These articles are written to apply to the northeastern corner of Illinois. Problems and timing may not apply outside of this area.
Late Summer Tree Insects
August 12, 1999
As summer moves along, some insects become less common while others start to appear. The Japanese beetle is fading away, for example. Typical late summer insects present now include fall webworm, annual cicada, and katydids.
Cicadas and katydids are heard more than seen. The annual or dog-day cicada makes considerable noise while sitting up in shade trees during the heat of the day. Some refer to them as "sewing bugs." This is not the same insect as the periodical cicada, which is also called either the 13 or 17-year locust. Annual cicadas rarely cause noticeable damage to trees, so control is rarely suggested.
Katydids also make noise while sitting in trees, but at night. Male katydids "sing" through much of the night from the tops of trees. Katydids are green, about 2 to 3 inches long, and have both long antennae and legs. These insects do not cause any significant damage to trees. Depending on your point of view, their singing may be annoying or just a soothing sound of the summer night.
Of the three, fall webworms are the most visible. Silk tents have been
showing up in a variety of shade trees. Close inspection reveals caterpillars
inside. As they feed and grow, these caterpillars make the web larger
to cover more foliage. When young, the caterpillars will only eat the
upper surface of the leaf, but as they get older they will eat the entire
leaf except larger veins and the midrib. When full grown, they drop to
the ground to pupate.
It may be unsightly, but fall webworm is rarely a threat to tree health in northern Illinois because it is late in the season. Although still green, trees are actually preparing for dormancy so having some defoliation now is not considered very damaging. If reachable, clip off the web and destroy it. If using insecticides, spray into the web to get control. Insecticides that will control fall webworm include Bacillus thuringiensis (Dipel, Caterpillar Attack, etc), carbaryl (Sevin), malathion, and acephate (Orthene).