Cankerworms and other similar insects are called inchworms because of the way they move. They have legs only on the front and back of their bodies. With this type of anatomy, they form a loop as they creep along leaves, said Jerry Goodspeed, Utah State University Extension horticulturist. They look like they are measuring the leaf as they move along, which likely explains how they got the name inch or measuring worm.

“It is amazing how much this small caterpillar can eat,” said Goodspeed. “I thought kids were voracious eaters, but they are mere amateurs compared to these caterpillars. The tree that gets the brunt of the attack from these caterpillars is the oak.”

Many inchworms or cankerworms are green with small dark strips running the length of their bodies, he said. These markings act as a camouflage and make them very hard to detect, especially when they are small.

“Often cankerworms can do major damage before anyone notices,” said Goodspeed. “I have seen them defoliate a small grove of Gambel oak in a matter of days. If gardeners do not pay attention, they can strip a tree before gardeners even have time to swear and spray.”

Adult cankerworms lay their eggs in late fall to early winter or early spring, he said. In both cases, the young larvae emerge in late April to early May and begin satisfying their voracious appetites.

“It is bad enough that these insects eat and weaken our trees, but then they become cocky about it,” said Goodspeed. “After they have eaten for a while, they hang from the branches on a thin thread, mocking those who walk by. Fortunately this is mostly bravado, because they rarely kill the trees. I have seen clumps of oak completely defoliated, but within a couple of weeks they recover and send out a new flush of growth.”

These insects can be difficult to control, he said. The best time to spray is when they are young and vulnerable. However, gardeners often fail to notice them until they are big and brash. Some years the infestation is worse than others, and many times there are so few cankerworms, they never get noticed.

The best and safest product for cankerworm control is one that contains Bt (Bacillus thuringiensis), Goodspeed said. Sold as Biobit, Dipel, MVP, Steward or Thuricide, Bt products are a naturally occurring bacterial pest for certain insects. As a general rule, it does not harm beneficial insects and is safe for humans and pets. It must be applied every five to seven days while the worms are feeding.

Other registered insecticides can also be used to control them, he said. Despite their impudent ways, cankerworms are fairly vulnerable to most insecticides. For the best control and to minimize damage, spray as soon as they are detected.

One tricky part of controlling these worms is getting the Bt or other spray up into the tree, Goodspeed concluded. These products are generally stomach poisons, so the worms must eat the product to do any good. This makes it especially important to spray the leaf surface where these pests are eating.