This is the third part of a series on spring tree insects. This article examines eastern spruce gall adelgid, eastern tent caterpillar, and elm leaf beetle.
In spring, deciduous trees break dormancy, and resume the growing process. During this period, many insects become active, emerging from their overwintering sites to plague their hosts. The following describes some of the insects that may be commonly observed in spring, and how they can impact susceptible trees.
Eastern Spruce Gall Adelgid (Adelgis abietis)
Eastern spruce gall adelgid is an insect that inflicts considerable damage to ornamental spruce trees. It is one of the most important species of adelgid. The larvae induce the formation of cone-like galls on developing twigs, which deforms and girdles them. Severe infestations can disfigure, and weaken trees, rendering them more susceptible to invasion from tree pathogens, and other insects.
Eastern spruce gall adelgid most commonly infests Norway and white spruce. Infestations may also occur on blue and red spruce, albeit with less frequency.
Symptoms of Infestation
Eastern spruce gall adelgid causes small pineapple-shaped galls to form at the base of new shoots. These galls stunt the growth of the shoots. Severe infestations can disfigure, and deform trees. Galls are an aesthetic nuisance as well. They reduce the ornamental value of host trees.
- When planting, select resistant varieties of spruce. Black, red, and Englemann spruces exhibit an increased resistance to eastern spruce gall adelgid.
- Prune, and dispose of green galls to reduce adelgid populations.
- Applications of dormant oil in early spring will effectively control overwintering nymphs, and prevent gall formation. Oil should not be used on blue spruce, as it can alter the tree’s coloration.
- Contact insecticides applied at budbreak will limit nymph populations. Additional applications can be performed in late September or early October to eliminate overwintering nymphs, and prevent gall formation the following spring.
Eastern Tent Caterpillar (Malacosoma americanum)
Eastern tent caterpillar is a species of moth that is native to North America. It is often mistaken for a bagworm, and sometimes erroneously referred to as gypsy moth, fall webworm, or forest tent caterpillar. Forest tent caterpillar is similar in appearance, but does not produce a tent. Eastern tent caterpillar is notable for being a social insect. The larvae form communal nests, or tents, in the branches of trees, where they feed in groups. Larval populations tend to fluctuate each year, with outbreaks occurring every eight to ten years.
Eastern tent caterpillar commonly infests apple, crabapple, and wild cherry. Hawthorn, maple, peach, pear, and plum are also infested, albeit with less frequency.
Symptoms of Infestation
A colony of eastern tent caterpillars can rapidly defoliate small trees and shrubs. While this may temporarily reduce the aesthetic value of the host, most plants will produce a second flush of growth within two to three weeks. The silken tents produced by the larvae are one of the most conspicuous symptoms of infestation. As the tents enlargen, they become noticeable on the branches of infested plants. When present, the larvae can be frequently observed crawling on infested plants, walkways, furniture, and buildings.
- Insecticides can be applied to susceptible plants to manage larvae populations. Applications should be performed as soon as the larvae hatch. Once the larvae have established their tents, they become more difficult to combat.
- In early spring, tents can be pruned out and destroyed.
- Egg masses can be stripped from trees during winter to reduce populations the following spring.
- Several natural predators, including various tiny braconid, ichneumonid, and chalcid wasps, can drastically reduce larvae populations when present.
- Maintain tree vigor to encourage recovery from defoliation. Ensure that plants are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around vulnerable plants to improve the soil quality, moderate the soil temperature, and retain soil moisture.
Elm Leaf Beetle (Xanthogaleruca luteola)
Elm Leaf Beetle is a significant nuisance of elm trees. It is a member of the family Chrysomelidae. Elm leaf beetle is indigenous to Europe. It was inadvertantly introduced to North America and Australia, where it has since become widespread. The adults and larvae feed on the foliage of elms. When populations are dense, elms may be completely defoliated. Repeated defoliation reduces tree vigor, and may result in tree mortality.
Elm leaf beetle infests American elm, Chinese elm, English elm, Scots elm, Siberian elm, and Zelkova, as well as numerous cultivars. Landscape trees are typically more susceptible to infestation than those in forested settings.
Symptoms of Infestation
The adults chew through leaves, creating small holes in the leaf surface. The larvae skeletonize the leaf surface. Damaged foliage will generally turn brown to white. Dense adult populations can partially or completely defoliate a large elm tree. This disrupts the photosynthetic process, impeding the tree’s ability to grow. Severely weakened trees are often bereft of growth. Trees that are defoliated will usually produce a second flush of growth, which may be readily consumed by subsequent generations of elm leaf beetle.
- Maintain tree vigor through sound cultural practices. Ensure that elms are sufficiently watered, especially during periods of extended drought.
- Apply a layer of organic mulch around the base of trees. Proper mulching will improve the soil quality, conserve soil moisture, and moderate the soil temperature.
- Promptly remove any dead or diseased branches. Prune trees periodically to improve air circulation through the crown, and facilitate a rapid drying of the foliage.
- When planting, select cultivars that exhibit an increased resistance to elm leaf beetle.
- Elm leaf beetle has several natural predators, including certain flies, wasps, earwigs, lacewing larvae, and a predaceous ground beetle. The small black tachinid fly parasitizes the larvae. It lays its eggs in the larvae. Once the eggs have hatched, the larvae are devoured from within. The tiny wasp, Oomyzus gallerucae, feeds on the eggs.
- Tree trunks can be banded with insecticides. Banding elm trees can limit beetle populations by killing the larvae as they navigate down the main trunk prior to pupation.
- Buildings located close to infested trees should be sealed to prevent the beetles from overwintering in them.
- Systemic insecticides can be injected into the soil. Perform initial applications in spring, as leaf expansion begins.
Photo courtesy of Katja Schulz CC-by-2.0