Documents: Special Interest: Orchids:

Tent Caterpillars
by Des Kennedy
by Des Kennedy

Des Kennedy is a celebrated public speaker, having performed at numerous conferences, schools, festivals, botanical gardens, art galleries, garden shows and wilderness gatherings in Canada and the U.S. His humour, irreverence and passion for gardening and the natural world have made him a 'must see' speaker in demand across the country.

June 2, 2002

We were strolling the gardens one evening when I saw a dreadful thing: our little purple-leaved sand cherry, barely past blooming, stood entirely defoliated. The mood of tranquil evening stroll destroyed, I hurried over to see what had caused this calamity.

A moment’s glance revealed the agent of mischief:: a tent caterpillar nest cunningly nestled among the lower twigs of the little shrub. Platoons of small caterpillars were wriggling their way down the branches, returning to the nest for night. The sense was of a plague like the ones wrathful Jehovah visited on the stiff-necked Egyptians.

It’s been a banner year for tent caterpillars at our place; I’ve probably destroyed two dozens nests already. I suspect we’re at or near the high point in a cycle of population fluctuations.

The silken nests, seething with dozens of small hairy caterpillars and dotted with their excrement, are undeniably repulsive. One’s reminded of teeming maggots in a corpse. I remember first encountering them years ago as a young gardener, going to extraordinary lengths to destroy them without touching them. At one point I went so far as scorching the caterpillars in their nests with the flame of a small butane torch.

This squeamishness has long since surrendered to the demands of the moment. Crouched by the defoliated sand cherry, I didn’t hesitate to reach in, scoop out the nest and squeeze its squishy inmates in my hand. Then I followed along each branch systematically squeezing caterpillars between thumb and forefinger until there wasn’t a sole survivor. This technique is what’s known as manual control in its purest form. 

Tent caterpillars are the gregarious larvae of a moth called clisiocampa, a member of the genus bombyx which includes the silk-worm moth. The moth lays her eggs on the branches of selected plants that will feed her offspring the following spring. Here on the coast, native red alders are a favourite host tree, while in our gardens, the moths are drawn to apple, cherry and plum trees. In the East, wild cherry, apple, pear and hawthorn are favourite nest trees.

Careful observation during the dormant season might reveal clusters of eggs on certain branches. The eggs can be scraped away, or the branches may be pruned off and burned. Winter spraying with dormant oil and lime sulphur is the surest control method -- it works by smothering the eggs.

Egg clusters that elude control hatch in spring into colonies often containing many dozens of larvae. They collectively spin the telltale cobweb-like tents which offer them protection from bad weather and predators. In poor weather, a colony might extend its tent along the entire length of a branch, enclosing the leaves for easier eating. Voracious eaters, the dozens of larvae in each tent can quickly defoliate small shrubs or whole branches of trees.

You’d think these plump larvae would make appealing food for predators, but they appear to carry on largely unmolested. Toads are said to feed on them, but I doubt many toads could make it to the highest tips of fruit trees where nests are frequently located. My handbook tells me there’s a sarcophagid fly that preys on them, as does a beetle with the marvellous name fiery searcher.

When a major outbreak occurs, as we’ve had this spring, prompt response is of the essence. Even for those of us who like our gardens au naturel, when it comes to tent caterpillars, a zero tolerance policy is best.

Some experts recommend spraying with carbaryl, malathion, rotenone or trichlorfon, but manual control, if at all feasible, is both a wiser and more gratifying approach. Nests are best assailed while the caterpillars are still inside -- on a cold day, or early morning or late evening. For nests located on the tips of tree branches, I take my pole pruner and snip off the branch just below the nest. When it falls to the ground, roiling with excited larvae, I grind it underfoot and there’s an end to it.

Some nests are very hard to spot, and it’s prudent to check host trees from different sides and angles to be sure there isn’t an extra nest tucked in somewhere. When a nest is located in such a way that pruning off the infected branch is not realistic, one resorts to the hands-on approach. Alternatively, one can take a small stick and twirl it gently to gather up most of the nest in the same way carnival venders gather cotton candy on a stick. 

By mid-spring, the small larvae of the gooseberry sawfly, although not tent dwellers, can be found munching on gooseberry leaves with the same devastating effect. Late spring brings on the leaf rollers or tortrix caterpillars, which use their silken threads to pull a protective leaf around them. By summertime, some gardeners find themselves confronting a truly awesome caterpillar, the daunting tomato horn worm.

With all these voracious caterpillars, constant vigilance, backed up by a horny thumb and forefinger, is the order of the day.


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