We have planted many hundreds of trees and shrubs on our property to encourage wildlife of all sorts, so I keep a careful eye out for pests that eat these plants. Some of the worse problems are due to insects that come in groups, such as caterpillars of various moths. I am used to dealing with army worms and web worms but came across a new enemy recently. Groups of hungry caterpillars appeared on our treasured black walnut trees (and sumac also) and started a full-scale attack (see background photo). In their young stage they were white with black spots and a black head, and were quite hairy. Now a fuzzy caterpillar often means “KEEP AWAY” since the hairs can be irritating and even toxic. These turned out to be hickory tussock moths or hickory tiger moths which are well known to possess stinging hairs and even to be toxic for birds to eat. They group together, likely with their siblings that hatched from their mother’s eggs, and remain together for some time until they get much larger when they live alone (see photo inset left of a much older caterpillar from our yard- not a typical coloration- often they are more black and white).
It is interesting to think about why not only insects but many types of animals group together in flocks or swarms. It is usually considered to be a defense, potentially to confuse and divert the attack of predators. In the case of these caterpillars, it is also likely that distasteful and/or toxic creastures ban together with siblings to gain an additional advantage if a predator does attack and learns to avoid this color pattern in the future. Siblings that share many of the same genes will benefit more by being together with their brothers and sisters than with strangers, if they are attacked and some are sacrificed to teach the predator to avoid the others.
Caterpillars have a variety of defenses other than stinging hairs and toxic flesh. They tend to be active at night and group together in daytime under protective webs. When I touched the leaf on which the young hickory tussock moths were sitting, a surprising thing happened- numbers of caterpillars began to drop down to the ground quickly on web lines (see second photo inset). You might wonder why this would be necessary when these caterpillars seem to be so well protected. However virtually all good defenses have spawned a specialized predator that can circumvent the defensive strategy- in this case it is the yellow-billed cuckoo (see photo inset right) which is a fuzzy caterpillar eater. Not coincidently we have noticed far more cuckoos on our property this year than last when caterpillars were not so numerous. So the hickory tussock moth caterpillars drop from the tree at the slighest indication of a cuckoo being present, and can then crawl back into the tree after the cuckoo leaves.
The intricate natural history of something so seemingly inconsequential as a tiger moth caterpillar tells us once again that the adaptations of this larval moth are subject to very specific design criteria to maximize its chances of survival and subsequent reproduction. So even if we destroy some of these creatures that eat our yard plants, we cannot fail to marvel at their remarkable methods of existence.