URBANA, Ill. – University of Illinois Extension horticulture educator Kelly Allsup cautions to watch out for insect pests on favorite landscape trees this late summer and fall. “If you don’t take necessary management actions at the appropriate time, the battle against them may be hard to win,” she says.
Allsup provides the following information:
Tree pests like fall webworm and oystershell scale have some control management practices that can be implemented in late summer and fall.
Fall webworm (Hyphantria cunea) attacks a large number of tree species but especially hickory, ash, birch, walnut, crabapple, apple, elm, maple, oak, and pecan.
There are two generations of the fall webworm in the southern portion of Illinois. The first generation emerges in late June or July and again in August and September. In the northern portion of Illinois, only one generation emerges in August and September. As far as the overall health of the tree, only the first generation is of concern.
Pale green and yellow caterpillars with thick white hair tufts begin to hatch. They feed for several weeks in tents on the tips of the branches. They can skeletonize leaves and even defoliate trees. After six weeks of feeding, they fall to the ground or find a nice crevice in the bark to pupate.
The adults, which are a pristine white moth (with or without black dots) emerge again in August to lay white egg masses on the bottom sides of leaves. Sometimes she lays them on branches or the trunk and they look like dark brown oval knobs. At this time, the most efficient method is to prune out webs of caterpillars, scout for, and remove egg masses.
The Pest Management for the Home Landscape says Bacillus thuringiensis var. kurstaki (Btk), among other pesticides, can be applied when caterpillars are young and tents are new for the best efficacy. This bacterial pesticide must be ingested to be effective, so open up the tents and spray on leaves inside.
Or, just let nature take its course. According to Michigan State University Extension, there are over 50 species of wasps that parasitize the eggs or caterpillar and over 30 percent of predators will devour these late-season threats.
Oystershell scale (Lepidosaphes ulmi) begins to emerge and attack a large species of trees in early May, especially lilac, ash, privet, beech, and viburnum.
Yellow crawlers emerge to feed causing yellowing, stunted foliage, and branch die back. A second generation of this pest can be controlled when Queen Anne’s lace is blooming in August with an insecticidal soap or a summer spray of petroleum oil. Spraying the crawler stage of this pest is best because they have not yet developed the waxy coating that prevents penetration insecticide. Heavily infested branches can be pruned out as the eggs will overwinter under the dead female’s waxy covering.
Scale are also great attractors of the beneficial insects. If lady beetles and other predators are present, a spray may not be needed.
Ultimately, good tree health is more crucial for tree insect management than any other practice. Good tree management includes watering in times of drought, averting soil compaction, adding mulch ring to prevent weeds, and preventing physical damage by lawnmowers and string trimmers.