Forest Insect and Disease Newsletter

Forest tent caterpillars and our late spring

Forest tent caterpillar egg mass

Forest tent caterpillar egg mass with newly emerged caterpillars. Photo by M. Albers, DNR

Quite a few people have been calling, wondering and hoping that the cold, snowy, late spring will be detrimental to the forest tent caterpillar. The short answer is no. Forest tent caterpillars are hardy and well adapted to Minnesota's variable weather. They are able to survive most weather conditions thrown at them just like the rest of us. One year the caterpillars emerged and then we got a heavy snow that hung around for 2 or 3 days. The forest tent caterpillars survived just fine.

The adult moths laid eggs in the crowns of trees last July. The embryo inside developed into tiny caterpillars last summer. The caterpillars have been patiently waiting inside the eggs in a dormant condition until temperatures warm up enough to tell them spring had arrived and the tree leaves are starting to emerge. If you collect some egg masses now and bring them into a warm house, you can see the caterpillars hatch out of the eggs in a day or two. This is always good for hours of entertainment. Towards the Twin Cities, they have no doubt hatched by now and begun feeding on the new leaves.

Once the caterpillars hatch they can be killed if we get a cold spell where the temperatures are too cold for them to move around and feed (since they are cold blooded) and it last long enough for them to starve. Also if they hatch and the temperatures get cold enough to freeze the new leaves on the trees, they will starve because it will take a while for a second set of leaves to emerge. I don't know anyone wishing for either of those to happen anymore this spring.