A short time ago I received an email encouraging me to write about bagworms. I haven’t written about them here and they are quite evident now so I’ll gladly address the subject.
There are numerous types of bagworms, but the one of greatest importance to our area is the evergreen bagworm. This is somewhat of a confusion to many. Evergreen bagworms prefer coniferous evergreens like junipers, arborvitae and Leyland cypress, but they also attack broadleaf evergreens and even deciduous trees and shrubs.
These insects are more likely to cause lasting injury or even death to coniferous species though. Bagworm larvae attack branch tips first. As larvae grow larger, feeding damage becomes more noticeable. Severe infestation can kill host plants, especially evergreen species because their leaves do not sprout and replenish as readily as those of deciduous species.
Despite the name, bagworms are not worms at all. They are moths. The larval stage is wormlike, so that’s where the name originates. The bagworm lives its entire life inside the security of its bag, which it constructs with its own generated silk and interwoven bits of foliage.
When the hatchling larvae emerge from their cocoons, they spin a strand of silk that catches the wind and drift to another place. Then they begin work on their protective cocoon. They weave foliage fragments into the silk they produce. The result is what we call the bags.
These voracious insects are often hard to spot until they have caused substantial damage, especially on conifers. The little camouflaged bags appear at branch tips. These structures look a little like buds or cones until we scrutinize them. Once the bags begin to turn brown and nod downward they are much easier to spot. However, at that point major damage could already have occurred.
Only the adult male moth leaves the security of the bag when it is ready to mate. Adult males are black with roughly clear wings. Length and wingspan are each about an inch long. Neither the male nor the female adult feeds. Females live a week or two while males emerge, find a bag with a female, breed and are dead within two days.
Females produce a pheromone to attract males or this process would be far less efficient. When females are finished laying their eggs they either mummify around them or exit the bag and die. Either way the eggs rest safely in these bags and each bag could contain up to 1000 eggs.
Bagworms do have enemies. Several species of parasitic wasps can help control them. Many birds also like them, but usually they don’t eat sufficient amounts early enough to prevent damage.
Several insecticides are effective at killing bagworms but early treatment is necessary. Biological insecticides, such as those containing the bacteria Bacillus thuringiensis can be effective too, but early treatment is still necessary. Once bags develop major damage has occurred. Inspect plants early in the growing season and hand pick and destroy bags whenever you see them. It’s a challenge.
Ted Manzer teaches agriculture at Northeastern High School (firstname.lastname@example.org).
What would be your reccomendation for prevention?
If plants are relatively small (10 feet tall or less I would try to build up a healthy Bt population by multiple applications of Dipel. If trees were too tall to apply a unifrm application of dipel I’d supplement it with a soil applied systemic. All the real effective ones are either very toxic restricted chemicals or ones that have been removed from the market. Temik worked great, but it’s been banned for years. I used to use oxamyl granular but it has been banned in the US too. Timing is important for effective control. That’s why the long term granular synthetics were so effective (and extremely toxic). Bi-weekly applications of Bt in May and June might be your best bet to keep bacterial populations high and emerging larva populations down. Sorry I couldn’t make things more simple, but these are some difficult to control critters.