various grasses. The larval stage of the insect constructs a spiny case that measures ¼ of an inch to 1 inch in length. The case is composed of plant material and silk. It is initially flush with color, but turns brown as the grass decomposes. While the insect is seldom destructive to plants, it is considered an aesthetic nuisance that can plague landscapes and gardens when abundant.
Grass bagworm primarily infests grasses. It may also occasionally infest conifers.
Symptoms of Infestation
The bags produced by the larvae can generally be observed on different surfaces in mid-summer. Infested grass will turn brown. Plants that have been infested by the larvae may be partially defoliated. The silk secreted by the larvae can girdle branches, causing them to die back.
- Grass bagworm inflicts minimal damage to host plants. As such, control is not generally required.
- When necessary, pesticides can be utilized to control the larvae. Applications must be performed in spring, as the eggs begin to hatch. Spray the entire plant, ensuring that the branches are thoroughly drenched.
- Individual bags can be plucked from plants, and destroyed.
Balsam Wooly Adelgid (Adelges piceae)
Balsam wooly adelgid is a small, wingless insect that infests and kills true firs. It is invasive to North America. Balsam wooly adelgid appears as a white, wooly spot on true firs. The insect damages vulnerable trees by feeding in bark fissures. As the insect feeds, it releases toxins contained within its saliva. The toxins disrupt the tree’s vascular system, inhibiting the distribution of nutrients and water throughout the crown. This often culminates in tree mortality.
Balsam wooly adelgid infests all true firs. Balsam fir, Fraser fir, and subalpine fir are the insect’s preferred hosts.
Symptoms of Infestation
The most visible symptom of infestation is the white wooly material the adults produce. The waxy covering is conspicuous on the bark or branches in summer and fall. Infested trees will often form a layer of hard wood over the damaged bark. While this layer can help to repel the nymphs, it may also disrupt the tree’s vascular system. Over time, the lack of sufficient water and nutrients to the crown will cause plant growth to become distorted. Eventually, the tree may expire. Swellings may develop on the outer branch nodes and terminal buds. If the swellings partially enclose the buds, new growth will be stunted. When the buds are completely enclosed by the swellings, no new shoots or needles will be formed.
- When planting, select trees that exhibit an increased resistance to balsalm wooly adelgid. Grand fir is one of the least vulnerable fir trees.
- Monitor potential hosts for signs of infestation. The insects are generally easiest to detect in summer and fall.
- Dense populations can severely damage the bark on infested trees. Many trees will form a protective layer over the damaged bark. After a few years, the nymphs will be unable to penetrate the thick layer, causing them to rapidly expire.
- Abonormally frigid temperatures in fall can decimate nymph and adult populations.
- In Europe, several natural predators and parasites help to limit adelgid populations. The most effective are three beetles, and three flies.
- Cull severely infested trees in winter to prevent the insects from spreading.
- Insecticides registered for use on balsalm wooly adelgid can be employed to provide some control. Applications must be performed in spring, before bud break occurs. To ensure that applications are effective, thoroughly drench the bark and branches.
Black Turpentine Beetle (Dendroctonus terebrans)
Black turpentine beetle is a pine bark beetle that is native to the United States. It is the largest pine bark beetle in the southern part of its range. When mature, the adults exhibit a black to red coloration. Black turpentine beetle is typically a secondary pest of vulnerable trees. It colonizes freshly cut stumps, and trees stressed by environmental factors. Infestations are generally confined to the base of susceptible trees. Dense populations are capable of killing infested trees. The most severe infestations tend to occur in the southeastern United States.
All southern pines are potential hosts. The insect is most partial to slash pine, loblolly pine, shortleaf pine, and longleaf pine. Black pine, pitch pine, Scots pine, and red spruce may also be infested, albeit with less frequency. Trees that have incurred fire damage, or are suffering from environmental stressors such as drought or flooding are more vulnerable to infestation.
Symptoms of Infestation
The most conspicuous symptom of infestation is the large pitch tubes that arise due to the insect’s feeding. The pitch tubes will appear on the bark surface of infested trees. At first, the pitch tubes are red to white in coloration. As the pitch tubes age, they turn gray. White sawdust may be observed accumulating around the trunk of infested trees. Trees that are killed will shed their needles. About two months prior to being cast, the needles will fade in color, turning from a yellowish-green to a reddish-brown. Heavily infested trees are often attacked by ambrosia beetles.
- Abundant rainfall may decimate larvae populations.
- When other insects, such as engraver beetles, pine borers, weevils, or termites are present, the larvae may starve.
- Infested trees may be treated with horticultural oils. While this may not eliminate the insects altogether, it can drastically reduce beetle populations. To increase the efficacy of the oils, ensure that the trunk is thoroughly drenched.
- Insecticides registered for use on black turpentine beetle can be utilized to provide some control. Applications should be performed before the adults have infiltrated the bark. Multiple applications may be required for sufficient control in warmer locations.
- As the adults bore into the bark, they may be smothered by the ensuing pitch flow.
Photo courtesy of Gilles San Martin CC-by-2.0