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Spruce trees with Bagworms

Check for bags that look like pinecones

By: Sandy Feather ©2009
Penn State Extension


Q. I have several young blue spruce trees that seem to be dying from the top down. Please look at the enclosed picture and see if you can tell me why this happening and what I can do about it.

A. The picture shows a blue spruce tree (Picea pungens ‘Glauca’) that appears to be 8-10 feet tall, and the top fourth of the tree is bare of needles. The bottom of the Spruce looks normal. The bare top branches have what appear to be “pinecones” hanging from them. The "pinecones" are actually bag cases that last year's generation of bagworm larvae constructed.

Adult bagworms are moths. This pest has a wide host range that includes arborvitae, spruce, pine, Douglas-fir, true firs, hemlock, juniper, larch, birch, black locust, honeylocust, maple, oak, sweetgum, sycamore and others.

Bagworms on Spruce
"Pinecones" near top are actually bagworm cases

 


LIFE CYCLE OF BAGWORMS

Bagworms overwinter as eggs laid inside the adult female's bag case. In our area, the eggs usually hatch from mid-May to early June, depending on moisture and temperature. The small larvae begin feeding on the host plant, and start to construct a bag case that is made up of silken strands spun by the insect and bits of the host plant. Consequently, the bag cases are well camouflaged, especially when they are still small. The larvae move the bag cases along with them, and emerge from the top as needed to feed.

They continue to feed and grow, and enlarge their bag cases as needed through the remainder of spring and most of the summer. They pupate in late summer, and hatch out as adults in seven to ten days. The male bagworm adult is a small black moth with almost clear wings. The female bagworm adult does not look like a moth at all - she lacks eyes, antennae, legs, wings, and functional mouthparts. She never leaves the bag case she constructed as a larva. The male leaves his bag case, flies to the female and mates, and then dies shortly thereafter. The female lays 500 - 1000 eggs inside her bag case, and then dies.

Since their mobility is limited, a large population of bagworms can build up on a single host plant. You can easily miss them when their numbers are limited, but once a population builds up on a host plant, you cannot miss the spectacular defoliation caused by the larvae feeding.

Bagworm cases on Arborvitae
Bagworms are often seen on Arborvitae after it's too late

Handpicking the bag cases from the trees in winter and early spring provides good control because you remove the eggs before they hatch. You may need to use a pair of hand pruners to cut the bag cases off of twigs to avoid damaging the plant - the silk these insects produce to tie the bag cases to twigs is surprisingly strong. Your spruce trees appear to be small enough for that approach to be practical.  Put the bag cases into a plastic bag, seal it up tightly, and send it out with the trash. My only concern with this approach is that we are very close to time for them to hatch.


SPRAYS FOR BAGWORMS

If they have started to hatch and you miss some with the handpicking, you can spray to control the escapees. While they are still very small – under ˝ inch - Bacillus thuringiensis (Bt) provides good control. Otherwise, Sevin (carbaryl), Bayer Multi-Force Insect Killer (cyfluthrin), Bonide Caterpillar Killer (lambda-cyhalothrin), pyrethrins plus piperonyl butoxide, and Captain Jack’s Deadbug Brew (spinosad) are labeled to control bagworms on spruce.


MORE

Insects on trees and shrubs

Spraying pesticides

Tree sprout elimination

 

       


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