A Pictorial Guide to BTM

There are no native pests that specifically defoliate boxwoods in Ohio and only a small number of caterpillars that may occasionally nip boxwood leaves. Defoliation by a caterpillar would point towards BTM.

Striped Caterpillars

The distinctive BTM caterpillars are green to greenish-yellow with black stripes and black dots. They display this color motif throughout their development. There are no native caterpillars that defoliate boxwoods.

Strange “Curlicue” Leaf Damage

As BTM caterpillars get larger, they start consuming the entire leaf. However, they often leave behind the leaf edges and sometimes the midvein. The tissue curls to produce a characteristic hair-like curlicue symptom that turns brown. The unusual damage is easy to spot and there is nothing else that produces this symptom on boxwoods.

Silk Webbing

BTM caterpillars are capable of producing silk; however, they are not heavy silk producers like fall webworms (Hyphantria cunea). BTM caterpillars may be found in wispy collections of silk presumably anchoring them to their host. Their webbing may also entangle feed debris and frass giving infested boxwoods a dog-eared appearance.

BTM caterpillars pupate on their boxwood host. Before pupation, the last instar caterpillars use silk to wrap themselves in whole live or dead leaves as well as leaf debris. They then spin a loose silk cocoon and pupate. These pupation structures remain evident for some time.

Frass

Insect excrement is called “frass.” BTM frass looks like small, light green to greenish-brown pellets. The frass may collect beneath infested plants or as noted above become trapped in caterpillar silk.

Bedraggled, See-Through, “Stick Shrubs”

Ultimately, if left unchecked, the voracious feeding by BTM caterpillars produces complete defoliation on boxwoods. It’s the “signature” symptom of BTM and it’s easy to spot from a distance. A closer examination will reveal that there are few leaves on the ground, only debris. Heavy BTM damage may kill boxwood plants.

Whether or not leaves have been consumed provides an important distinction between leaf damage caused by BTM and leaf damage from lingering winter injury, or damage from an infectious disease. If a disease is suspected, samples should be sent to our OSU C. Wayne Ellett Plant and Pest Diagnostic Clinic (CWE-PPDC) for confirmation before action is taken.

Damage by the non-native Boxwood Leafminer (Monarthropalpus flavus, family Cecidomyiidae) is beginning to appear on its namesake host in Ohio. Overwintered larvae (maggots) of this leaf-mining fly have awakened and their feeding is in hyper-drive. It’s important to note the damage is confined to the spring and typically becomes masked by new growth later this season.

Gazing Into the Boxwood Chrystal Ball

With the multiple discoveries of BTM in Canada and the U.S., it’s unlikely that BTM will be eradicated in North America. However, it’s also unlikely that BTM will follow the same destructive trajectory as in Europe.

Boxwoods are not native to the temperate zones of North America. We don’t find boxwoods growing in our forests to serve as BTM bridges between our landscapes. Rather than a sea of boxwoods, our collections of boxwoods are more like islands. They are commonly somewhat isolated from one another.

BTM can be effectively managed with insecticides. The caterpillars are not super cats; they’re easy to kill. However, many of the effective insecticides available in the U.S. are no longer labeled for use in Europe. So, we have more weapons against BTM.

Other management options are also being pursued. As noted above, a synthetic version of the sex pheromone used by females to attract males is being used in traps to detect and monitor BTM. The pheromone may also be used in the future for “mating disruption.” The goal is to flood an area with wafting pheromones to confuse male moths so that can’t find and mate with females. Thus far, preliminary trials look very promising.

It's also becoming evident that all boxwoods are not equal as BTM hosts. Significant differences in BTM host suitability between boxwoods have been observed in other BTM infestations as well as our own in southwest Ohio.

Indeed, the authors of a Virginia Cooperative Extension publication (2022, see Selected References below) note, “Field observations indicate that certain cultivars, such as ‘Winter Gem,’ are prone to BTM defoliation. It was also documented that other cultivars such as ‘Green Mountain’ appear to be less susceptible to damage from this insect.”

