This is the time of the year when Common Bagworms (Thyridopteryx ephemeraeformis) come into clear focus owing to their size and noticeable damage. Overwintered eggs hatched in southwest Ohio in early June (see "Be Alert to Bagworms!" posted on June 6). However, it's amazing how well these native moth caterpillars crawl below our radar until their burgeoning appetites finally gives them away.
I'm speaking from personal experience. I looked out our kitchen window this past weekend to see a hungry horde of bagworms chowing down on needles at the top of my prized Canaan fir (Abies balsamea var. phanerolepis). How dare they! The cheeky bagworms are no more; I have a sprayer.
Bagworms are so-named because the caterpillars wrap themselves in silk bags festooned with pieces of their plant host material. The arrangement provides highly effective camouflage.
The bagworms spend the winter as eggs inside dead females that remain in their bags. A single female can produce 500 - 1000 eggs meaning that populations can climb rapidly. Just a few undetected females can spawn damaging numbers of caterpillars next season.
Bagworms also have a habit of seeming to appear out of nowhere. That's partly because of their cryptic lifestyle but also because 1st instar caterpillars can produce a strand of silk to catch the wind and "balloon" to new locations. This behavior is one of the reasons bagworms often appear on hosts that were not infested last season.
Bagworms can also feed on deciduous trees and shrubs as well as evergreens. It is a common misconception that bagworms only eat evergreens. If fact, they are called "Evergreen Bagworms" in many southern states. However, the caterpillars may be found feeding on over 130 different species of deciduous trees and shrubs.
Bagworms can be controlled through physical removal as long as they are destroyed by squeezing or stepping on them once they are plucked from an infested plant. If simply dropped to the ground, they will crawl back upon a host plant. I would have used this method if the top of my Canaan fir wasn't so far out of reach. There's a certain satisfaction with doing the "bagworm dance" to dispatch the troublesome caterpillars.
Early instar bagworms can be killed using products based on the naturally occurring biological insecticide Bacillus thuringiensis var. kurstaki (Btk) (e.g. Dipel, Thuricide, etc.) which preserves bio-allies. The image below shows bagworms collected from the same host over the weekend. The age range is not unusual and occurred because of an extended egg hatch. However, it shows that Btk products will only be effective against a small segment of the population. Btk is most effective on small bagworms and becomes much less effective when bags surpass 3/4" in length.
The clock is also rapidly ticking down for standard insecticides to remain effective. Late instar bagworms can detect insecticide toxicants causing them to hasten pupation, but they do not die. However, they do stop feeding which leads to the perception they were killed. Early-pupating females produce fewer eggs, but they still produce enough to continue the infestation next season.
It's important to remember that bagworms may remain attached even if the caterpillars were successfully killed with an insecticide. They look just like bags with live caterpillars. The deception may lead to needless repeat applications, but they can only die once.