by Susan Ndiaye, Community Horticulture Educator

Have you ever noticed one of these structures hanging on a Colorado blue spruce or an arborvitae?  They kind of look like pine cones, but not exactly.  Well, they aren’t pine cones, but silken bags spun and decorated by bagworms (Thyridopteryx ephemeraeform).

Bagworms are moths whose larvae feed on evergreens such as spruce, juniper, pine and arborvitae.  The larvae can also feed on deciduous trees such as maple, elm, birch and sycamore.  Bagworms defoliate the trees and shrubs they infest.  In large numbers, bagworms can cause significant defoliation, which can lead to the death of the plant.

Bagworm Lifecycle

In late spring, bagworm eggs, which overwinter in their mother’s silken bag, hatch and caterpillars emerge.  These caterpillars begin to form new silk bags, and as they eat, they cover it with bits of leaves.  As the caterpillar grows, it expand its bags.   Then in late summer the caterpillar firmly attaches its bag to the plant and pupates.

Complete metamorphosis from caterpillar to moth takes about four weeks.  Adult male bagworms emerge from their bags as clear winged moths and begin to search for a mate.  Adult female bagworms are wingless moths and never leave their bags.  After mating females produce 500-1000 eggs before dying.  Their eggs overwinter inside their mother’s silken bag and the whole cycle begins again.

Management

Because bagworms are protected by their silken bag, management can be tricky.  For smaller trees and shrubs the best tactic is to remove and destroy the bags by hand.  Unfortunately, this is not possible in all instances, especially on larger trees and shrubs.  Insecticides are most effective right after bagworm eggs hatch, when the caterpillars are small.

But how does one know when the eggs are going to hatch?  Well, it turns out that there is a “Bagworm Forecast” that you can check in the spring to determine  the best time to apply insecticide.  The maps provided by this forecast are updated daily and available six days in the future, so you can plan ahead.

For recommendations on pesticides, check out the resources below.  And as always, make sure you read and follow all the instructions on the pesticide label including the use of personal protective equipment.  The label is the law!

If you need to spray a larger tree, you may need to contact an arborist.  Click here to find a certified arborist near you.

Fun Facts

As females don’t fly, you may wonder how bagworms spread.  Bagworm caterpillars can balloon, or use their silk threads to catch the wind and travel long distances.

Despite relatively little protection for overwintering bagworm eggs, research at Purdue University found that it takes a 24 hr period at -0.6 ° F or below to kill the eggs.  So if you live in Orange County New York don’t expect a cold winter to kill off your bagworms.

Here is a video of a bagworm feeding!

Video from Purdue University Landscape Report (https://www.purduelandscapereport.org/article/824/)

Resources

Bagworm – Penn State University

Bagworms – Cornell University

Bagworm Forecast – USA National Phenology Network

Bagworms on Landscape Plants – University of Kentucky

Cold weather in January 2018 may have killed bagworms in some parts of Indiana – Landscape Report, Purdue University