MANITOULIN—Long-time residents of Manitoulin Island will recall forest tent caterpillars becoming a nuisance about every 10-12 years. This insect is native to Canada and a natural member of forest ecosystems, but when its population explodes enormous numbers of caterpillars cause extensive defoliation of deciduous trees and create an overwhelming nuisance to people encountering them.
We are at a peak level this year resulting in misery for those who spend time near deciduous trees. This misery comes in the form of writhing masses of caterpillars on our homes, trailers, decks, sidewalks and roads along with falling fecal pellets.
The forest tent caterpillar, with the scientific name Malacosoma disstria, is unusual in that it does not make tents. Instead of making tents, forest tent caterpillars spin silken mats on branches of trees where they congregate to moult and rest.
Another common species on Manitoulin is the eastern tent caterpillar, with the scientific name Malacosoma americanum. This species makes tents which are currently visible on shrubs along the side of Manitoulin roads. (See article in The Manitoulin Expositor on June 7, 2017.)
Eastern tent caterpillars feed on shrubs such as chokecherries, wild roses, raspberries and hawthorns, whereas forest tent caterpillars feed on leaves of poplar, birch, oak, ash, and sugar maple, but will eat other trees when their populations are high. They also eat leaves of apples and garden roses. The caterpillars of both species are harmless to people. They do not bite or sting; however, some people have an allergic reaction to handling them.
Both species have a biology characterized by cyclical increases in the size of their populations and at which time their host trees or shrubs are defoliated. Caterpillars of both species are social and travel together to feed. Forest tent caterpillars have markings along their backs which look like exclamation marks whereas the eastern tent caterpillar has a white line along their backs.
Both species have one generation per year. They overwinter in the egg stage and the larvae hatch in the spring synchronized with the opening of leaf buds. Larval development takes from four to seven weeks with feeding starting from the time leaf buds appear in the spring until mid to late June.
Larvae go through 5-7 instars (growth stages separated by moulting) and when full grown are about 7 cm in length. The caterpillars are not yet full grown so we are stuck with the misery they cause for about two more weeks.
As hungry forest tent caterpillars crawl around branches of host trees in search of food, they lay down strands of silk then others move along them like tightrope walkers. Sometimes the larvae lower themselves to the ground using strands of silk they secret from their mouths. Unfortunately, hanging larvae commonly get caught in our clothing and hair.
Once the larvae have eaten all the leaves on a host tree, they crawl down the trunk and walk across the ground in search of new trees. This is when we see them crossing roads and sidewalks and crawling up the sides of buildings.
So many caterpillars have attempted to cross the highway in some parts of the Island, the pavement has been stained brown with their squashed bodies. Drivers need to be cautious in such areas as it is easy to lose control; slow down when you see patches of stained highway.
Full grown caterpillars spin silken cocoons between leaves or in crevices of the bark, or on buildings, where they turn into moths by early July. Moths are beige to brown with a stout body and a wingspan of about three cm.
Moths fly only at night, mate and then deposit egg masses which are the size and shape of metal eraser holders at the end of pencils. Eggs are laid in masses of up to 300, which are stuck to twigs and covered with a substance which prevents them from desiccating and freezing in the winter. The moths then die and the eggs overwinter ready for the young first-instar larvae to hatch early next spring.
Although defoliated forests look bad at this time of the year, trees have evolved the ability to tolerate these cyclic events. Healthy trees have enough nutrients inside their trunks to put out a new complement of leaves a few weeks after defoliation. However, some trees are weakened by repeated defoliation making them more susceptible to stresses such as drought. Two or more years of heavy defoliation can result in severe reduction in radial growth.
When you walk in hardwood forests over the next two weeks you will hear, on clear, sunny days what sounds like raindrops. This is not raindrops, but the fecal pellets of caterpillars feeding above. Studies have shown that high levels of nitrogen in the pellets contribute to more luxuriant foliage the following year.
However, the rain of fecal pellets is a problem if your deck, sidewalk or camp site is below trees with feeding larvae. Last year the deck at our trailer near Sheguiandah was stained with pellets when we were away for a few days and it rained.
What will cause caterpillar numbers to go down?
As with all insects that undergo eruptive cycles, something triggers the populations to start increasing and then after several years, to crash. Scientists do not clearly understand the events that trigger huge populations, nor the reasons for the sudden crashes. However, we do know that warm spring temperatures when the larvae are hatching and feeding are beneficial to the insects as are warm and dry conditions in July when the moths are flying.
Forest tent caterpillars are subject to a variety of mortality factors including low winter temperatures and late spring frosts, diseases, predation by birds and small mammals and parasitic insects.
We know that as populations of caterpillars increase, a counterbalance in the form of natural enemies and diseases begin to appear. About 50 species of parasitic and predacious insects feed on tent caterpillars and as the caterpillar populations increase, so do the populations of insects that feed on them.
In time there will be so many parasites, they cannot all find caterpillars and their populations will in turn decrease. Previous peaks were in 1989 and 2003 so depending on the weather, impact of disease and abundance of parasites, the 2018 population should start to crash this month and there will be fewer caterpillars next year.
What homeowners can do
Homeowners can help reduce the impact of forest tent caterpillars on their trees by physically removing colonies of young larvae at the end of branches or older larvae clustered on tree trunks. This is best done in the evening or on cool days when the caterpillars are resting in large clusters. Adhesive bands such as Tanglefoot on tree trunks can help trap caterpillars crawling up or down trees.
Vulnerability of trees in your yard to caterpillar defoliation can be reduced by making sure trees are well watered. Egg masses should be removed during the winter when absence of leaves makes them easier to spot.
Larvae and fecal pellets can be swept from walls and outdoor furniture with a stiff broom or pressurized water. Fecal pellets will stain wooden decks when they are dissolved by rain so try and remove the pellets as often as possible.
Covering your patio or deck with sheets of plastic, along with outdoor furniture, boats and vehicles under attacked trees will keep fecal pellets from staining the surface. Cocoons are difficult to remove from buildings but a stiff broom is helpful.
Because the forest tent caterpillar is a natural member of forest ecosystems, and long-term damage is usually minimal, forest managers are reluctant to engage in control measures. The application of insecticides over an area the size of Manitoulin is cost prohibitive. Besides, most citizens are against chemical control in forests recognizing that forests are valued reservoirs of biodiversity and contribute multiple ecosystem services.
However, when protection of large trees is considered necessary, a number of insecticides are available. Microbial insecticides, such as Bacillus thuringiensis (Bt) are effective and have minimal effects on nontarget insects compared to synthetic insecticides.
Even though forest tent caterpillars are annoying, little can be done to control them other than let nature take its course. In the meantime, check your clothing when returning from outdoor walks to avoid bringing caterpillars into your home. Cover your picnic tables for the next month when eating outdoors beneath deciduous trees; remember, the pinging you hear is not rain drops!
EDITOR’S NOTE: Dr. Joe Shorthouse is a retired entomologist with Laurentian University in Sudbury and a summer resident of Manitoulin.
