A Simple Truth About
FOREST TENT CATERPILLARS
or How To Prepare A Feast In Five Easy Steps

I've about had it fighting off the forest tent caterpillars. It seems they're everywhere. I think it's about time someone published some information about how to identify and kill these bains of man. I thought we weren't going to get any in our area this year until I spotted one on my neighbour's tree last weekend. When I looked a little closer around my yard, I found a bunch of webs full of caterpillars in my apple tree! After spraying down the apple tree with malathion (after which I was laid up for about 4 days with terrible sweats and shakes), I thought I had no worries. Boy, was I wrong.

The caterpillars came back with a vengeance. On my clothes. In my hair. Up my chimney. I decided to use stronger measures this time. First, I caught one of the critters climbing my garage wall and threw it into a small box I'd constructed. The box had screen all around it so I could see the caterpillar. But more importantly, the screen allowed other caterpillars to see what might happen to them. When I discovered large forest tent caterpillars invading my crab apple tree, I hung the bug box near it for all the loose caterpillars to see. Unfortunately, they didn't seem scared.

Next, I grabbed my high pressure car washer and started spraying my crab apple tree. The caterpillars fell like frogs from the sky. Then I pounced on them one by one and crushed them with a mighty twist of my foot. This time they wouldn't get away.

I'm going to raise the caterpillar I caught, as my own. I'll keep you updated as to the proceedings of the cocooning and the emergence of the dreaded moth.

Here's a little research I did on the forest tent caterpillar along with some pictures to help you identify (and kill) the insect:

The forest tent caterpillar (Malacosoma disstria Hubner) may be found throughout the United States and Canada wherever hardwoods grow. It is a native insect that has attracted attention since colonial times. Region-wide outbreaks have occurred at intervals varying from 6 to 16 years in northern areas. Southern gum forests in southwest Alabama and southern Louisiana have had continuous infestations, especially in water tupelo "ponds," since 1948. Here, varying degrees of defoliation occur annually in 1.4 million hectares of gum forests.

Hosts

The favoured hosts of this insect are broadleaved trees: in the Northeast, sugar maple and aspens; in the Lake States, quaking aspen and oaks; in the Appalachians and in the Central States, oaks; in the Mid-south and in southern coastal States, water tupelo, sweetgum, and swamp blackgum; in the Mississippi Valley, cottonwood and elms; in Texas, oaks; and in the Northwest, red alder and willow. Other tree species fed upon include birch, cherry, basswood, and ash. Species not fed upon are red maple, sycamore, and most conifers. After they have stripped trees, the caterpillars feed on wild and ornamental shrubs and even the leaves of cultivated fruits and vegetables.

Injury

The forest tent caterpillar often defoliates extensive areas. Outbreaks in the Lake States typically last for 3 years, then subside. Diameter growth may be reduced as much as 90 percent. Such defoliation kills few trees except for those that are suppressed. Unusual outbreaks lasting 5 to 7 years have caused mortality up to 59 percent where aspen grew over a high water table. Water tupelo in southwest Alabama subjected to annual defoliation for nearly 20 years grew only 1.3 mm in diameter per year on the average. This represents approximately 25 percent of that normally expected. Sweetgums in adjacent areas began dying after three successive defoliations. Tree flowers may be eaten, nectar gathering by honeybees may be reduced, and seed production is diminished. During years when larvae hatch before leaves unfold, caterpillars mine buds. The quantity and quality of sugar maple sap are greatly reduced as a result of defoliation. New foliage appearing after spring defoliation may be stunted and thin.

An average of 19 egg masses on an aspen tree 15 cm in diameter indicates that complete defoliation could occur. Similar relationships between the number of egg masses and defoliation apply to the southern tupelo-gum forest.

Life History and Description

There is one generation a year. Young larvae appear when the leaves are beginning to unfold. The time varies with weather and locality. Newly hatched larvae are nearly uniformly black, are less than 3 mm long, and bear conspicuous hairs. Colonies stay together and move about in a file, following silk trails laid down by leaders. With each successive moult, markings of pale bluish lines along the sides of a brownish body and a row of footprint-shaped, whitish spots on a black background become more evident. When full grown, caterpillars are about 50 mm long.

The caterpillars do not spin a tent; instead, they form a silken mat on the trunk or branch, and here they congregate when at rest or during moulting periods (fig. 1). During the early instars, these congregations are usually in the upper part of the tree; later they are more commonly found lower in the crown and on the trunk.

