Bagworms - Friend or Foe?

While walking my yard the other day, I noticed interesting brown sacks hanging from my crabapple tree. They were pretty cool looking and reminded me of miniature hornets' nests dangling from the branches. The sacks were an inch or two long and looked like they were made of dead leaves. I had noticed later in the summer my crabapple tree had lost its upper leaves. Could this be the reason? It was time to investigate. Likely it was an insect, but was it friend or foe?

The culprit has been identified: bagworms (Thyridopteryx ephemeraeformis). Inside the bags are eggs waiting to hatch in early spring. Found throughout the United States, bagworms infect a variety of evergreen and deciduous trees and shrubs.

The principle damage caused by bagworms is defoliation. The caterpillars hatch in late May to early June and start chomping. In August the larvae will spin and enclose itself in the bag and 4 weeks later the males emerge as a moth. The females remain in the bag to lay their eggs and then die. Deciduous trees such as my crabapple will incur mostly aesthetic damage since it grows new leaves each year, but evergreen species such as arborvitaes can be killed if the infestation is extreme.

What to do about these bagworms? Since bird and insect predators like to feed on the larvae, if the infestation is not too bad, you should let nature take its course. Remember to have beneficial bugs and birds, you need to provide them with food. However, if your tree is not looking too good, the most eco-friendly method of eliminating the bagworms is to handpick them off the tree and put them in your garbage before they hatch in the spring. You might need a pair of scissors - those buggers are tied on there tight. Looking at my tree however, I am not sure about how to get up top to cut the bags down, so I may need to resort to a spray if the aesthetics of the tree is really compromised. The suggested organic control is to spray in early spring right after the larvae hatch with Bacillus thuringiensis (Bt) which is commercially available under a variety of brand names. Bt products will not harm the majority of beneficial insects, but can affect butterfly larvae so limit the application to the trees with actual pests and only use when you feel it is absolutely necessary for the health of your tree.

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