This is the second BYGL Alert this season focused on Bagworms (Thyridopteryx ephemeraeformis, family Psychidae). You can read the first Alert titled, “Bagworm Egg Hatch: The Game’s Afoot!” that was posted on May 29 by clicking this hotlink: https://bygl.osu.edu/node/2363
This Alert is based on a site visit that I made yesterday and centers on the common misconception that bagworms only feed on evergreens. Although bagworms are frequently called "evergreen bagworms" in many southern states, the caterpillars may feed on over 125 species of evergreen and deciduous woody plants in 45 plant families.
My friend Mark Webber (Mark Webber’s Arboricultural & Horticultural Consultancy, LLC) alerted me to heavy bagworm feeding damage on columnar sweetgums (Liquidambar styraciflua 'Slender Silhouette') in southwest Ohio. Nearby conifers remained untouched further emphasizing the point that the bagworm’s ravenous appetite isn’t confined to evergreens.
Several years ago, I took pictures of bagworms destroying a small crabapple in northern Kentucky. After the caterpillars had defoliated the tree, they started chewing through the bark on small branches to consume the sugar-rich phloem. The canopy dieback was so severe the tree was replaced the following season.
In 2017, I posted a BYGL Alert about a “drive-by diagnostics” blunder involving bagworms on European hornbeams (Carpinus betulus ‘Fastigiata’). I had driven past four hornbeams that appeared to be riddled with Japanese beetle (Popillia japonica) damage. Hornbeams are common table fare for the beetles, and we had high beetle populations that year.
I turned my car around to snap a few shots of the damage as well as the coleopteran culprits. However, during the time it took for me to park and walk to the trees with my camera, the coleopteran damage somehow morphed into lepidopteran bagworm damage. It was the most complete of any complete metamorphosis that I’ve ever seen!
Life in a Silk Sock
Bagworms are one of the sneakiest general defoliators found in Ohio landscapes. The caterpillars fashion “bags” made of silk and festooned with pieces of their host plants. As the caterpillars mature, they begin weaving more host plant debris into the silk which provides structural stability as well as highly effective camouflage.
Adding to the subterfuge, male and female bagworm caterpillars spend their entire larval development inside their silk bags. So, unless you look closely at the bags (and have patience), you don’t see the caterpillars, just the bags.
The bags have two openings at opposite ends. The opening at the top of the bag gives the caterpillars access to their food. They create temporary “tie-offs” out of silk to attach their bags to their host.
This means the caterpillars don’t need to hang on with their tarsal claws giving them the freedom to extend their bodies outward to feed. The anchor points also allow the caterpillars to retreat into their safe silk havens when threatened without the bags and caterpillars crashing to the ground.
The opening at the bottom end of the bag serves as a toilet allowing the caterpillars to shove out fecal pellets (frass). Otherwise, their bags would gradually become loaded down with frass and the bag-o-poo would eventually pull caterpillars from their hosts towards extinction.
Check Your Deciduous Trees!
The bagworm’s cryptic life style isn’t the only thing that allows bagworms to creep beneath our radar. The take-home message is the mistaken belief that that bagworms only eat evergreens can allow considerable damage to develop on deciduous trees and shrubs before the caterpillars are detected.
Overlooking deciduous trees and shrubs during bagworm inspections also increases the risk that infested plants can become bagworm reservoirs supporting the future spread onto evergreen as well as deciduous hosts. The picture below shows an abandoned bag on sweetgum left-over from last year next to a bag occupied by a ravenous caterpillar this season. The old bag was a harbinger of bad things to come.
Direct Management: The Bagworm Two-Step
Early-instar bagworms can be effectively controlled with a range of insecticides including biorationals. However, larger later-instar caterpillars are more difficult to kill, and bagworm development may be asynchronous. At this time of the year, small caterpillars are commonly found feeding alongside larger caterpillars making insecticide efficacy problematic.
Fortunately, there is another highly effective management option at hand that involves digital extraction coupled with compression; also known as the “bagworm two-step.” Of course, this management approach is most effective on small trees and shrubs.
The hands-on bagworm suppression technique is demonstrated through a serious of images below. The first image shows bagworms that have been digitally extracted.
Warning: Viewer Discretion is Advised. The following images depict violence and are intended to be viewed by a mature audience.
The first step of the “bagworm two-step” is positioning the bagworms on a firm surface. The image below shows the proper positioning to maximize the efficacy of the second step (a.k.a. the “compression step”).
The second step of the two-step is shown below.
As shown below, the compression step is highly effective. Thus far, no bagworm populations have developed resistance to the bagworm two-step.
