Why do outbreaks occur and what brings them to a close?   All organisms, even those that produce the fewest young and have the longest life cycles have a capacity to reproduce that greatly exceeds the capacity of the environment to absorb them.  The forest tent caterpillar has a life cycle of one year and is far from a slow breeder.  FTC’s produce egg masses that vary in the number of eggs they contain but a reasonable average number is 175.  Assuming unopposed exponential growth, a single pair of FTC moths producing this number of eggs could generate a population of caterpillars that numbered close to 100 million in just four years. The capacity for exponential growth is the reason why population explosions of tent caterpillars seem to arise out of nowhere.

No species is capable of uninterrupted exponential growth and although populations of the FTC may expand for several years they inevitable collapse. There are two types of mortality factors that bring outbreaks to an end, those that are density dependent and those that are density independent.

Density dependent mortality factors
.  Density dependent mortality factors that limit population expansion of forest tent caterpillars are the following:

Furia gastropachae a fungal pathogen
(The crustose coating of conidiophores on the caterpillar pictured above is often absent)
Killed by the fungal  pathogen
Nucleopolyhedrosis virus

The caterpillars to the right are the indirect victims of starvation.  The larvae defoliated the entire forest then wandered about for days in a fruitless search for food until they were overtaken by  a pathogen and killed.  The most likely pathogen is Furia whose spores accumulate on the ground and are picked up by the wandering caterpllars.

This starling is feeding on the pupa of a forest tent caterpillar. Starlings will also eat the caterpillars but first brush them repeatedly against the ground to knock off the hairs.   Although many birds will eat the pupae, few species eat the hairly larvae to any extent.  The exception to this is the cuckoo which is a hairy-caterpillar specialist and is often found in areas where there are tent caterpillar outbreaks.  Overall, however, birds, including the cuckoo, can have little importance as mortality agents during outbreaks when populations of the caterpillars number in the millions.

maggot in pupa
parasitoid flies

Sarcophaga aldrichi adult fly
Maggot of  S. aldrichi  inside eviscerated pupa
The parasitoid is often very abundant during extended outbreaks
Sacrophaga aldrichi is a pupal parasitoid that has often been credited with making a significant impact on forest tent caterpillar populations.   The flies emerge in May then mill about with the caterpillars until the latter construct their cocoons in late June.  The fly then lays an egg on the cocoon and the fly maggot burrows into the pupa and feeds on it. The maggot typically remains in the cocoon of the caterpillar for 2-3 weeks, then burrows into the ground where it stays until the following May. 

 Several years into an infestation the flies become extremely numerous and can become as bothersome to people as the caterpillars.
As noted above, Jens Roland estimated the biomass of tent caterpillars during outbreaks in caribou equivalents.  Likewise, he notes that the various species of parasitoid flies, of which S. aldrichi is the most abundant,  may achieve a biomass per square kilometer of forest during the peak of a tent caterpillar infestation that is equivalent to that of 82 wolves .

 Right: S. aldrichi on a leaf wrapped around a cocoon of the forest tent caterpillar.

The impact of density dependent mortality factors varies with the size of the caterpillar population. When there are few caterpillars, the effect of density dependent agents is minimal, enabling the population of caterpillars to expand. But, as the population of caterpillars grows they become increasingly more effective.  Thus, when there are few caterpillars, there is plenty of food, but as the population of caterpillars grows, there is less food and the caterpillars must compete for a limited resource with the potential consequence of mass starvation.  Likewise, biological mortality agents (disease, predators, and parasitoids) become much more numerous and effective in containing the caterpillars as the number of caterpillars increases because biological mortality agents rely on the caterpillars for their sustenance and the more caterpillars there are the more energy that flows to them and the more they reproduce. 

The stinkbug (Pentatomidae) is a generalist predator of  caterpillars, killing its victim by piercing and sucking.

Density independent mortality
factors that limit population expansion of forest tent caterpillars are elements of weather, most importantly, temperature.  Although the tiny caterpillars that are the basis for the next infestation spend the entire winter inside the shells of their eggs, they are rarely killed outright by extremes of temperature.  Studies of the FTC in Canada show that the small caterpillars that lie sequestered in the egg mass over the winter are cold hardy to about -40 F, an extreme of temperature rarely seen. 

