POPULATION OUTBREAKS OF FOREST TENT CATERPILLARS

Approximately 70% of the forest Lepidoptera that achieve periodic outbreak status deposit their eggs in batches and of these over half are social as larvae. It has been suggested that the ability of social species of caterpillars to undergo periodic population explosions is due to enhanced predator defense, thermoregulation, and foraging efficiency.  In North America, the forest tent caterpillar Malacosoma disstria is one of  the best known of these “outbreak” species.  

defoliation
 tree defoliated red maple
Forest stripped of leaves by the
 forest tent caterpillar
 Hardwood defoliated by the
 forest tent caterpillar
 Red maple (Acer rubrum) is not eaten by the
forest tent caterpillar







During outbreaks, the biomass of caterpillars is enormous, greatly exceeding the collective biomass of all the other animals in the forest.  Dr. Jens Roland, of the University of Alberta, Canada, estimated the biomass (total weight) of forest tent caterpillars per square kilometer of forest during the peak of an outbreak in the aspen boreal forest of Canada to be the equivalent of that of 657 caribou.
 caterpillar on tree caterpillars




                                                           
                                FTC gut                       
This population of caterpillars in this forest consumed all available foliage and, in a state of near-starvation, searched frenetically for food.  These caterpillars are all on non-palatable plants and, as was the fate of most of the caterpillars at the epicenter of this population, eventually starved to death or were consumed by disease.
 The state of hunger of a caterpillar can be determined by inspecting the gut.  The caterpillar on the right has a full foregut (light green) and midgut while that on the left has an empty foregut and a partially filled midgut.


Are the population outbreaks of tent caterpillars cyclic?  The answer is no if cyclic is considered to denote a regularly recurring event.  It is often stated that region-wide tent caterpillar outbreaks occur at about ten year intervals but the actual interval between successive outbreaks varies. Both the forest and eastern tent caterpillars are native to North America and there are records of outbreaks going back as far as the 1600’s but reasonable accurate records are not available much before the 1900’s.  The most complete records of forest tent caterpillar outbreaks are from Ontario.  From 1867 to 1987,  province-wide outbreaks occurred at intervals of 9-16 years. Caterpillars remained in outbreak numbers for 1-8 years (average 3 years).  In Manitoba and Saskatchewan, outbreaks occurred at 6-16 year intervals from 1923-1957 but some areas in these provinces experienced only one outbreak in the 34-year period.  Thus, information from the historical record can not be used to predict, with certainty, when the next outbreak will occur in any particular area, nor exactly how long it will last.  The lack of true cyclicity in the population dynamics of tent caterpillars reflects the independent randomness of the elements that come together to promote population explosions.
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