The army worms are coming! The army worms are coming!

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Malacosoma disstria

Don’t let the winter get you down…

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Susan Bourne

The airborne unit of our native insect is on the alert to head into action in 2014.

The last mass deployment of these phenomenal eating machines was seen in Ontario between 2000 and 2002. These paratrooping caterpillars wrought devastation on deciduous trees then, and scientists are pretty sure we are on the cusp of another invasion with the first wave of leafy annihilation commencing this May.

Known in this neck of the woods as ‘army worms’, the Forest Tent Caterpillar (FTC), Malacosoma disstria, is a native pest to the majority of deciduous trees in the region. Aspen, Birch, Basswood, Ash and Mountain Maple are on the menu of the voracious vegetarians. (Red maple, Acer rubra, which can be found throughout Quetico, has some natural protection: its release of a phytochemical, Ethyl m-digallate, inhibits caterpillars from feeding on the foliage of these trees. This is an adaptation also seen in Sugar maples, Acer saccharinium, but to a lesser extent.)

In mid-May, the first battalion of tiny troops will set out and gorge themselves on the verdant shoots of the  trees their mothers left them on, in egg-form, the summer before. FTCs are considered one of the most social of caterpillar species, travelling and eating in colonies. This social grouping allows for the species to thrive as a whole and accounts for the complete defoliation of broad-leaved tree stands.

From the instant they hatch, the caterpillars are spinning silk strings and eating non-stop. By the fifth and final instar (shedding of its skin), they are consuming up to seven leaves per day! They will consume every last leaf on a tree and then, when faced with certain starvation, they drop to the ground and frantically travel by land until they find another host.

Army worm is definitely an apt name as they scour the ground en masse in search of a new host. This is when the species can become dangerous to humans and occurs usually around 4-5 weeks into the infestation.  As the starving caterpillars risk all crossing roadways, there are many who don’t make it to the other side… The carcasses of the few who have fallen for the many create a very slippery path, so be particularly cautious from the beginning of June until July when driving.

Sometime five to eight weeks after hatching (usually the end of June to mid-July), the caterpillars will form individual cocoons, from which they emerge as moths two weeks later. They then mate, lay their eggs in bands around twigs, and die. The resting eggs wait until the following spring.

Though the forest may seem to take a hit from the infestation, the species of the area have adapted. Plant species that are usually shaded take advantage of the pockets of light the naked deciduous trees provide. Young Red and White Pine, which need shade when they first germinate, are able to sprout up once they have more light. Understorey species such as Ostrich Fern and Ribes species will most likely seem more lush this year. Additionally, the feces of the caterpillars are an excellent source of nitrogen and in the Boreal, that can be in short supply.

The host trees usually will leaf again after defoliation, but the leaves will be much smaller. The loss of leaves limits growth and increases stress in the host trees. Around 10% of the Trembling Aspen die after an infestation. This creates gaps in the canopy and allows other tree and plant species to flourish and diversifies the age and composition of the forest.

The peak of infestation can last three years. Eventually, the caterpillars succumb to starvation, weather (cold springs), disease and predation. Bay-breasted and Black-throated green Warblers, Blue Jays, White and Red-breasted Nuthatches, Red-winged Blackbirds, Black-capped Chickadees and Hermit Thrush all enjoy supplementing their diet with juicy caterpillars and will be seen and heard in greater numbers.

To most of us who have experienced an infestation, we also know the presence of Forest Tent Caterpillars will inevitably result in an explosion of the dreaded ‘friendly fly’. Friendly flies, Sarcophaga aldrichi, are native to Ontario and respond to increases in the population of FTCs. The flies lay their eggs in the cocoons of the caterpillars and their maggots feed on the developing moth. Despite rumours that began in the 1930s, the friendly fly was not introduced by the government! Mother Nature takes all of the credit for this highly adapted critter. And, no, the MNR does not release hoards of these flies during an infestation.

Whether or not we actually have an explosion of Forest Tent Caterpillars this season or next, remember there is more than just the silky webbing of the caterpillars linking themselves from tree to tree. The entire ecosystem has adapted to these outbreaks. One species’ demise is another’s boon!

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