Forest tent caterpillar
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 a. Clutch on twig; b. Adult; c., d. eggs
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| Malacosoma disstrium Hübner, 1820 |
The forest tent caterpillar (Malacosoma disstria) is the larva of a North American moth, found throughout the United States and Canada, and most common in the eastern regions.
These tent caterpillars do not make tents, rather they weave a silky sheet where they lie together during molting. They lay down strands of silk as they move over branches and travel along them like tightrope walkers. However, it has been shown that a trail pheromone secreted from the ventral surface of the posterior tip of the abdomen rather than the silk guides and stimulates trail following. The caterpillar are social and travel and feed en masse. The caterpillars live in deciduous trees, which they strip of leaves after emerging from their eggs. The moths favor oak, sweetgum and tupelo, aspen trees, and sugar maple for oviposition but the larvae can be found feeding on many other species of woody trees or shrubs when they disperse from ovipositional trees during outbreaks. The females lay eggs in masses of up to 300, which are stuck to twigs and covered with a gluey cement called spumaline which prevents them from desiccating or freezing over the winter. The eggs hatch the following spring.
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[edit] Outbreaks
The caterpillars are considered problematic when their populations explode in the springtime. They can completely defoliate a tree. The trees re-foliate quite quickly (within two weeks to a month) and produce enough new leaves to carry on photosynthesis. Under most circumstances, little lasting damage is caused to the trees; however the disappearance of foliage is an eyesore and can be an agricultural nuisance. On those rare occasions when infestations last for three years or more, significant levels of tree mortality will begin to emerge during the years following outbreak collapse. Large-scale tree mortality has been reported in only one instance, in northern Ontario, Canada, after two outbreak cycles in the early and late 1990s occurred back-to-back, resulting in more than six consecutive years of aspen defoliation in some areas.
One outbreak in upstate New York and Vermont began in 2002, with 650,000 acres (2600 km²) defoliated in New York and 230,000 acres (930 km²) in Vermont in 2005.
[edit] Appearance
Forest tent caterpillars are just over 2 inches (5 cm) in length, black or dark brown or gray with blue and faint yellow longitudinal stripes. Each abdominal segment bears a white spot. The caterpillars have long setae, giving them a furry look. The adult moth that emerges after pupation is yellow or tan with a thick, short, furry body. The wingspan is about 1.5 inches (3 cm). It is rather strictly nocturnal, starting to fly soon after nightfall and by and by returning to rest in the latter half of the night (Fullard & Napoleone 2001).
It is not known with certainty how far egg-laden female moths tend to fly. There is one credible report of moths flying hundreds of kilometres with the assistance of an unusually strong wind.
[edit] Toxicity
A closely related species, the eastern tent caterpillar (Malacosoma americanum), has been linked to a phenomenon called 'Mare Reproductive Loss Syndrome' (MRLS). Experimental studies have shown that when pregnant mares are fed eastern tent caterpillars they abort. The caterpillars of this species often feed on the highly cyanogenic black cherry tree (Prunus serotina) and it was originally hypothesized that the mares were aborting in response to the cyanide they consumed along with the caterpillars. However, that hypothesis was disproven. In another study, the necropsy of a mare fed eastern tent caterpillars showed that fragments of the caterpillar's setae had embedded in the gut wall leading investigators to hypothesize that these invasive fragments may allow infective agents to pass into the animal's blood stream then travel to the placenta, initiating an abortive event. It remains to be determined whether this is indeed the cause of MRLS.
[edit] Outbreak cycles
Forest tent caterpillar outbreaks tend to recur at reasonably regular intervals every decade or so, with the precise interval varying somewhat in time and space. Outbreaks usually last two to four years. Although the insect's distributional range is quite large, the area over which decadal outbreak cycles are synchronized (i.e. oscillating with the same phase) varies substantially. Outbreak cycles are more strongly synchronized in eastern Canada than in western Canada. Where spatially separated populations are phase-synchronized, the synchronization is thought to be due to the process of entrainment, that is the synchronization of a circadian clock with the external environment.
[edit] Mortality agents
The cause of the outbreak cycle is not known with certainty. There are a large number of natural mortality agents which could be responsible for population cycling - including, but not limited to: parasitoids, predators, starvation, disease, and severe spring, summer, or winter weather. Most infestations subside after one or two years, as a result of a combination of these factors. The most common parasitoids associated with population decline are Diptera (flies) of the family Tachinidae and Sarcophagidae.
[edit] Control measures
Larvacides applied in early spring can be effective, but once the caterpillars emerge little can be done. They can be removed from trees by hand and killed by dropping them into a bucket of soapy water. (The soap reduces the water's surface tension. Caterpillars placed in non-soapy water can literally crawl across the surface to escape.) Dusk is a good time to attack caterpillars as they are gregarious and form dense groups on tree trunks and structures at the end of the day.
[edit] References
- Fitzgerald, Terrence D. (1995): The tent caterpillars. Cornell University Press.
- Fullard, James H. & Napoleone, Nadia (2001): Diel flight periodicity and the evolution of auditory defences in the Macrolepidoptera. Animal Behaviour 62(2): 349–368. doi:10.1006/anbe.2001.1753 PDF fulltext
[edit] External links
Terrence D. Fitzgerald: Forest Tent Caterpillar
