A type of orangish caterpillar that cuddle together in silk tents at night are proliferating in parts of Whidbey Island.
The population of the aptly named western tent caterpillars tend to explode on the island every six or seven years, according to Loren Imes, coordinator of the Master Gardener program for WSU Extension Island County. In years past, creepy-crawly hordes have eaten trees bare and even made roads slippery with their squished bodies.
Imes said it’s unclear why caterpillar infestations tend to go in cycles, but it could be influenced by the corresponding population of their natural predators, particularly a type of parasitic wasp that lays eggs on caterpillars’ heads.
This year, the tent caterpillar overpopulation seems to be focused on North Whidbey and into Skagit County. Jason Armstrong, manager of Deception Pass State Park, said the caterpillars are everywhere in the park while nearby Kukutali is positively infested.
Imes said the caterpillar population explosion tends to be specific to areas, so the rest of Whidbey might see the phenomenon in the next couple of years.
Imes explained that the very hungry caterpillars like to feed on leaves from fruit trees as well as trees in the alder family. They build silken tents at the end of branches where the proverbial social butterflies return en masse each night. They will eventually transform into brownish moths.
The caterpillars may defoliate trees, which usually isn’t a problem unless the trees are facing other stressors, like drought. Unfortunately, the northern half of Whidbey Island is considered to be “abnormally dry,” according to NOAA.
“Seems like in a dry year, plants are a little bit stressed and it makes them more susceptible to insects,” Imes said.
According to Imes, controlling caterpillar populations can be difficult unless people catch them when they are still young. He said a spray containing a naturally occurring bacteria called Bt, or Bacillus thuringiensis, can help. In addition, he suggests cutting the tents off of trees and throwing them away in plastic bags. He said they should be taken down after the caterpillars return to their homes at night or before they venture out in the early morning. He’s wielded a weed torch against the caterpillars, but he said it’s not recommended, particularly now with the dry conditions.
Imes suggests that people should leave caterpillars alone if they have a white dot on their heads, which is a tell-tale sign that a wasp has laid eggs. The eggs will hatch and the wasp young’uns will feed on caterpillar-flesh, thereby helping to control the population.
“If they don’t have the dot, just step on them,” Imes advised.
In addition to tent caterpillars, Imes said other pests are causing concern. A number of people have brought in branches from trees and shrubs that are mysteriously browning. With the help of a microscope, he was able to identify tiny spider mites as the culprits. In addition, cypress trees at the Central Whidbey Sportsman’s Association appeared to be dying and the problem turned out to be the tiny caterpillars of cypress tip moths.
Residents with plant-related problems and mysteries can call the Master Gardener hotline at 360-639-6058. In addition, Master Gardner Clinics are scheduled throughout Whidbey Island this month. The schedule is on the Extension website at extension.wsu.edu/island.