Be on the watch for eastern tent caterpillars. There was a report of these insects at the end of last week in southeast Minnesota and it was reported in the Twin Cities at the beginning of this week. The caterpillars are bluish black with yellow and a white stripe running the length of the top of its body. They are also mostly smooth except for a series of hairs sticking out along the sides of their bodies. They are two inches long when fully grown.
However, the first sign you'll notice are the silken tents they create in the forks of branches. After the caterpillars first hatch, they'll construct this webbing which serves as a shelter they use at night and during rainy weather. The tent will be small at first but will increase in size and can eventually become quite conspicuous. During the day they crawl out of these tents and feed on tree leaves. Although they are found on a variety of hardwood trees, eastern tent caterpillars are particularly fond of fruit trees, including apple, chokecherry, crabapple, plum, and cherry.
Healthy, well-established trees can tolerate eastern tent caterpillar feeding. Their feeding, as well as the presence of their webs, is just a cosmetic problem and only affects the trees' appearance. However, young trees, as well as unhealthy, stressed trees, are more susceptible to feeding damage and should be protected.
Also consider the size of the caterpillars. As long as they are no more than half their full grown size, i.e. one inch or less, it is worth considering whether to treat them. This is not an issue now as they are just emerging and they are all small. However, if an infestation is discovered later, it is important to check to make sure they are not too large (larger than an inch). The larger they are, the closer they are to being done feeding and then it is not worthwhile to treat them.
if it is permitted where you live).
There are a variety of residual insecticides that you can use if it is desirable to protect your trees. Consider using products that have a low impact on the environment, such as Bacillus thuringiensis, spinosad, and insecticidal soap. Bacillus thuringiensis is a particularly good product if the tree is flowering since it will not harm visiting honey bees. If you use insecticidal soap, the product needs to directly contact the insects. There is no residual activity so you may need to repeat the treatment.