Summer Tree Care: Identifying Common Tree Insects in Summer, Part 5
This is the fifth part of a series on summer tree insects. This article examines fall webworm, and the myriad species of insects called lace bugs.
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.
Fall Webworm (Hyphantria cunea)
Hyphantria cunea, also referred to as fall webworm, is a native insect of North America. Fall webworm passes through four stages during its life cycle, with the larval and adult stages representing its most notable forms. The larvae appear from late summer to early fall, and feed on the leaves of various trees and shrubs. As they feed, the larvae cause minor to significant defoliation of host plants. Fall webworm is a relatively innocuous pest. Host plants generally recover from defoliation within a single growing season. Dense populations can cause extensive defoliation, which may result in a temporary reduction in plant vigor.
Fall webworm feeds on over 636 species of plants, including alder, ash, cherry, cottonwood, elm, maple, persimmon, sweetgum, walnut, willow, and birch. In the eastern United States, the insect’s preferred hosts are pecan, walnut, American elm, hickory, crabapple, and maple. In the western United States, fall webworm commonly infests alder, willow, cottonwood, and fruit trees.
Symptoms of Infestation
The larvae envelop branches in large webs, and consume the foliage. When present in forested settings, they generally cause minor defoliation. Dense populations can cause extensive defoliation, rendering host plants more susceptible to invasion from disease pathogens and secondary pests. The webs that the larvae construct are not injurious to plants, but their unsightly appearance can reduce the host plant’s ornamental value.
If the webs are opened, the larvae may be observed feeding from within them. Depending on the geographic location, their coloration and markings will vary. Fall webworm is generally separated into two unique races: a northern race and a southern race. Larvae from the northern race have black heads with pale yellow or green bodies, and long white hairs that arise from red or black tubercles. Northern larvae also feature a black stripe that runs along their abdominal segments. Larvae of the southern race are tan and yellow with red or orange heads, and brown hairs that grow from reddish-brown tubercles.
- Most plants generally recover from substantial defoliation, especially if it occurs late in the growing season.
- Fall webworm has several natural predators that help limit its populations. Birds, insect predators, and parasitoids prey on the eggs and larvae.
- Webbed branches may be pruned and destroyed. Pruning is not advised if it would interfere with the aesthetic shape of the plant.
- Larvae can be controlled using a registered insecticide. Applications should be performed in July, when larvae are small. Larvae become more resistant to insecticides as they mature.
- Avoid burning any webs or larvae. Fire can damage or kill the host plant.
Lace Bugs (Tingidae)
Lace bugs are a group of insects that belong to the family Tingidae. There are approximately 140 species of lace bugs in North America. Most lace bugs target a specific host. Some of the most common lace bug species are Stephanitis pyrioides, which infests azaleas, Corythucha cydoniae, which is partial to hawthorns, Teleonemia scrupulosa, which feeds on lantana, and Corythuca celtidis, which attacks hackberry. Lace bugs are generally considered a minor nuisance. Dense populations are capable of causing mild to moderate defoliation of host plants.
Some of the most common lace bug hosts include alder, ash, avocado, azaleas, basswood, birch, buckeye, ceanothus, cherry, coyote brush, elm, fringetree, hackberry, hawthorn, lantana, London plane, photinia, poplar, rhododendrons, serviceberry, sycamore, toyon, walnut, and willow.
Symptoms of Infestation
The adults and nymphs can be observed feeding on the underside of leaves. Due to their small stature, a magnifying lens may be required to see them clearly. As the insects feed, they cause a stippling of the infested leaves. The stippling initially appears as a series of tiny, white spots on the leaf surface. The spots quickly coalesce, resulting in extensive leaf discoloration. By mid-summer, the leaves will have turned yellow. By the end of the growing season, they will become necrotic.
As they feed, the adults and nymphs expel specks of dark excrement, which infested leaves can become laden with. When abundant, the excrement may drip onto surfaces located beneath the host. Severely infested plants may prematurely shed their leaves. Prior to dropping, the leaves will often wilt and curl. Avocados that experience premature leaf drop may incur sunburn damage, resulting in a significant reduction in fruit yield. Broad-leaved evergreens that are exposed to full sun may expire if heavily infested.
- Begin inspecting vulnerable plants for lace bugs in late winter. Continue checking for lace bugs at least once a week throughout the growing season.
- Lace bugs do not often cause significant damage to their hosts. As such, most lace bug infestations can be tolerated, and offset by maintaining plants through sound cultural practices.
- 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.
- Stippled foliage should be promptly removed, and disposed of.
- When nymphs are abundant, they can be dislodged from plants with a strong stream of water. Direct the water at the underside of the leaves. Initiate this practice by late spring, and repeat it every 1 to 2 weeks to inhibit the growth of lace bug populations.
- Various insecticides may be employed to combat lace bug populations. Non-persistent, contact insecticides are recommended for use on lace bugs, as they are not as detrimental to beneficial insects. Begin applying insecticides by late spring. Continue to perform applications at three to four week intervals, until the growing season has ceased.
- When planting, select trees and shrubs that are suitable for the location. Plant susceptible trees and shrubs away from sidewalks and buildings. This will prevent the excrement produced by the lace bugs from accumulating on nearby surfaces. Replace or transplant trees or shrubs that are not thriving in a particular setting.
- Avoid exposing vulnerable plants to full sun, as it can render them more prone to infestation, and environmental injuries.
- Lace bug populations may be limited on various shrubs by removing mulch and leaf litter from beneath host plants during the winter months. Mulch can be replaced the following spring.
- In fall, rake and compost leaves that have been shed beneath plants infested by lace bugs.
- Lace bugs have a multitude of predators and parasites that help to suppress their populations. Some of the most effective include assassin bugs, lady beetles, lacewing larvae, jumping spiders, mites, and parasitic wasps.
- Horticultural oils can be sprayed on leaves to deter lace bug populations.
Photo courtesy of Katja Schulz CC-by-2.0