pine cone like cocoons on arborvitae - Knowledgebase Question

Dexter, MI
Question by andrea98
July 27, 1999
I noticed my newly purchased (and expensive) 8ft "nigra" arborvitae looking a bit stressed on the top. When I looked closer I saw that it had "gone to seed" or so I thought! I opened up one of the "pine cones" and it had a grub or caterpillar like creature in it and it has been devouring my new tree! There are tons! I also noticed a few on my (even more expensive) 8ft hemlock! I have been picking them off and burning them, but the task seems endless! What should I do? and what the heck are these things?!!! The cocoons really look like little pine cones, they are made of the arborvitae foliage and the pattern is shingled, pointed and all go in the same direction, it is mostly beige and green foliage, it is untrue how "real" these things look! inside is a beige/brown "grub" (more grub like than caterpillar) and the head pokes out where it attaches to the branch to feed. They are horrible! I wish I looked sooner, they have been feasting for weeks! The infected trees are in a new burm and I am worried about the other trees.

p.s. the top of the tree is far more damaged than the bottom, should I severely prune?

Answer from NGA
July 27, 1999


Bagworms have found your garden! They don't always have such expensive tastes, but they sure found a treasure at your place! Bacillus thuringiensis (Bt), sold under the trade name Dipel and Thuricide, is a great weapon against bagworms. It is basically a disease you give them (and other critters like gypsy moths, tent caterpillars, webworms, tomato hornworms, and other moth larvae). It is a great organic remedy that will not harm the birds, bees, animals and other insects in the garden. Since the pests must consume it for it to be effective, wait until you see them. It's most effective when the larvae are small. BT is very popular and is widely available in mail order catalogs and most garden centers.

Plants can take quite an assault and still recover so don't prune until you have the infestation under control. If new needles don't appear next spring, prune the damaged branches back.

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