Because my standard answer is Memorial Day, and Memorial Day was two days ago, I thought I would explain why my standard answer is standard.
Besides picking bagworms off infested trees and shrubs, the only way to kill them is to spray with an ingestible insecticide while they are feeding. Below is the life cycle of the bagworm so you can understand why now is the time to spray for control.
At this time of year eggs are hatching inside last year's bag. Worms emerge and start feeding. As they feed, they form a protective and camouflaged bag around their bodies. The longer the worm feeds, the larger the worm and the bag become. The appearance of the bag depends upon the foliage that the bagworm is feeding on.
Feeding continues until sometime in August. At that time larvae change to adults. The female is wingless and stays in her bag. The adult winged male emerges from his bag and flies to the female and mates. After mating, the adult male dies.
After mating the adult female closes the opening in the bag and lays about 750 eggs inside the bag. After laying eggs, the female dies.
Eggs incubate over winter in the bag, emerge as worms in late May, and the cycle begins all over again.
You also want to spray for bagworms at this time because of the damage that 750 larvae can do in just a short time. Inspect your landscape at this time of the year to detect bagworm presence. If you do find them, spray as soon as possible.
If you spray for bagworms in July or August, make sure that the worms are still feeding. If feeding is not taking place, no application of pesticide will kill the worms.
The preferred host of the bagworm is arborvitae, juniper and white pine. If the bagworm count in your landscape is extremely high, you may find bagworms on any plant growing in your landscape, include deciduous trees and shrubs, perennials and annuals. I have even seen them hanging from telephone poles and metal gutters.
Send your gardening and landscape questions to Paul Schnare at P.O. Box 699, Cape Girardeau, Mo. 63702-0699 or by e-mail to email@example.com.