50March 16, 2017

I don’t think that anyone is too happy with the recent snowstorm. I know I was hoping for an early spring.

One of the issues that is on the mind of many of our customers is how to control the winter moth caterpillar. People also have expressed concern about the resurgence of the Gypsy Moth caterpillar. The question is, can people do something now before the caterpillars arrive. Let me give you a suggestion on what you can do.

For the past few years, there has been a debate over the use of dormant oil to control the egg cases of both the winter moth and the gypsy moth. The female moths lay egg cases on the trunks of the trees that are the favorite food of the caterpillars. The theory is if you spray the trunk of the trees with dormant oil it will kill the eggs in the egg cases. I have had customers try this approach and some have reported back that the dormant oil works, while others say not so much. Let me give you my take on this.

Dormant oil is horticultural oil that is mixed with water and sprayed onto the trunk of the trees. To have a chance at being effective, you need to thoroughly get the oil into all the nooks of the bark. The dormant oil needs to be applied when the temperatures are above 40 degrees. This allows the oil to flow into all the places where the female moths lay their eggs. Once the oil comes into contact with the egg cases, it is absorbed and it coats the eggs and kills the eggs. The spray needs to be done before the caterpillars hatch out. The eggs hatches at about the same time that the young leaves emerge on the trees. Oaks and maples are two of the favorite trees that will have gypsy moth egg cases. The winter moth prefers to lay its eggs on any of the flowering ornamental trees. It also likes to lay its eggs on blueberry bushes and some of the fruit bearing trees.

My feeling on this is that the oil spray alone will not protect your plants 100% of the time. Caterpillars can blow in from other trees.  However, my feeling is that the more eggs you can kill, the fewer caterpillars there will be to eat the leaves and flower buds of your plants. The horticultural oil is relatively inexpensive and a little goes a long way.

Once the weather warms up, my feeling is that you should do a dormant oil spray on any of the plants that are susceptible to caterpillar damage. You may still need to spray the trees once the leaves come out, but you may find that there are fewer caterpillars to kill with a later spraying of your plants with an insecticide that kills the caterpillars.

Well, that’s all for this week. I’ll talk to you again next week