Photo: Joe Boggs
The non-native Box Tree (Boxwood) Moth (BTM) (Cydalima perspectalis, family Crambidae) was confirmed in southwest Ohio last year. Maps of the BTM quarantine in Ohio as well as quarantined areas elsewhere in the U.S. can be viewed by clicking this hotlink:
https://go.osu.edu/btm-quarantinemap
It’s important to periodically inspect boxwoods for BTM feeding damage regardless of whether they are in quarantine. To aid in detecting BTM, please re-read the following BYGL Alert: “Box Tree Moth (BTM) Update #1,” posted on April 17, 2024. This Alert covers BTM biology and includes a pictorial guide on what to look for in detecting BTM:
https://bygl.osu.edu/index.php/node/2335
Last season, BTM male moth traps revealed two general “hot spots.” One was in and around Loveland, an eastern suburb of Cincinnati, and the other was in and around Kettering, a southern suburb of Dayton. Subsequently, heavy caterpillar infestations have been discovered in and near these hot spots.
This season, we’ve observed some expansion of the BTM hot spots. Whether this is due to natural spread, or the discovery of undetected infestations is not known. However, owing to BTM’s multiple generations, damage is rapidly ramping up to produce some dramatic impacts and questions about BTM management.
BTM caterpillars feed exclusively on boxwoods. BTM can produce rapid defoliation and the rapacious caterpillars may feed on the bark of the twigs once all of the foliage is consumed. This reduces the capacity for the shrubs to sprout new growth and leaves from buds.
If the caterpillars exhaust their boxwood food supply in a landscape, they may be observed crawling on or dangling from other plants. The image below doesn’t show a magical levitating BTM caterpillar. The caterpillar was dangling from a silk thread attached to a magnolia. Of course, there was no evidence of feeding damage on the magnolia leaves.
BTM can kill boxwoods, but only if BTM remains undetected and action is not taken. This Alert focuses on BTM management. What should you do now if you live in or near a location with heavy caterpillar infestations?
BTM Suppression: Tips and Tools
Host plant resistance provides the best long-term management option for any significant insect plant pest. Although anecdotal, evidence that all boxwoods are not equal in the compound eyes of BTM continues to be observed in Ohio. The images below were taken in landscapes where other boxwoods were being heavily damaged by BTM.
Where boxwoods are under threat from BTM, there is a wide range of insecticides that have proven effective against caterpillars if used properly. Lists of insecticides are presented at the end of this Alert.
However, don’t make insecticide applications unless the boxwoods are infested with BTM caterpillars, or if BTM presents a clear and present danger because BTM caterpillars have been found in nearby landscapes. Preventative insecticide applications where BTM has not been found are not justified. Such applications waste money and present environmental risks including killing beneficial insects that can help to keep boxwood pests in check.
There are multiple examples of BTM being successfully suppressed in southwest Ohio and elsewhere using insecticides. However, there have also been examples of dramatic failures with using insecticides to protect and support boxwood health.
Insecticide Failures. Let Me Count the Ways
1. Failure to Detect. The goal behind making insecticide applications is to halt BTM caterpillar damage to promote boxwood recovery. It’s critical to apply insecticides early in a BTM infestation to reduce substantive damage such as bark stripping that can hinder recovery.
However, BTM caterpillars can be sneaky. Infestations commonly start with the caterpillars feeding deep within the canopy on older leaves. They gradually move upward on the stems to feed on newer leaves. This “inside–out” feeding pattern may allow BTM caterpillars to remain hidden until their damage becomes significant.
The images below illustrate the point. The first image shows boxwoods that appear healthy and free of BTM. However, parting the foliage revealed a sizable BTM caterpillar population.
BTM caterpillars can even crawl beneath our radar under high population conditions. The image below shows heavy BTM feeding damage on boxwoods that appeared green only a few weeks before I took these pictures on Monday in Kettering.
There was a high caterpillar population. However, the hungry horde remained cloistered within the lower foliage until the caterpillars burst forth to consume the newest foliage. This is illustrated in the images below. Note the “inside-out” feeding pattern. The boxwoods below will soon look like the boxwoods above unless the BTM caterpillars are suppressed by an insecticide application.
The take-home message is that boxwoods growing in a “hot spot” should be thoroughly inspected periodically to detect BTM. Parting the foliage may reveal a massive infestation lurking just below the surface.
2. Failure to Use the Proper Rate. Effective insecticides can be rendered ineffective if applied at the wrong rate. Insecticide labels provide clear instructions on mixing rates that will deliver an effective dosage of the product to achieve the desired results.
However, it’s easy to make an error in the calculations, particularly with “stepping down” the rate for low-volume applications. The application rate on the product label may be given as the amount of the product to be mixed in 100 gallons. Of course, if the application method is a 4-gallon backpack sprayer, applicators must do a little math to reduce the amount for the smaller spray volume.
Different parts of a boxwood planting may receive different application rates through skips and costly oversprays. It’s important to provide thorough, consistent coverage of the entire planting. An easy solution is to use a colored spray indicator that shows applicators where they have sprayed. Many spray indicators are designed to break down quickly so the colorant doesn’t linger after it has done its job.
3. Equipment Use Failures. Another common error is a failure to spray using sufficient pressure to penetrate to the target. For example, Btk is a “stomach poison” that must be consumed by the BTM caterpillars. Young BTM caterpillars commonly feed within small structures made from leaves and debris tied together with silk, so the application must be made at sprayer pressure that will penetrate such structures.
It's helpful to make some test applications over a small area to assess spray penetration before making an area-wide application. A colored spray indicator can clearly show whether or not the pressure is sufficient to penetrate to all target areas.
Another equipment error is a failure to keep the spray mix agitated. Btk products again provide a good example. The spray mixture is a diluted suspension of the bacterium in water. The product may settle to the bottom of the sprayer if the spray mixture is not agitated periodically. The first half of the application delivers a higher rate of the product compared to the last half of the application. Some backpack sprayers have built-in agitators to mitigate the problem.
4. Failure to Re-Apply if Needed. Although the insecticides listed below have proven to be effective against caterpillars, providing season-long protection may require two or more applications. It’s rare for an insecticide application to provide 100% efficacy, so BTM survivors may rekindle an infestation after the insecticide residue has lost potency. Also, female moths may migrate into a landscape from a nearby infestation to lay eggs.
For example, the naturally occurring biological insecticide Bacillus thuringiensis var. kurstaki (Btk) (e.g., Dipel, Thuricide, etc.) is a biorational insecticide that’s highly effective against young caterpillars. If there’s a mix of instar stages, later instars may survive. Thus, it’s imperative to closely monitor infested boxwoods long after a Btk application to gauge whether a second, or even third, application is required to eliminate a BTM infestation. Of course, the same is true with any insecticide.
BTM Insecticides
Insecticides that have proven effective against caterpillars if used properly are listed in the following tables. The lists were drawn from a multi-state IR-4 project fact sheet (Frank et al. 2022) and from the N.Y. State Integrated Pest Management Program (Eshenaur and Leeser 2024).
The first set of tables are insecticides commonly available to home gardeners. The first table shows “biorational” insecticides with relatively low mammalian toxicity and few environmental side effects including reduced impacts on non-target arthropods. The second table shows “traditional” insecticides. The next two tables show biorational and traditional insecticides for horticulture professionals.
DISCLAIMER: The information provided in the tables above is presented with the understanding that no product discrimination is intended, and no endorsement of any product mentioned, or criticism of unnamed products is implied. Labeling, registrations, and uses can change. The label is the law: applicators are responsible for reading and following all label directions.