Example
of the larvae of Eastern Forest Tent Caterpillar and the silken mat. Click
on photo to make it larger.The forest tent caterpillar (Malacosoma disstria) is found throughout the broad-leaved forests of the United States and Canada. During 1957-1987, the forest tent caterpillar had the largest outbreak area of any North American folivore (13.5 million acres). Outbreak periods usually range from 6 to 16 years in northern areas. However, southern gum forests in southwest Alabama and southern Louisiana have had continual, annual infestations that encompass an area of approximately 3.5 million acres (1.4 million hectares) since 1948. Because it can have a significant impact on several species of hardwoods species, it is of concern to forest managers all over the U.S. and Canada.
There is only one generation of forest tent caterpillar each year. The larvae hatch just as the leaves begin to open (exact timing varies with weather and locale). The new larvae are black with conspicuous hairs and less than 1/8 inches (3 mm) long. Larval colonies remain together and move in files down silk trails left by the leaders. After each molt, pale blue lines and footprint-shaped, whitish spots become more evident. Once they reach full size, adult forest tent caterpillars are approximately 2 inches (50 mm) long.
Unlike their name implies, forest tent caterpillars do not spin a tent. Instead, they form a silken mat on the trunk or branches of the host tree where they congregate for rest periods and molting. Early on, these communal areas are found in the upper area of the tree, however, as the life cycle progresses they usually move to the lower crown and trunk.
Example
of the larvae of Eastern Forest Tent Caterpillar and the silken mat. Click
on photo to make it larger.
Forest tent caterpillar larvae have five instars. If there is a large population that completely defoliates the host tree, the fourth and fifth instars increase their movement to search for food. The larvae also move around to find suitable sites for spinning cocoons. The larvae have often been termed 'armyworms' because of this motility.
In order to pupate, the larvae spin cocoons in a folded leaf, bark fissure, or other sheltered place 5-6 weeks after hatching. Approximately 10 days later, the moths emerge. They are buff-colored with two darker lines on each forewing. They have a wingspan of 1-1.5 inches which allow them to be carried by strong winds for many miles. They are attracted to lights, and only live for a few days after they emerge from the pupae. During this time, the moths lay masses of 100-350 eggs in the branches of the upper-crown of a host tree. The eggs are covered with a glue-like substance that hardens into a glossy dark brown mass. The embryos develop into larvae within three weeks. The larvae then overwinter in the eggs and emerge in the spring.
Several broad-leaved trees are the favored host of this insects. Some of these tree species include:
Red maple (Acer rubrum), sycamore (Platanus occidentalis), and most conifers are not utilized by the forest tent caterpillar. However, they have been known to feed on shrubs and the leaves of cultivated fruit/vegetable after they have completely defoliated an area of trees.
As stated above, the forest tent caterpillar can defoliate extensive areas. Outbreaks in the Lake States can typically last three years before they subside. During this time, as much as 90 percent of the diameter growth can be reduced. Usually, this defoliation only kills a few trees, except for some that are suppressed in the understory. However, if an outbreak lasts 5-7 years, there can be up to 59 percent mortality in some species in certain areas such as aspen stands within an area with a high water table. Species that are subjected to annual outbreaks, such as the water tupelo in southwest Alabama, only grow an average of only 1/2 inch per year (1.3 mm/yr.) in diameter; this growth rate is 25 percent of normal. Also, successive defoliations can begin to kill individual trees.
Defoliation by forest tent caterpillars can precipitate a decline in tree flowers and/or seed production, and can cause a decrease in quality and quantity of sugar maple sap. They also have been known to 'mine' buds if the larvae hatch before leaf flush, and new foliage after a spring defoliation can be thin and stunted. Complete defoliation can be expected if there is (for example) an average of 19 egg masses on a 6 inch (15 cm) diameter aspen tree. This relationship has also been noted in the southern tupelo-gum forests.
The forest tent caterpillar is a defoliator, and can substantially impact a forest stand. For example, heavy defoliation outbreaks can cause tree and stand mortality and changes in stand composition. Lesser outbreaks can induce loss of diameter growth, reduced leaf size, twig dieback, and weakening of the trees so that they become more susceptible to secondary insects and diseases. The removal of foliar biomass can cause a reduction in photosynthesis, and interference with evapotranspiration and the translocation of nutrients within the tree. This can negatively impact watershed values, and reduce the aesthetic value of the trees in urban and forest recreation areas.
These effects are mediated by the site and weather conditions. Trees on poorer sites can show the effects of defoliation before those on better sites, as will trees that experience adverse weather conditions such as floods or droughts. Overmature stands also seem to deteriote quicker after an outbreak, and suppressed trees are more likely to die as a result of heavy defoliation. So, forest tent caterpillars could be seen as a natural thinning agent for stressed and/or suppressed trees within a stand.
Wildlife can be positively impacted by the defoliation, but negatively impacted by control measures. There is concern about the use of Bacillus thuringiensis (see below). B. thuringiensis is lethal to all types of caterpillars, and environmental organizations fear that spraying with this insecticide will lead to a drastic reduction in the food available to neotropical migrant birds, thereby reducing bird populations in areas sprayed.
Extreme temperatures severely affect the forest tent caterpillar. Freezing weather just prior to, during, or following hatching has been associated with high mortality of larvae in the eggs and newly emerged caterpillars. Temperatures below -42o F (-41o C) during the winter also can kill most of the larvae in the eggs. Likewise, high temperatures (above 100o F, 38o C) when the moths emerge and/or are laying eggs result in the death of adults and low viability of the eggs.
The forest tent caterpillar also has several parasites. Species of flies and wasps parasitize the eggs, larvae, and/or pupae. For example, the female large gray flies (Sarcophaga aldrichi in the north, and S. houghi in the south) deposit maggots on the cocoons. These maggots enter the pupae and kill them along with any other parasites present. S. aldrichi in northern aspen forests can become very numerous and terminate an outbreak within an area. S. houghi is less conspicuous (in the southern gum forests), but can still be an effective control agent. Wasps are also important parasites, and egg wasps have been known to parasitize up to 20 percent of the forest tent caterpillar eggs in Alabama. There are also wasps (e.g. Itoplectis conquisitor) which parasitize the pupae stage.
There are several beetles, ants, true bugs, spiders, birds, and small animals that consume the caterpillars and/or the pupae, but the extent that they affect a population is unknown. There are also viral and fungal infections that can destroy large numbers of caterpillars. Finally, the larvae will starve if they completely defoliate an area.
Small trees can be protected if the egg masses are collected and destroyed, if the larval colonies at the ends of branches are destroyed, or if the larvae clustered during molting or resting are eliminated. Burning the infected areas of the trees may be used if caution is employed so that the tree is not irreparably damaged. Also, there are several chemical insecticides and one microbial (Bacillus thuringiensis) insecticide that can be used to control a forest tent caterpillar outbreak.