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They defoliate alders, strip apples, devour ash.

They chew cherry, masticate willow, nibble wild rose.

They denude hawthorns, an outcome no one wants.

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The Western tent caterpillar is a voracious insect with an insatiable appetite for succulent leaves. The caterpillar is an uninvited guest that shows up in trees and bushes – and on decks and picnic blankets – every year at this time. They are yellow and black with brilliant blue spots. Fuzzy, but not cuddly.

They have a boom-and-bust population cycle. Some years, they are barely noticeable. This is not one of those years. On some of the southern Gulf Islands, the past two weeks have resembled a pestilence you'd find in the Bible.

"Like living in a horror movie," said Linda Richards, a Galiano Island author. "Caterpillars everywhere."

Richards has been completing The Indigo Factor, a thriller with a paranormal edge, while coping with a real-life nightmare – hungry caterpillars that have made a coffee on the deck unpalatable while stripping bare three towering alders in the front yard.

"They look like winter. The base of the tree was cocooned. It was horrifying."

There has been one other casualty: "My poor little plum tree."

The caterpillars spin tents on trees, emerging to feast on buds before moving on to leaves. They are social insects, congregating to bask in sunshine. Sunning or eating, they are an unsightly, creepy, squishy mass.

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At Fir Hill Farm on North Pender Island, the insects have overrun fence posts, farm equipment and even the gate to the driveway.

"Sometimes we forget," said Barbara Johnstone Grimmer, "and just grab the gate and [get] a handful of crushed caterpillars."

Her sheep farm includes one of the many apple orchards to be found on the island, some of which this year have been abandoned to the insects.

"They've completely wiped out orchards. Most people just gave up. The alder trees are all just skeletons. Now, [the insects] have dropped and they're all over the ground. We pick them off ourselves. It's like it's raining caterpillars."

Her husband, Glenn Grimmer, remembers as a boy other infestations so great as to have been able to hear a muffled munching in the still island air.

"They've taken all the foliage off our Kings, our Granny Smiths, Yellow Transparents, Spartans," he said. "They're all nuked. There's just too many to deal with."

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The caterpillars do not represent much of a threat to humans, other than being a nuisance. They do not bite, or sting, or pass disease.

Judith Myers, a University of British Columbia entomologist, who has spent half a lifetime studying Malacosoma californicum pluviale, is a rare voice in defence of the tent caterpillar, which she joking calls "the world's greatest species."

"As individuals, they're pretty," she said. "They have interesting designs going down their back."

She has been studying their behaviour since 1976, starting at Mandarte Island, east of Sidney Island, noting periods of activity and sluggishness. One day while doing research, she coughed, and noticed the caterpillars had lifted up while shaking their heads. That began a study of their response to sound frequency. (The caterpillars' action was a defence against fly parasitoids that try to lay eggs on their heads.)

Ms. Myers, who owns property on Saturna Island, said the greater hours of sunshine, as well as a rain shadow, make the southern tip of Vancouver Island and the neighbouring Gulf Islands a haven for tent caterpillars.

The good news is there will almost assuredly be fewer insects next year. The caterpillars are at the peak of their population cycle. When they reach a high density, as they have this year, the population is driven back by viral disease, the competition for resources, and their worst natural enemy – not a gardener armed with a blowtorch, but a tachinid fly that deposits tiny white eggs on the caterpillar's body. When they hatch, the maggot burrows into the caterpillar to begin feeding from the inside out, a real-life horror movie.

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