Participants in today’s OGIA/OSU Greater Cincinnati Diagnostic Walkabout held at the Boone County Arboretum, Union, KY, discussed the current developmental condition of bagworms (Thyridopteryx ephemeraeformis, family Psychidae). The caterpillars have completed their development and “tied off,” marking a significant shift in management options.
On the upside, it means the caterpillars have stopped feeding, so bagworm damage has ceased for this season. On the downside, insecticide applications will no longer work this season even though the bags remain evident.
Don’t be fooled! The caterpillars are no longer feeding, so they won’t consume insecticides and no toxicant can penetrate the bags. Management now centers on a more hands-on approach.
Life in the Bag
Both male and female bagworm caterpillars develop through 7 instars inside silk bags festooned with pieces of their host plants. The host plant debris woven into the silk provides structural stability and highly effective camouflage.
The bags have two openings at opposite ends. The caterpillars use the large upper opening to poke their heads out to feed and enlarge their bag abodes. They avoid plummeting from their plant hosts by anchoring their bags to their plant hosts with a small bit of silk.
The lower opening serves as a toilet, which allows the caterpillars to shove out fecal pellets (frass). Otherwise, their bags would gradually become a hefty bag-o-poo, pulling the caterpillars from their hosts towards extinction.
The male and female caterpillars take very different paths as they develop. Male bagworms develop into a fly-like moth, and they are capable fliers. Their pupae look much like other moth pupae, and they wiggle part of the way out of the potty hole to allow the male moths to emerge. Seeing the empty pupal skins means male moths are on the wing.
Female bagworms never develop into a moth-like insect. They remain inside their bags and develop into something that looks more worm-like than moth-like. The mature bagworm females have no wings, no apparent mouthparts, no antennae, and three pairs of very short, dysfunctional legs. Since the females can’t leave their bag, they draw in males by emitting a chemical attractant (sex pheromone).
The males have large antennae used to detect and track the parfum de l’amour wafting from the female bags. Once found, the males mate with the bag-bound females.
Soon after mating, the female produces overwintering eggs that fill the female’s pupal case snug inside of their mother's old bag where they remain until the following season. Each female is capable of producing 500 – 1,000 eggs, which explains why populations can build rapidly.
Since females never fly, the spread of bagworms occurs in two ways. A percentage of the 1st instar caterpillars will produce a strand of silk to catch the wind and "balloon" the tiny caterpillars to establish new bagworm beachheads. This behavior is a key reason bagworms often appear out of the blue (literally!) on hosts far from trees and shrubs that were infested the previous season.
Bagworms also exploit new horizons by the caterpillars simply crawling from the hatched eggs. This produces “hot spots” within and between infested plants. It’s the reason landscape managers should apply the BAT management method described below.
The 4-Step Bagworm Annihilation Technique (BAT)
This season’s bags are a handy target for removal and creative destruction. Removing and destroying the bags can deplete the population and avoid damage next year. Indeed, Kris Stone (Director, Boone County Arboretum) noted that a mass removal project had already been applied, which was evident by the low number of bags found by Walkabout participants.
Following are the four steps in the BAT method. The management technique can be applied anytime from now until eggs hatch late next spring. Of course, using the BAT is only practical on small trees and shrubs.
Step 1:
Closely examine infested plants. Keep in mind that bagworms can be camouflaged. Also, a common misconception is that bagworms only eat evergreens. In fact, bagworms may feed on over 125 species of evergreen and deciduous woody plants in 45 plant families. Overlooking bagworms on deciduous trees and shrubs allows the plants to become reservoirs for infestations next season.
Step 2:
Remove the bagworms and place them in a bag (bag-o-bagworms). It’s important to be thorough. Missing one live female, or a dead female with eggs, means there is the potential for up to 1,000 new caterpillars to arise from overwintered eggs next year.
Step 3:
Place the bagworms on a hard surface.
Step 4:
WARNING: This step is rated “V” for graphic insecticidal violence. Some viewers may find it disturbing. Viewer discretion is advised.
Apply direct compressive compaction to the plucked bagworms. 5.5 lbs. per square inch* was applied in the demonstration pictured below and was more than sufficient to overcome the compressive strength of bagworms (i.e., squash resistance). Thus far, no bagworm populations have become resistant to the BAT.
* Calculation: Compression load of 220 lbs. divided by 40 sq. inches, which is the average size of a 13 shoe sole area.
