Question: We simply can't believe it! We have caterpillars in our fir tree! We thought for sure the cold weather would have killed them, but they appear to have thrived on it! What are they? Should we spray them with something?
Answer: Based on your description, the caterpillars you are referring to are most likely silver spotted tiger moths. These insects are common throughout western Washington. The main host for silver spotted tiger moth is douglas fir, but it is known to also feed on spruce, pine and other conifers.
The chewing damage caused by silver spotted tiger moth caterpillars is limited and usually not very serious. The damage is not unique; it is consistent with that of other caterpillars. Aside from the chewing damage, the presence of these caterpillars is most detectable by their gregarious nature and by dense webbing on branches. These "tents" are somewhat similar to those of tent caterpillars - a pest that does not feed on conifers. These tents and the caterpillars within appear on trees from now until spring.
The caterpillars continue to feed throughout the cold winter months eventually reaching about 1 1/2 inches long. They are covered with a combination of dense black, reddish brown and yellowish hairs that may cause a rash or dermatitis upon contact. In most cases simply pruning out the infested branches and destroying the caterpillars provides an effective control.
Question: Our neighbor told us that the very cold temperatures we experienced earlier this month killed off a lot of insects resulting in fewer garden pests next spring. Is this true?
Answer: Probably not and for some very good reasons. First of all, we know that some bugs actually make their own anti-freeze. Sub-freezing temperatures do slow down insect activity, but when temperatures rise, many of the insects will become active again. This interesting phenomena can best be explained as follows: Some insects manufacture glycerolan effective anti-freeze similar to the product made by man for use in car radiators and the like. When the mercury drops below the freezing point, these insects, by some process not fully understood, gradually adjust to the low temperatures. Apparently the glycerol they manufacture gives them the hardiness to withstand all but the most extreme cold weather. Among the insects that can make their own anti-freeze are certain species of ants, beetles, aphids, cankerworms and flies.
Nature gives insects another protection from the cold too. Most go through a metamorphosis which includes a dormant period when they are completely inactive. If insects stay dormant in the pupal or larval stage during the winter, they often survive the frigid weather. Some insects have been found frozen in solid ice and thawed to full activity! When the insect pests that have been bothersome all summer take a powder in the fall, many of them are not gone for good. They have just signed a truce until spring.
On a final note, if we do experience frigid temperatures for an extended period, it's usually the beneficial predator bugs that fall victim to the elements. In many cases they are not as winter hardy as the destructive ones. Due to this fact, we may well experience a greater destructive insect population following an unusually cold winter than would be normal.
EDITOR'S NOTE: For answers to local gardening questions, contact Master Gardener Rachel Gana at 642-8723.