Somewhat counter intuitively, mild winters have the potential to do more harm to the caterpillars than cold temperatures. Indeed, the milder winter temperatures likely to become more prevalent with global warming could lead to the region-wide elimination of tent caterpillars, at least temporarily.  This is the case because warmer winters can cause the caterpillars to emerge too early in the spring, well before the buds of their host trees have begun to expand sufficiently to allow the caterpillars to gain sufficient nutrition from them.  In the spring of 2006 in central New York State, caterpillars eclosed from their egg masses in mid-April and it wasn’t until the beginning of May before the leaves of sugar maple began to appear.  Had the caterpillars emerged much earlier they may have perished before there was sufficient food for the caterpillars to survive.  A similiar phenomenon has been documented recently for the winter moth caterpillar in Europe.

maple buds

Condition of the buds of maple at the time forest tent caterpillars hatched.  The caterpillars mined the buds for food until the first leaves appeared.

It is also the case that cool and cloudy spring weather kills caterpillars.  Studies of the closely related eastern tent caterpillar have shown the when a tent caterpillar’s body temperature drops below about 59o F, the insect can not digest its food and will starve to death if it is maintained for long at that or a lower temperature.  Although air temperature in the spring is often less than this threshold temperature, caterpillars bask in the sun to raise their core temperatures.  However, basking is not possible if there is dense cloud cover, and a prolonged episode of cool, cloudy weather can lead to the death of the caterpillars due to starvation.  


cloudy sky

Such an event was documented for the eastern tent caterpillar during the spring of 1917 in Syracuse, New York.  The caterpillars hatched from their eggs by April 22  following a period of mild and warm temperatures.  For the next 26 days the air temperature rarely exceeded 55o F and the skies were almost constantly overcast.  When mild weather returned some 38 days after egg hatch, nearly all the caterpillars had perished due to apparent starvation, the result of the cold temperatures forestalling the flushing of host leaves.


Density dependent and density independent mortality factors may interact to bring a population explosion to a close.  Thus, an epizootic may sweep through a population of caterpillars, precipitated by the cool and wet weather that favors the pathogen Furia.     

virus caterpillars

The association of tent caterpillars and trees is an ancient and natural one and trees have evolved the ability to respond adaptively by growing new leaves shortly after they are defoliated by tent caterpillars.  There are typically no long term consequences to occasional bouts of defoliation. However, trees defoliated in the spring then stressed by drought in the summer may experience some dieback of branches and in some cases these conditions have resulted in the death of trees.

defoliated tree
maple leaves
Refoliation of sugar maple: Left , June 9, 2006, Right, July 10, 2006

tent caterpillars


Filotas, M.J.F., A.E.Hajek, and R.A. Humber. 2003. Prevalence and biology of Furia gastropachae (Zygomycetes: Entomophthorales) in populations of the forest tent caterpillar (Lepidoptera: Lymantriidae). Can. Entomol. 135: 359-378.

Filotas, M. J., and A. E. Hajek. 2004. Influence of temperature and moisture on infection of forest tent caterpillars (Lepidoptera: Lasiocampidae) by the entomopathogenic fungus Furia gastropachae (Zygomycetes: Entomophthorales). Environ. Entomol. 33: 1127-1136.

Fitzgerald, T. D. and Costa, J. T. 1986. Trail-based communication and foraging behavior of young colonies of the forest tent caterpillar Malacosoma disstria Hubn. (Lepidoptera: Lasiocampidae). Ann. Entomol. Soc. Am. 79: 999-1007.

Fitzgerald, T. D. and F. X. Webster. 1993. Identification and behavioral assays of the trail pheromone of the forest tent caterpillar Malacosoma disstria Hubner (Lepidoptera: Lasiocampidae). Can. J. Zool. 71:1511-1515.

Fitzgerald, T. D.  1995. The Tent Caterpillars. Cornell University Press. 303p.

For an extensive database built around a recent forest tent caterpillar outbreak in central New York State, click here.

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