Caterpillars of Fall

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Many people enjoy catching a glimpse of butterflies and moths as they float and flutter through the landscape. Monarch butterflies are minor celebrities these days. Swallowtail butterflies are impressive and eye catching. Caterpillars are the larval form of butterflies and moths, and in this form may be much less charming than after their metamorphosis. For example, the tiger swallowtail caterpillar is called “parsley worm”, and can decimate plantings of herbs such as parsley and dill. However, for those who love the beautiful swallowtail butterfly, it is worth the sacrifice.

Fall is the season when many caterpillars are maturing and causing damage sufficient to cause concern for some homeowners. Webworms, orange-striped oak worm, and azalea caterpillar are some of the caterpillars commonly found this time of year, munching away on trees and shrubs. Azalea caterpillars feed on – you guessed it – azaleas, and often show up in late August and early September. Now is the time to start scouting your azaleas several times a week, keeping a sharp eye out for the black and yellow striped (sometimes appearing black and white) caterpillar with a reddish head. Young azalea caterpillars are more yellow than black, and feed all in a bunch. They spread out on the bush as they mature. They have a unique reaction to disturbance, as they will turn up their heads and tails in unison, forming a “u”, when disturbed. The easiest way to control azalea caterpillars is by pruning off the branch where a cluster is feeding, and place in a trash bag, or, for those with a greater need to express retribution, stomping on the pruned branch. Once the caterpillars get larger and start to disperse, it is still possible to prune several branches and get rid of offenders. Knocking the caterpillars into a pail of soapy water is also effective. While scouting, look for branches stripped of leaves: this is a sign of azalea caterpillars at work.

The orange-striped oakworm is another common caterpillar seen in late summer. The rather lovely, velvety Oakworm moth lays hundreds of eggs in mid summer on the undersides of oak leaves. Pin oak and willow oak are preferred hosts, but they will feed on other hardwood trees as well. The caterpillars hatch and begin feeding in a cluster on the tips of the branches. Orange-striped oakworms are black with orange or yellow stripes, except when just hatched, when they are green. Because they are so numerous, the oakworm can cause extensive defoliation of the host tree. If caught during the gregarious feeding stage, the branch can be pruned off and caterpillars destroyed. If the caterpillars have already dispersed, control may be more difficult. Depending on the size of the tree, it can be shaken or branches hit with a pole to dislodge caterpillars. Just take care to protect yourself from falling caterpillars!

Like orange-striped oakworms, webworms feed on hardwood trees, preferring persimmon, sourwood, and pecan. They, too, are aggregate feeders, although they feed within the protected covering of a web. This web expands to encompass more of the tree’s leaves as the caterpillars get larger. Unlike tent caterpillars, which create webs at the trunk and limb crotches of the tree, fall webworm builds webs at the end of tree branches. Caterpillars are pale with black spots and dark heads, with a number of stiff hairs covering the body. The moth is stark white, sometimes with a couple of black spots. The web protects the caterpillars within from predators and insecticides. The web must be torn open before treatment. Never try to torch webs with fire, as this may start an unintended blaze.

Often fall caterpillars are first noticed by homeowners when the caterpillars are mature and dispersing from the feeding site in search of a place to pupate. They crawl across sidewalks, over roads, and up the sides of houses. Insecticides are not very effective at this point: they are contact sprays (must be sprayed directly on the caterpillar to be effective) and large caterpillars are hard to kill with insecticides. Once caterpillars are dispersing they are no longer a problem to trees and shrubs. Spraying insecticide on a large tree infested with caterpillars can also be quite a challenge. Fortunately, a mature tree is able to handle quite a bit of defoliation, especially at the end of the season when the tree has already produced the energy reserves it needs for the winter. Rarely is a healthy tree done in by a season of caterpillar infestation. Usually caterpillar pests are brought into balance by wasps and other parasitic and predatory insects, birds, and adverse weather. So enjoy all the butterflies and moths and don’t worry too much about the caterpillar pests. After all, even teenagers eventually grow up to be enjoyable adults.