Is a Bagworm Munching on Your Arborvitaes?

The common bagworm, also known as the Thyridopteryx ephemeraeformis, is a voracious leaf-feeding caterpillar that eventually becomes a clear-winged moth. If you are one of the millions of property-owners who have planted a living fence of Arborvitaes, you should be on the alert right now for these destructive little pests.

The astonishing mimicry that this insect has accomplished makes it practically invulnerable and, to most people, invisible. The feeding caterpillar produces a silken cocoon that is covered with chewed-up bits of plant material – from two feet away, you would swear the cocoon was a naturally occurring cone, a part of the plant itself.

Instead, this cocoon contains the feeding caterpillars, it is home to the adult female who mates while in it, and it provides protection for up to 1,000 eggs per female.

Unfortunately for those who have tall Arborvitaes, the insect often starts at the top of the trees and chews its way down, often denuding the plant as it goes.

At this time of year, the only defense is to hand-pick the cocoons off the trees and destroy them. Late spring is the time to spray for them; both organic and chemical controls exist, but the cocoons make it fairly difficult to reach the females.

And a last word about “living fences.”  If you really want privacy, consider installing a fence. No tree or shrub benefits from being planted in massed quantities. The more of one species you plant – what’s known as a monoculture – the happier some insect or disease will be when it locates the all-you-can-eat buffet on your property.

Many insects, and almost all fungi, are species-specific. So if the tree they land on is one of a kind, that insect or fungus will spread slowly, if at all. Our newest insect threat, the Emerald ash borer, gathered tremendous momentum in Michigan because ash trees were planted there in huge quantities to replace elms lost in the 1950s and 60s. And the elms were a monoculture, too! Will we ever learn?