Alert and curious people in Iowa have noticed that the bagworm eggs have hatched and that the small, bag-carrying caterpillars are feeding on tree foliage.
Bagworms are a pest of trees – mostly spruce, arborvitae, cedar and other conifers but also deciduous trees such as honeylocust and crabapple. Each caterpillar makes a small case or bag out of silk and bits of plant foliage. The caterpillar carries the bag as it feeds on the tree foliage, expanding the bag as the caterpillar grows. Bags eventually become 1.5 to 2 inches long. Attacked plants may be partially defoliated, weakened or rendered unsightly though it is not uncommon for complete defoliation to occur resulting in death of conifer trees.
Picture 1 shows what the small caterpillars/bags look like on the tree. They are effectively camouflaged and may be hard to see. The suggested monitoring technique is to stare intently at the tree and wait to notice the wiggling bags.
Bagworm caterpillars disperse by a process called ballooning. The tiny caterpillars spin a long, single strand of silk that if caught by an updraft breeze, lifts the caterpillar into the air and they blow where the wind blows. They land wherever the wind stops blowing. If they land on a host tree, they survive. If they land in a corn or bean field, on a road, or in a pond, they die. It’s a risky way to disperse but it works often enough that the species is perpetuated and new trees become infested. A Rutgers University article states bagworms may disperse “many hundred yards downwind" and that as many as 75% of bagworms will disperse by ballooning.
Pictures 2 and 3 show an interesting situation I had not seen previously. Tiny bagworrms, plus bags, were on a car 20 feet from a nearby spruce tree. It appears to be an unusual situation where bagworms that had already started their bag were blown - or they walked - from a nearby host where they were feeding.
These two reports are reminders that now is the time to check your trees for bagworms and treat if warranted. Application later in the summer will be less effective and treatments become worse than worthless after the caterpillars are grown and the bags tied shut while the caterpillars transform to the adult moth stage. See our online article for more about bagworms.
Most commercial and homeowner insecticides labeled for ornamental trees and shrubs are effective against small caterpillars, including formulations of Bacillus thuringiensis kurstaki.