My friend, you have the most dreaded summertime plant-eating insect of all time — the bagworm.
Bagworms are one of the more curious and interesting insect pests of trees and shrubs. Its upside-down ice cream cone looking bag is constructed from bits and pieces of leaf material from the plant it’s feasting on. Should you be adventurous and look inside the bag, you’ll discover a little worm known as a larva; as the larva grows so does its bag. The larva carries its bag wherever it goes. When disturbed, it quickly pulls its head back into the bag for protection.
The bagworm is especially fond of junipers, cedars, arborvitae, and white pine along with other trees and shrubs if really hungry. It’s been reported they are fond of feeding on some 128 different species of plants.
Damage occurs from the larva devouring the foliage. Young bagworm larva are highly mobile in their search for food; crawling or floating on wind currents in search of a plant to victimize. If not controlled when they are small, the maturing larva will do some extensive defoliation during the summer, eventually leading to plant death.
High populations of bagworms can easily strip a plant of its foliage. This is particularly a problem on evergreens because the defoliation alters the shape and beauty of the plant. Defoliation will kill needle leaf evergreens.
The bagworms will continue to feed and grow in size usually until late July. When fully grown, the bag containing the larva will be about 2-and-a-half inches long. By this time they have stopped feeding and thus anchor themselves to one place in the plant and no longer venture about seeking food.
Hundreds of eggs are laid in each bag and proceed to overwinter. Not all of the bags will have eggs in them; some of the larva are males which do not lay eggs.
Oh yes, you better believe that if you do nothing, these overwintered eggs will start to hatch in late April to mid-May. Upon hatching, the young larvae crawl out from the bottom of the bag and start feeding and the process starts all over again with the construction of a brand new upside down ice cream cone shaped bag to cover their body.
How to Control
Chemical control must be done when the larva are young and actively feeding. The bags offer not only a camouflaged refuge but a water repellent cover as well. Being that the bags repel water, they are water-resistant to insecticide sprays, which seldom penetrate inside the bag to kill the larva, thus they must be feeding to come in contact with the controlling chemical.
One of the best ways to control bagworms is to pick them off the plant and destroy the bags. A thorough job must be done. On large trees, handpicking may be dangerous and impractical. A number of natural enemies feed on the larvae and eggs, apparently this explains why populations of bagworms fluctuate from year to year.
A few recommended insecticides for control are: carbaryl, malathion, bifenthrin, cyfluthrin, Spinosad, or Bacillus thuringiensis and should do the job. If you choose to use Bacillus thuringiensis, you need to know that it will not work very well on large more mature larvae.
Read the label carefully to ensure the correct amount of insecticide is used and you follow application instructions and safety precautions.