Figure 1) Fifth-instar larvae at rest on an aspen trunk.

Larvae usually pass through five instars. When high populations result in complete tree defoliation, the fourth and fifth in-stars often move around a great deal in search of food. The larvae wander in search of suitable sites for spinning cocoons, and their movements have caused them to be called "armyworms" by some.

Five to 6 weeks after hatching, the larvae spin cocoons of silk coloured yellow by a powdery material dispersed between the strands. They are constructed in a folded leaf, bark crevice, or other sheltered place.

In these cocoons, the larvae change to pupae. The stout bodied moths, which emerge about 10 days later, live for only a few days. They are buff-coloured and have a wingspan of 25-40 mm. The forewings have two darker oblique lines near the middle (fig. 2). Strong winds can carry the moths many miles, and great numbers are attracted to lights.

The eggs are laid mostly on upper-crown branches in masses of 100 to 350, which encircle small twigs. Each mass is cylindrical with truncated ends. The eggs are cemented together and are coated with a frothy, glue-like substance, which hardens and turns a glossy dark brown. Within 3 weeks the embryos develop into larvae that over-winter in the eggs and hatch in the spring.

Figure 2) Forest tent caterpillar moths are buff-coloured, with two darker oblique bands on the forewings.

Control

In some years hatch is low. High mortality of larvae in the egg is associated with temperatures below -41°C. Freezing weather just prior to, during, and following hatching kill many of the young caterpillars. When trees are completely stripped of leaves, larvae starve. In the North, temperatures above 38°C in the shade during moth emergence and egg laying have caused death of adults and low viability of eggs.

Several species of flies and wasps parasitize the eggs, larvae, and pupae of the forest tent caterpillar. Most important are large gray flies, Sarcophaga aldrichi Parker, in the North; and S. houghi Aldrich in the South. Female flies deposit maggots on cocoons. The maggots penetrate the silk and move into the prepupa'e or pupae, killing them as well as any other parasites that may be present. S. Aldrichi becomes extremely abundant and contributes greatly to the termination of out breaks of the caterpillars in aspen forests. Although the flies do not bite, they annoy people by lighting on them and regurgitating on clothing and laundry hung outdoors. In southern gum forests, S. houghi is much less conspicuous and usually goes unnoticed. Nevertheless, it can be an important control agent. Itopletis conquisitor is an important ichneumonid wasp parasitoid of the pupal stage. Parasitization up to 20 percent by five species of egg wasps has been recorded in Alabama.

Predatory beetles, ants, true bugs, spiders, birds, and small animals feed on caterpillars and pupae, but it is not known to what extent they affect populations of the forest tent caterpillar. Polyhedral virus diseases often destroy large numbers of caterpillars. A fungus disease, Entomophthora sp., is common in the South, and a protozoan infects larvae in the North.

Small trees can be protected by collecting and destroying egg masses, destroying colonies of young larvae at the end of branches, or killing larvae clustered on the trunks of branches during moulting and resting periods. Several chemical insecticides and a microbial insecticide, Bacillus thuringiensis are registered for control of this insect.

Whatever.

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Short "Snappers"

• Only female bees work. Males remain in the hive, their only mission in life being to fertilize the queen bee on her maiden flight. After they have served their function, the males are not allowed back into the hive but are left outside, where they starve to death.
• Ten percent of men are left-handed and eight percent of women are left-handed.
• From the smallest microprocessor to the biggest mainframe, the average American depends on more than 264 computers per day.
• The word "live" spelled backward is "evil."
• One out of every three British males between the ages of 17 and 35 was killed in World War I.
• Interestingly, thirty-two percent of Bahamians are Baptist.
• In the 1990s, in response to a quiz conducted at the National Science Foundation, only 48 percent of Americans answered the following true/false question correctly: "The earliest human beings lived at the same time as the dinosaurs." The correct answer, as you know doubt knew, is "false." This quiz was used to determine how much American adults know about basic science.
• Socrates, Nero, Mark Anthony, Cleopatra, Vincent van Gogh, and Adolf Hitler all committed suicide.
• The colour combination with the strongest visual impact is black on yellow. Next to follow black on white, yellow on black, white on black, dark blue on white, and white on dark blue.

Don't forget about the "Meat Treat" submissions. Let us know what kind of meat that you've never eaten but always wanted to. We had a couple of excellent ideas this week - click here to find out what they were!

As always, send me your suggestions for future columns on crang.com. Check back soon for an update on the transformation of our little prisoner.

© June 10, 2001