Question
Last year, bagworms caused considerable damage to my blue spruce before I noticed them and sprayed to stop the carnage.  To prevent this from happening again, what time(s) during the year do you recommend spraying to kill off the bagworms before they start defoliating the spruce?

Thanks,
Joe

Answer
I would wait until you see the bags before spraying.
Bagworms have a single generation per year and overwinter as eggs inside the female bag. There may be 300-1000 eggs in a bag. The eggs delay hatching until late-May or early-June. As they hatch, the small blackish larvae crawl out the bottom of the bag and spin down on a strand of silk. These larvae on a string are often picked up by the wind and ballooned to nearby plants. When a suitable host plant is found the young larva immediately begins to form a new bag over its body. This bag is only about 1/8 inch long and is soon covered with sawdust-like fecal pellets.

I would check the tree in the first couple of weeks of June for any signs of more bags and then spray if any are found. Since they have only one generation per year the chance are you wiped them out with the spray last year. There is not reason to waste the spray unless something is there.