July 8, 2022
As we approach the middle of summer, those in the more southern regions of Illinois may see leaf defoliation and some early silk webbing on the leaves of trees. In general, you can blame two types of web-forming caterpillars and two types of mites for this problem.
Fall webworm. This pest is very common mid- to late season. The fall webworm feeds on nearly a hundred different tree species, but prefers walnut, persimmon, hickory, birch, cherry and crabapple. The hairy, straw-colored, half-inch-long larvae begin feeding on the new growth at the end of branches, and enclose the foliage with fine silk webbing engulfing larger and larger areas of foliage.
Look for two generations in southern regions and only one generation in more northerly areas. In most cases, defoliation is not all that detrimental to the tree, but chronic, multi-year defoliation events can lead to tree stress, making trees vulnerable to lethal pathogens and wood-boring insects.
Control fall webworm by pruning out the webs when they are small and spraying with an insecticide. If you choose to use chemical control, you will need to penetrate the web with the spray or rip the web open to make contact with the larvae. Just spraying the outside of the web will not be effective. Since the caterpillars feed later in the growing season, control is usually not warranted, as the tree has generally produced the majority of its food for that season. Mature trees can weather the storm; however, new plantings might warrant better control of fall webworm.
Walnut caterpillar. The second common defoliator feeds on walnut, but also pecan, butternut and hickory. When young, the larvae are black. Then they turn to a dark red with white hairs and thin white lines running the length of the body. Like the fall webworm, they feed in groups, consuming the entire leaf. They also spin silk, but not as extensive as the fall webworm.
Fortunately, the walnut caterpillar does most of its feeding in late summer and does not have a major impact on nut production. However, in more southern regions, there can be as many as three generations, resulting in earlier feeding and more damage to tree health and fruit production.
Two-spotted spider mite. Mites are very small insect relatives, so will need a hand lens or some really good “cheaters” to see them. These critters are sap-feeders and cause a bronzing and yellowing of the foliage. Additionally, they produce a fine silk webbing, engulfing leaves and stems or plants.
The two-spotted spider mite feeds on hundreds of hosts, including vegetable, flowering and woody plants. They really like roses. The two-spotted spider mite is easily identified by the two dark spots on either side of the body, along with a tannish body.
Honeylocust spider mite. While also a yellow to light brown to tan, this mite lacks the black spots and only feeds on honeylocust trees.
Both mites are warm-season pests, showing up in mid- to late summer when it gets hot and dry. Trees growing in parking lots, confined root spaces and other tough conditions will be affected the most. Like the defoliators, these mites will not kill a tree (they may kill herbaceous plants) but are just another stressor the tree has to deal with.
Mites can be controlled with miticides or horticultural oil or soap sprays. To check for mites, hold a sheet of paper under a branch and give it a good shake — the mites will drop onto the paper. Be alert for predaceous or “good” mites, as they will feed on the pest mites. The good guys will usually have longer legs and move faster than the pest mites. Remember, if you use chemical control, you may kill the beneficial mites and allow the pest mite populations to explode.
Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at [email protected]. The opinions of this writer are not necessarily those of Farm Progress/Informa.
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