Outbreak cycle

After a period of years of very low populations, favorable forest conditions and weather trigger a rapid increase in tent caterpillar survival resulting in an outbreak. Once an outbreak begins, it usually continues for 2-5 years before the population collapses. In general, forest tent caterpillar outbreak cycles begin every 6 to 16 years in northern Wisconsin.

Once tent caterpillar populations rise high enough, they often consume all available tree foliage before they are fully-grown. Starvation is a main factor to end an outbreak. Furthermore, hungry caterpillars that migrate in search of food become more visible to their natural enemies.

After two to four years of forest tent caterpillar outbreaks, populations of natural enemies increase in response to rising tent caterpillar numbers. The increase in the population of natural enemies helps put the populations of the forest tent caterpillar in balance. Beetles, true bugs, birds, and small animals feed on caterpillars and pupae. It is reported that crows consumed approximately 20% of forest tent caterpillar pupae during outbreaks in Minnesota.

High populations become susceptible to parasitism and disease. For example, a parasitic fly (Sarcophaga aldrichi), called “friendly fly” (PDF, 99KB) because it often lands on humans without biting, is a great contributor to lowering the populations of forest tent caterpillars. The adult fly parasitizes the pupal stage by laying an egg in it. A maggot hatches from the egg, consumes the inside of the pupa. Parasitized pupae will die prior to becoming a moth. During the final years of an outbreak, pupal mortality by the friendly fly can reach more than 95%. Adult flies become very numerous during the later stage of the forest tent caterpillar outbreaks. It is similar in appearance but slightly larger than a house fly and does not bite. The friendly fly is one of the main contributors to the collapse of outbreaks. Larvae of forest tent caterpillars are also susceptible to infections by viruses, bacteria, and fungi.

Adverse weather, such as spring frost or prolonged cool and wet springs can kill newly hatched caterpillars. Starvation plus increased predation and disease cause the eventual collapse of tent caterpillar populations. Whatever the causes are, nature eventually brings the population in balance.