Box Tree (boxwood) Moth (BTM) (Cydalima perspectalis, family Crambidae) caterpillars (larvae) develop through as many as 7 instar stages with the caterpillars molting between each instar. This is the damaging stage with the caterpillars feasting on boxwood leaves and sometimes stripping the bark of small twigs.
BTM caterpillars in southwest Ohio have undergone a rapid pace of development over the last two weeks. On June 28, the BTM caterpillars were in very early instar stages making them hard to find. Indeed, it was even difficult to detect new damage to boxwood leaves.
As shown in the lead photo as well as the photos below, by this past Friday (July 7), caterpillars had developed to a size making them easy to detect. Although there remained some caterpillars in the early instar stages, the majority had reached middle instar stages if not beyond. No doubt temperatures played a significant role in this rapid spurt in caterpillar development with daily high temperatures consistently topping 85F.
The rise of larger caterpillars means leaf-feeding damage is more apparent compared to just a week and a half ago. The ravenous caterpillars were focusing their attention on any green foliage left over from the previous generation. It illustrated the mounting damage that occurs with each successive generation.
Webbing was also much more apparent compared to previous observations. BTM caterpillars are not heavy silk producers like fall webworms (Hyphantria cunea); however, caterpillars may web leaves together to create a rudimentary protective structure.
Caterpillars were also observed on top of webbing or entangled within random strands of webbing. The silk commonly gave the foliage a slight halo appearance in slanting sunlight.
A Generational Thing
The total seasonal damage by a plant-feeding insect depends on the number of generations and the population densities for each generation. Normally, the overall population densities increase with each generation meaning more damage occurs with each successive generation. Knowing the number of generations is critical for developing and applying effective management programs.
We do not yet know the number of complete generations that BTM will have in Ohio. The scientific literature notes that BTM may have as many as 5 generations per season depending on climatic conditions. BTM spends the winter as 3 – 4th instar diapausing caterpillars which produces a split generation between years.
Two overlapping complete generations have been consistently observed in Canada. However, a PennState fact sheet notes that if BTM were found in Pennsylvania, the moths may have as many as 3 generations. It’s reasonable to assume we will see at least 2 generations in Ohio, possibly a 3rd or even beyond.
We suspect that we’re currently observing 2nd generation caterpillars based on pheromone trap catch data, caterpillar development observations, and boxwood damage. Traps were consistently capturing male moths until around two weeks ago when trap catches dropped off dramatically.
Only early instar caterpillars were observed on June 28; however, there was clear evidence of heavy boxwood damage that had occurred earlier in the season. This signaled that a generation had been completed. Whether it was the overwintering generation, or the 1st full generation is not known with certainty, but the overall timing relative to boxwood damage and caterpillar development makes the latter more likely.
We are also observing a range in caterpillar size. Whether this is an early indication of the development of overlapping generations is also not yet known.
What Should You Do About BTM?
1. DON’T Make Preventative Insecticide Applications to Boxwoods
BTM has only been found in one location in southwest Ohio. Thus far, that’s the only location where BTM presents a clear and present danger to boxwoods. Preventative insecticide applications are not justified where BTM has not been found.
Making unwarranted insecticide applications is not only a waste of money and product; the applications can also have unintended consequences including producing secondary pest outbreaks. For example, many “caterpillar insecticides” will also kill predatory mites that help keep Boxwood Spider Mites (Eurytetranychus buxi) in check. This mite has been on the rise over the past several years.
2. Monitor Boxwoods
BTM in southwest Ohio is far removed from the other known BTM infestations in North America. This means it can pop up anywhere. We must be aware of what to look for with BTM and remain vigilant. However, we also must be aware of the symptoms produced by other boxwood problems.
In our previous BYGL Alert titled, “What to Look for with Box Tree (Boxwood) Moth,” we covered how to separate BTM symptoms from those produced by other boxwood problems such as from winter injury and various boxwood diseases. You can access this Alert by clicking on this hotlink:
https://bygl.osu.edu/node/2188
3. If you see it, report it!
We need your help with discovering the geographical distribution of the known BTM infestations as well as learning if there are other BTM infestations in Ohio. This knowledge will guide the development of BTM management plans.
The Ohio Department of Agriculture (ODA) has developed an online reporting tool that includes many helpful features including an interactive map that you can use to place a "pin" on the exact location of your observation. The reporting tool requires a picture, so make certain you take clear, in-focus pictures to upload with your report.
Here is the hot like to the ODA reporting tool:
https://survey123.arcgis.com/share/1b36dd2cf09e4be0a79776a6104ce1dc
4. Act ONLY AFTER BTM is Officially Confirmed.
Again, do not spray unless you know that you’re targeting BTM. Preventative insecticide applications are a waste of money and can create more problems.
If BTM is confirmed in your landscape, the caterpillars are easy to kill; they’re not super cats! If you only have a few boxwoods, a highly effective option is to crush the caterpillars with your hand (gloves are optional). It’s doubtful BTM will develop resistance to this insecticidal effort.
Knocking the caterpillars off into a bucket of soapy water is problematic. The caterpillars are sticky. They are covered in a viscous material allowing them to stick to leaves. They are also commonly connected to silk in some form or fashion. They may be resting on silk or encased in a rudimentary silk nest. Either way, short, spikey hairs help to hold the caterpillars on the silk which makes knocking them off a challenge.
If boxwoods are too large, too many, or both to make the physical removal of BTM caterpillars practical, insecticide applications are an option. The multi-state fact sheet by Frank et.al. (2022) lists 21 products with numerous active ingredients “labeled for caterpillar management in nursery and/or landscape sites where boxwoods are grown.”
Selected References
Coyle, D.R., J. Adams, E. Bullas-Appleton, J. Llewellyn, A. Rimmer, M.J. Skvarla, S.M. Smith, and J.H. Chong, 2022. Identification and Management of Cydalima perspectalis (Lepidoptera: Crambidae) in North America. Journal of Integrated Pest Management, 13(1), p.24. Available at: https://academic.oup.com/jipm/article/13/1/24/6717904
Cook, J.C., T. Culliney, C.F. Funaro, and J.B. van Kretchmar (editor R. Hallberg). 2022. New pest response guidelines: Cydalima perspectalis, box tree moth. U.S. Department of Agriculture Report, 54 p. Available at: https://www.aphis.usda.gov/plant_health/plant_pest_info/box-tree-moth/box-tree-moth-nprg.pdf
Frank, S., D. Gilrein, M. Havers, and C. Palmer. 2022. Box Tree Moth: Fact Sheet, Management & Visual Guide, Fact Sheet, NC State/Cornell/Rutgers/IR-4 Fact Sheet, Available at:
https://ir4.cals.ncsu.edu/EHC/InvasiveSpecies/BTM_FactSheet_VisualGuide.pdf
Skvarla, M.J., 2023. Box Tree Moth. PennState Extension Fact Sheet, Available at: https://extension.psu.edu/box-tree-moth
Photo: Joe Boggs
Photo: Joe Boggs
Photo: Joe Boggs