If you have evergreens, such as Cedar, Juniper, or Arborvitae, there's a good chance that you will have to deal with Bagworms at some point.
Bagworms are small caterpillars that feed on plants, and especially evergreens. It's often hard to see the caterpillars because they disguise themselves with bits of silk and foliage from the plants that they're eating. What you may see are their bags, which look like cones or branches hanging from the plant. These bags are left over from last year and are used to protect the Bagworm eggs. The caterpillars will feed for up to six weeks, growing larger and building bigger bags to house new eggs for the next season, which will then hatch again in mid May.
If it's practical to do so, pick off any bags that you see so there are fewer eggs to be hatched next year. You could also apply an insecticide in may or early June to kill the caterpillars if necessary. Some sprays, such as promethium, are contact insecticides, which means it needs to directly hit the bug. You could also look at a systemic control like acephate so that as the caterpillar feeds on the plant it also feeds on the poison.
Whatever method you choose to control your Bagworms, be sure to check back every May or June for more and act quickly to keep your plants looking good.
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