The bag worm grows on the inside of a small bag that contains hundreds, if not thousands of eggs filled with developing bag worms. These bags can be found hidden inside the bark of the tree trunks. Inside of the bag, which may be camouflaged with foliage, bark, and other debris, the larva is tan or brown with black spots. Once it emerges, the larva is black. It spins down on a silk string in search of a host plant. In some cases, the larva never makes it all the way down to the bottom of the string. It may be picked up by the wind and whisked off to any number of nearby plants.
Once the larva has found a host plant either on its own or via the wind, it begins to spin a new bag over its body. The larva feeds and grows inside of the bag. It feeds by sticking its head out to eat, and then it retracts. It protects itself from any perceived threats by retracting into the bag and holding it shut. The bag worm, Thyridopteryx ephemeraeformis, prefers red cedar and arborvitae but it also likes apple trees, birch, black locust, cypress, elm trees, juniper, oak trees, pine trees, poplar, spruce trees, and sycamore. The bag worm occurs mostly from New England to Nebraska and south through the state of Texas.
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By around mid-August, the bag worm matures and it migrates to another area in search of a sturdy host or structure. After the move, the larva pupates. This is the non-feeding stage where the bag worm begins to transform into its adult form. The female bag worm transforms into a wingless moth, which actually looks much like its larva and the male transforms into a winged moth.
Because the bag worm goes unnoticed until it is mature, it is tough to control. If there happens to be pesticide residue on surrounding foliage, the mature larvae may pupate early. The bag worm has few known predators and even fewer known parasites, so some of the most effective control measures often involve chemicals.
According to the Ohio State University Extension Center’s Entomology Department, there are four effective control options including:
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·Timed Sprays Using Degree-Day Emergence
·Mechanical Hand Picking
·Bacterial Spray Bacillus thuringiensis (Bt)
Stomach insecticides are very useful for control of bag worms. Remember that the plant foliage is to be thoroughly covered because the larvae are protected from contact by being in the silk bag. Again, early sprays against young larvae are more effective than later applications. Products registered for bag worm control are: acephate (Orthene), bendiocarb (Ficam, Turcam)(*), bifenthrin (Talstar), carbaryl (Sevin), chlorpyrifos (Dursban), cyfluthrin (Tempo)(*), diazinon, dimethoate (Cygon), fluvalinate (Mavrik)(*), malathion, nicotine sulfate, pyrethrum, permethrin (Pounce)(*), rotenone and trichlorfon (Dylox, Proxol)(*).
*Timed Sprays Using Degree-Day Emergence
Using a base temperature of 14.4 degrees C (57.9 degrees F) the median first emergence is 380 DD base 14.4 degrees C (716 DD degrees F) and the median last emergence is 572 DD base 14.4 degrees C (1062 DD degrees F). Allow for an extra week of ballooning after the last emergence mark has been reached before applying Bt or an insecticide. A less precise degree-day model uses a 50 degrees F base, which calculates last emergence around 900 DD units.
*Mechanical Hand Picking
If the bags are few in number and easily reached they may be picked off the plant and squashed. This can be done easily in the late fall when deciduous foliage has been dropped or the bits of plant material on the bags turn brown and can be easily located on evergreens. Be sure to cut the attachment silk band so that the branch will not be girdled in the future.
*Bacterial Spray Bacillus thuringiensis (Bt)
Bt is effective against bag worms if it is used against young larvae. Applications should be made at the end of June after all the eggs have hatched and the larvae are through ballooning.
*From Ohio State University Extension Center, Entomology Department, D.J. Shetlar