When to Spray Beginning in April, check the leaves every two weeks for the yellow egg clusters and young larvae. Spray as soon as you can determine that the trees are infested. You may have to spray several times during the summer because there are several generations of the beetle each season. A community-wide control program to protect trees from the first generation larvae is the most effective. When only a few trees are sprayed in a neighborhood, later generations of beetles moving from untreated areas will frequently reinfest sprayed trees, thus making additional applications necessary.
Tree roots take up the insecticide and the plant moves it into stems and leaves.
Therefore, if the root zone is covered by paving or very compacted soil, the effectiveness will be reduced. Timing of the application also is important. If the insecticide is applied too early in the spring before trees are growing vigorously or late in the summer when tree growth is slow, their effectiveness will be reduced. This material must be thoroughly watered into the soil, and the tree should be kept well watered throughout the season. Merit insecticide is another soil-applied insecticide, and it is available to homeowners and other unlicensed pesticide users.