Sunday, June 7, 2015

Forest Tent Caterpillar Moth - Malacosoma disstria

Yesterday, while out walking in the Mont-Royal Cemetery and the Mont-Royal park, I encountered a couple of these:

Melacosoma disstria
Melacosoma disstria (forest tent caterpillar moth) is native to North America [1,2] and does eventually pupate and produce a moth. But the moth form is a comparatively short part of its life cycle which is essentially just for reproduction.

M. disstria is displays quite a number of interesting characteristics. It is one of those species which exhibits a boom-and-bust population cycle, which is where there are periods of great abundance and density, followed by periods of relative scarcity of the species.

This species has booms at intervals of 6-16 years [1] and is very notable and impactful during these booms; because this species subsists mostly on foliage and feeds together in groups, they can defoliate acres and acres of trees. For example, in Southwest Alabama's Southern Gum forests, this species defoliates 3.5 million acres of trees per year [1] (note that this species doesn't experience bust periods in these forests, possibly because annual flooding prevents predators from locating and consuming the population [1]).

One contributing factor to the destructive power of this species is its sociality. M. disstria caterpillars eat, molt, and travel together in groups [1,2]. Those individuals who travel with groups appear to grow more quickly, and therefore reach maturity more quickly than solitary individuals [2].

In most places where this species is found (eg Ontario, Quebec), the population boom attracts a wide array of predators and facilitates the spread of viral agents, resulting in a subsequent bust [1]. Although it can be unsightly, if the defoliation of a tree only happens over the common period of about 3 years, there is relatively little impact on the overall health of the tree/forest [1], so in most cases at least in Ontario and Quebec, these infestations are not a source of too much concern. Note, however, that trees do exhibit considerably reduced growth (reduced up to 90%) in the years that they are defoliated by M. disstria [1].

The common name of this species (forest tent caterpillar moth) refers to the quantities of silk that the caterpillars lay down on the branches of trees, but is somewhat inaccurate; M. disstria isn't specifically constructing tents with silk, but rather laying down silk threads to facilitate movement over the branches [1].

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