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Scouting Report, 8/5/98

August 5, 1998

Twig pruner is active. Be watchful for branches up to three feet long dropping from oak, elm, and other trees. If twig pruner is present, the wood will be smoothly severed and the bark will be torn and ragged. If twig girdler is present, the bark and sapwood will be cut off, but the heartwood or pith will have a ragged, broken end. Because attacked trees usually lose few branches relative to the number on the tree and because attacked trees tend to be very large and expensive to treat, insecticide spraying is usually not recommended. Once the branches start to drop, it is usually too late to treat anyway. Gathering up and destroying the fallen branches by burning, chipping, or burying may reduce the problem next year.

Annual white grub larvae are present throughout the state. They were found in northeastern Illinois on July 27. This is at least a week earlier than usual and reflects the adult beetles emerging one to two weeks early this year throughout the state. Scouting should begin now, particularly in irrigated turf in areas where nonwatered turf was dry and brownish in late June and early July. Usually at least ten to twelve grubs per square foot are needed to cause turf damage later. However, three to five grubs per square foot may be enough to attract raccoons, skunks, and birds if their damage has been a problem in the past. If numbers warranting treatment are found, use shorter-acting insecticides such as trichlorfon (Dylox), bendiocarb (Turcam, Intercept), or diazinon.

Fall webworm is numerous in central Illinois and is present throughout the state. In northern Illinois, all of the webs should be small enough to control by pruning out the web. Remember that this caterpillar stays within the web to feed, so pruning off the webs at any time of day will eliminate almost all of the caterpillars associated with them. In the central and southern areas of the state, insecticide spraying may be a better option than pruning on big webs where major branches are webbed on smaller trees. Bacillus thuringiensis kurstaki (Dipel, Thuricide, and others) and various chemical insecticides will be effective, particularly if spray pressure is high enough to get inside the web. Fall webworm spreads out its egg hatch over several weeks, so young colonies with small webs will still be produced for at least the next month throughout the state. Keep the pruners handy.

Sod webworm is likely to be found in areas of northern Illinois and in other areas where rainfall has been spotty. This insect does best in drier areas such as south- or west-facing slopes. Irregular brownish areas of turf are the result of thatch showing through after the leaf blades have been eaten off. Greenish caterpillar feces may also be present in the thatch.

Ash flower gall is numerous in areas of Chicago. Many of the galls appear as fuzzy green balls one-half to one inch in diameter. Some of the galls have already turned brown. There's really nothing that can be done at this time, but you might warn clients that the brown galls will persist into the winter and become very noticeable after leaf drop in the fall. These galls are caused by mites that attack the male flowers. They do not affect tree health.

Author: Phil Nixon staff at The Morton Arboretum


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