The inherent genetic diversity of boxwoods has played an important role in using resistance to respond to a range of issues from Volutella blight/canker to boxwood blight. BTM host preference trials are currently being planned. Thus, a long-term BTM management strategy may involve plant selection.

Finally, BTM has not been in North America long enough to assess the potential impact of the “3-P’s” (Predators, Parasitoids, and Pathogens). However, during multiple visits to our southwest Ohio infestation, we have observed yellowjackets (Vespula spp.) grinding up BTM caterpillars to feed the balls of protein to immature yellowjackets back in their nests.

What Should You Do?

1. DON’T Make Preventative Insecticide Applications to Boxwoods:

Insecticide applications should only be made to boxwoods with an identified BTM infestation, or to those in very close proximity to a BTM caterpillar infestation. Thus far, the most likely target would be boxwoods growing where BTM has been confirmed. Preventative insecticide applications outside the locations where BTM has been found are not justified.

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.

3. If you see it, report it!

It is essential that we discover the geographical distribution of BTM 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 such as 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 hotlink to the ODA reporting tool:

https://survey123.arcgis.com/share/1b36dd2cf09e4be0a79776a6104ce1dc

4. Management

As noted above, BTM caterpillars can be killed using a wide range of insecticides. Of course, as always, READ AND FOLLOW LABEL DIRECTIONS!

Homeowners: Over-the-counter products with the following active ingredients are effective against BTM:

- Synthetics: carbaryl, acephate, bifenthrin, cyfluthrin, esfenvalerate, permethrin, resmethrin, and tetramethrin

- Botanical: pyrethrin

- Microbial: Bacillus thuringiensis, Bt kurstaki, and Bt aizawai; most effective on small caterpillars, Spinosad.

Professionals: A complete listing is available by clicking on this hotlink:

https://ir4.cals.ncsu.edu/EHC/InvasiveSpecies/BTM_FactSheet_VisualGuide.pdf

Highlighted insecticides available to professionals include those listed above for homeowners as well as chlorantraniliprole (e.g., Acelepryn, Mainspring GNL) and dinotefuran (e.g. Safari, Transtect). Both are systemic and one Virginia Tech study indicated that dinotefuran may produce egg mortality.

Selective References

Nacambo, S., F.L. Leuthardt, H. Wan, H. Li, T. Haye, B. Baur, R.M. Weissand M. Kenis, 2014. Development characteristics of the box‐tree moth Cydalima perspectalis and its potential distribution in Europe. Journal of Applied Entomology, 138(1-2), pp.14-26.

Santi, F., P. Radeghieri, G.I. Sigurta, and S. Maini, 2015. Sex pheromone traps for detection of the invasive box tree moth in Italy. Bulletin of Insectology, 68(1), pp.158-160.

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

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

Bras, A., Lombaert, E., Kenis, M., Li, H., Bernard, A., Rousselet, J., Roques, A. and Auger-Rozenberg, M.A., 2022. The fast invasion of Europe by the box tree moth: an additional example coupling multiple introduction events, bridgehead effects and admixture events. Biological Invasions, 24(12), pp.3865-3883.

Sisk, E., J. Brindley, and A. Del-Pozo. 2022. Box Tree Moth in the United States, Virginia Cooperative Extension, ENTO-525NP. Available at:

https://www.pubs.ext.vt.edu/content/pubs_ext_vt_edu/en/ENTO/ento-525/ento-525.html

Skvarla, M.J., 2023. Box Tree Moth. PennState Extension Fact Sheet, Available at: https://extension.psu.edu/box-tree-moth

2022 Pest Alert: Box Tree Moth (Cydalima perspectalis), March, USDA PPQ. Available at:

https://www.aphis.usda.gov/publications/plant_health/alert-box-tree-moth.pdf

Ron Wilson