How to control bagworm infestation of arborvitae trees
Have read that it is now too late in the season for spraying to be effective - and do not want to use heavy chemicals that could be damaging to wildlife. BUT... the arborvitae trees are full of bagworm (nests) and picking them off results only in more appearing the subsequent morning or afternoon. The trees are showing areas of some yellowing and dessication of foliage. How do we prevent the bagworms from overwhelming and killing the trees? they have attacked these trees each year for the last 3-4 years, each time it seems as if there are more bagworms. The trees have recovered each year - but this year is the most intensive attack. Are there organic sprays or treatments that can neutralize or kill the bagworms and save the trees?
District of Columbia
You did not mention how large the bagworms are. If they are under an inch, you can spray with Bt, a biological control between mid June - mid July.
This works well on young bagworms, has low toxicity, and does not affect humans or other animals. If the bagworms are larger than an inch, you can spray with Spinosad. It is not too late to spray.
Keep your trees healthy by watering during dry periods. Make sure mulch is no thicker than several inches and away from the base of the trunk.
See our website for more information http://extension.umd.edu/hgic/bagworms-trees-and-shrubs