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Tent Caterpillars

 

By Ranger Steve Mueller

Tent caterpillars become abundant and then seem to disappear for years. During the recent Memorial Day weekend, I led ecological interaction walks in the Jordan River Valley for the Michigan Botanical Club Spring Foray. Members gathered from the state to explore the advance of spring ephemeral flowers, trees, shrubs and associations with insects, birds, fungi and other organisms. Organisms were busy at work in their nature niches. 

Eastern Tent Caterpillar (Malacosoma americanum) on bark. Image from U.S. forest Service.

Driving to the natural areas from home, many foray attendees noticed eastern tent caterpillar webs on cherry trees along freeways, highways, and back roads. The roads act like threads of silk to get us from where we work to places we rest in shelters at night. The tent caterpillars create their own highway with silk threads used to mark the way from where they feed to their nightly tent residence where they sleep protected and safe.

Many hazards prevent safe return as they go about work and travel. At times they reproduce in excessive abundance. Over 30 years ago, I interviewed Suzy for a position as interpretive naturalist at Howard Christensen Nature Center. We walked the trails discussing natural history and the work. Eastern tent caterpillars were abundantly feeding on cherries and had stripped most cherry leaves from trees. 

She asked if that would kill the trees. I suggested she conduct a scientific study to determine the answer. I told her to select a tree of her choice and report back to me whether it survived longer than her. She selected a particularly heavily infested cherry that was 20 feet tall and 10 feet wide. It was nearly nude from having its leaves almost entirely eaten. By mid-June the tree was looking much like it did in winter. Silk tents were woven among branches throughout the tree. 

The caterpillars had removed the organs responsible for providing life giving sustenance and seriously threatened its health. The tree had adequate stored energy to survive that summer and photosynthesis provided some added daily food to meet energy requirements. After the spring population eruption, the caterpillars spun cocoons that emerged as drab brown moths. The moths laid masses of 100 to 300 eggs glued to cherry branches. 

The next spring when new delicate leaves filled with water and sugars carried from roots through stems to buds, the leaves expanded for work capturing sunlight energy to produce more sugars and plant tissues. Caterpillars hatched from the egg masses and ate the soft new tissues. For a second year, the tree was stripped naked during May and June. By mid to late summer the tree produced more leaves while the moths were hidden in cocoons. 

During the third summer, tree branches were filled with caterpillar tents despite birds, ants and many predators eating their share and using them to feed young. Predators were not abundant enough to reduce the tent caterpillar population. Along came a virus that had been building its own population yearly. During this third year, it became abundant enough to kill the majority of caterpillars. The virus had its survival job and was doing to caterpillars what the caterpillars were doing to the trees—killing them—or were caterpillars killing trees? 

Back to Suzy. After 30 years, I asked Suzy if her selected tree was still alive and asked if she was still alive. She said both were living and both appeared healthy. After that third year the caterpillar population crashed and so did the virus. Every decade or so the tent caterpillar population builds and crashes with the virus life cycle conducting its ecological role. Some cherries already weak from over-crowding or other reasons, die during the moth eruption. It thins the forest providing more growing space, nutrients and health for remaining trees. 

In the natural areas where we hiked with botanical club members, forest tent caterpillars were abundantly feeding on sugar maple leaves. This species does not build tents like the eastern tent caterpillars but their life cycles resemble each other’s. We stood quietly and listened to their frass (poop) falling from tree tops. It sounded like a gentle rain on the 88ºF clear summer afternoon. I suggested participates return to see whether they or their selected trees lives longer.

Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at odybrook@chartermi.net – Ody Brook Nature Sanctuary, 13010 Northland Dr. Cedar Springs, MI 49319 or call 616-696-1753.

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