News release: Bagworms

Tim Baker, MU Extension Horticulture Specialist
102 N. Main, Suite 1, Gallatin, MO 64640
660-663-3232, bakert@missouri.edu

Release Date: March 19, 2015

Headline: Bagworms

I recently received my first call of the year about bagworms, so I thought it might be a good idea to remind everyone that in a few months it will be time to spray for them.

Bagworms can occur on a variety of trees and shrubs.  These infestations range from minor to severe.  A severe case of bagworms on a small tree can easily defoliate the tree, so it pays to keep them under control.

Bagworms are actually the larva of a moth, Thyridopteryx ephemeraeformis.  It is very common on evergreens, but can also attack deciduous trees.

Bagworms hatch in June in Northwest Missouri.  The eggs overwinter in the protective bag.  When they emerge, they are tiny, and very difficult to see.  However, once they emerge from their bags, they are most easily controlled.  It’s best to apply control measures at this time, before they have a chance to form their own bag.

When they start to feed, they also start forming a new bag, which they carry along with them.  The bag increases its size as it grows.  The bag itself is silken-like, with pieces of leaves and twigs attached to the outside to offer camouflage protection.  They carry this bag along with them throughout their entire life.

By mid-August, the bagworm has finished feeding, and firmly attaches the bag to a branch, closing it up.  It then pupates.  By mid-September, pupation is complete, and adult males emerge from the bag.  These are clear-wing moths.  The males fly to other infested plants, seeking a mate.

The females, however, remain inside their bags.  The adult female does not look like a moth.  She lacks eyes, antennae, wings, legs, and functional mouth parts.  She looks more like a maggot than a moth.  After mating, she lays between 500 and 1000 eggs inside her bag, and then dies.  Next June, those eggs will hatch, and the cycle starts over again.

Although some birds and insect predators feed upon bagworms, you will need to consider some means of control, if you have a large infestation.  For small trees, hand picking them is the easiest method.  If the tree is larger than what you can pick from the ground, you will need some type of spray equipment and use either a chemical or biological control.  If the tree is very tall, this presents a problem, because it takes specialized (and expensive) equipment to reach the heights of large trees.  And if you can’t treat the entire tree, those bagworms left untouched will re-infest the tree each year.

If you would like to use a biological or organic control, Bt (Bacillus thuringiensis) is your best bet.  This material effectively incorporates a caterpillar disease which the bagworms contract and eventually die.

If chemical control is your preferred method, there are several labeled chemicals available.  Liquid Carbaryl (Sevin) is labeled, and should work well. 

The trick with any control, biological or chemical, is getting good coverage.  The best time to spray is in June, when the newly-emerged larvae start crawling, and before they have had time to start making their own protective bag.  Control after that may not be as effective, but if they are still feeding, it’s worth a try.  Once they stop feeding in mid-August, spraying won’t help.