University of Illinois Extension


Barbara Larson
Unit Educator, Horticulture
Winnebago & Boone counties

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May Insect Problems

Periodically I pull out my crystal ball and predict insect and disease problems. However, I don’t need the crystal ball in May because every year a few insects routinely show up and may cause damage to plants. Remember most of the insects in your yard are beneficial but a few may cause problems.

Pine sawfly larvae are usually busy feeding in early May. Their favorite foods are mugo and scotch pines, but they will feed on other pines too. Although pine sawfly larvae look a lot like caterpillars, the adult is a non-stinging wasp-like insect. In early May the larvae are about 1/2 to 3/4 inch long with black heads. Fully-grown larvae are 1/2 to 1 inch long with several light and dark green stripes on their body. They feed for several weeks on older pine needles, finishing about the time the new needles elongate.

When disturbed, the larvae throw their heads back in unison, a behavior that may scare away birds and other predators. Since the larvae do not eat the new needles, their feeding doesn’t kill trees and shrubs, but plants look sparse for a year or two until the newer needles cover over the bare areas. Pine sawfly larvae are easy to control, because they feed in large groups only on a few branches. Hand remove by shaking the infested branch over a pail of soapy water. Most of the larva will fall in the pail and drown. Hand picking also works. If you choose to use an insecticide, spray only the infested branches. Spray the larvae with carbaryl (Sevin) as soon as they are seen. Products containing Bacillus thuringiensis (Dipel, Thuricide) will not control pine sawfly.

Birch leafminers are also a type of sawfly. Shortly after the birch leaves unfold, the female will lay her eggs in the leaves. After about a week the eggs hatch and young larvae begin to make mines in the leaves. Irregular brown blotches on the leaves indicate leafminer activity. If you hold a mined leaf up in the light you may see a tiny "worm" or dark pepper-looking excrement in the mined area. The larva feeds or mines for two or three weeks between the upper and lower leaf surfaces, then chews a hole in the leaf and drops to the ground to pupate. A new adult emerges several weeks later. In northern Illinois as many as four generations per year may occur.

The best way to reduce birch leaf miner damage is to watch for the first mines in May and then treat. Spray the foliage with acephate (Acephate, Orthene) or dimethoate (Cygon, Ferti-lome Ornamental & Evergreen Spray) when mines first appear and repeat in three weeks. Read and follow label directions.

Ash plant bug and Honeylocust plant bug populations fluctuate each year but we always have at least a few samples come into the Extension office in late May and early June. Both types are slender, flattened, quick moving insects that feed on the sap of leaves and stems. Adult ash plant bugs are brownish and 3/16 inch long. Honeylocust plant bugs are green and 1/8 inch long. Immature insects are similar in color, but smaller.

Ash plant bugs cause tiny light spots on leaves and small dark black spots on the underside of the leaf. Honeylocust plant bugs feed on expanding leaflets, causing distortion and twisting. These leaflets usually stay on the tree, marring the appearance for the growing season, but not harming the tree. Heavy infestations will cause leaves to drop from the tree during June but damaged trees will refoliate.

Treatment for plant bugs is rarely needed. Usually when damage is noticed the plant bugs are too large to effectively treat. The leaves and branches of small trees may be sprayed with acephate, carbaryl, or malathion when the insects are young. The possible environmental impact does not justify spraying of large trees for these insects.

April - May 2000: Gardening with Hebs - Part 1 | Discouraging Canada Geese | Needle Evergreen Diseases | May Insect Problems

Past Issues

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