Every year it seems like we get to learn about a new caterpillar that emerges and makes a pest of itself. Last spring, the oakleaf roller caterpillars hung from the oak trees by silk threads, and then in the fall the armyworms marched across the county eating our Bermuda lawns along the way. Fortunately, the various caterpillars do not reach infestation levels every year!
I had three phone calls about bagworms on Italian cypress or juniper trees within the last week. That is when I need to start brushing up on my entomology! Hopefully, this is just an interesting case study and not an infestation of bagworms.
The scientific name for bagworm is Thyridopteryx ephemeraeformis, and it is one of a few species of caterpillars that constructs bags or sacs. Bagworms are unique because they incorporate plant debris in the sacs, and they have an interesting life cycle.
Bagworms attack trees and shrubs like cedars, cypress, junipers, pines, and spruce. They will also go after apple, basswood, black locust, elm, Indian hawthorn, maple, some oaks, and a few other broadleaved plants. They are not abundant every year, but a plant that becomes infested can defoliate a tree or shrub and eventually kill the plant.
Bagworm eggs overwinter in the sealed sacs that are created by the females in the previous fall. Tiny caterpillars, only 1/25 inch long, emerge in the spring and lower themselves on silk strands to a new branch to create a tiny conical bag. The caterpillars can carry the bag upright, and they move it around the tree as they eat. The caterpillar goes through four or more stages (instars), and it enlarges the bag as it grows. The caterpillar grows to be about an inch long within the bag when it pupates in August or September.
The pupa of the male moth comes out the bottom of the bag, and it emerges with a hairy black body, ½ inch long clear wings, and feathery antennae. The males fly out to find a female and mate. The female remains inside the bag and looks like a maggot. The female does not develop functional eyes, legs, mouthparts, or antennae, and they emerge from the bag halfway to mate with the males. The female will deposit 500-1,000 eggs in the pupal case, then drop to the ground and die.
One good thing about bagworms is that they do not travel very far in the caterpillar stage because they must carry their bag with them. Unfortunately, this means that a big infestation of bagworms on a tree will not spread out and they can quickly defoliate a tree. Wind and birds are the primary means that bagworms get moved from one tree to another.
The cheapest way to control bagworms is to pick them off by hand and destroy or discard them. Dropping them on the ground is not effective because the eggs will hatch in the spring and could reinfest the plants.
If handpicking is not feasible, you can apply an insecticide in the spring after the eggs have hatched and the larvae are feeding on the tree. Bt kurstaki (bacillus thuringiensis var kurstaki) or Spinosad are two insecticides that are organic and safe for beneficial insects. Spray the tree thoroughly to ensure good coverage of the foliage. The caterpillars have to eat these insecticides to be effective. Other chemical control options are products containing carbaryl, pyrethroids, malathion, pyrethrins, or acephate. Follow the label instructions for applying any insecticide.
For more information about lawn and garden topics, contact Kate Whitney, Horticulture Extension Agent, at 512-943-3300.