Evergreen bagworm, Baltimore city
We seem to have a serious infestation of these on yews and Leyland cypress on our property. Is there anything we should do to avoid further damage at this point? Or are they somehow beneficial (provide food for birds/pollinators)?
A bagworm infestation can be a serious problem on evergreens. They have been feeding on the trees for most of the summer and their feeding activity will be ending shortly if it hasn't already. This is not the best time to treat for them because if they are not feeding they will not ingest the insecticide. Unfortunately, they are not beneficial at all and the bag they create really protects them from predators.
At this point in time, pick off and destroy as many as the bags as you can. Do not just throw them under the trees. This can help reduce their number for next year as the next stage of their life cycle is reproduction (which will happen soon). After mating the female lays eggs in the bag she is in and they overwinter (can be 200-1,000 eggs). The end of next May into June is when the eggs will hatch. That is your next opportunity to spray. For more details please see the following 2 links,