Box Tree Moth (BTM) (Cydalima perspectalis, family Crambidae) is a non-native pest of boxwoods (Buxus spp.). Yesterday, the Ohio Department of Agriculture (ODA) announced that a BTM sighting has been confirmed in Montgomery County (Dayton Area). The confirmation was based on adult moths.
BTM was first confirmed in Ohio by the ODA in late June based on adult moths. Several caterpillar infestations have now been discovered in the area where moths are continuing to be trapped. The location is in the southwest part of the state near the conjunction of Hamilton, Clermont, Warren, and Butler Counties. Coupled with the Montgomery County detection, the two BTM sites are the southernmost in North America.
Why Worry?
The current inventory of boxwoods being grown in Ohio nurseries has an estimated value of over $200 million. BTM could have a significant impact on the state’s nursery industry through the loss of sales coupled with costs associated with pest management.
BTM is also a proven boxwood killer. Boxwoods were some of the oldest plants used in U.S. landscapes with planting records dating back to the late 1700s. They are currently some of the most common plants found in Ohio landscapes. Quoting from the USDA APHIS, 2022 New Pest Response Guidelines (see “Selected References” below): “In introduced areas where box tree moth has 2 generations per year, boxwood stands have declined over 95% in 8 years or less.”
A BTM Life Cycle Primer
Like other moths and butterflies (order Lepidoptera), BTM develops from eggs to adults through complete metamorphosis (eggs, larvae (caterpillars), pupae, adults). The caterpillars must shed their skins as they grow. It’s something like needing to buy new clothes for our children as they grow. Each time the caterpillars molt, they advance into a new instar stage.
According to the literature, BTM caterpillars may develop through as many as 7 instars. Of course, as the caterpillars become larger, they produce more damage. Thus, each instar stage produces more damage than the previous stage.
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. Thus, knowing the number of generations is critical for developing and applying effective management programs.
However, we do not yet know the number of complete generations that BTM will have each season in southwest Ohio. BTM spends the winter as 3 – 4th instar diapausing caterpillars which means there is a split generation between years.
Not counting the split generation, the scientific literature notes that BTM may have as many as 5 generations per season depending on climatic conditions. BTM produces 2 overlapping generations in Toronto, Ontario, and in Niagara County (Buffalo), NY.
Thus far, it appears that we may experience as many as 4 complete generations in southwest Ohio. However, this is based on trap catches and observing caterpillar development rather than a scientific study.
The caterpillars cause the damage while the adults mate, generate the next generation, and support natural spread. BTM females produce a chemical attractant (sex pheromone) to gain the attention of male moths. The BTM pheromone blend has been synthesized and is highly effective in detecting BTM infestations. It may also be used to manage BTM through mating disruption, but more about that later.
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.
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.
Caterpillar Hibernaculum Structures
As the BTM season ends, the caterpillars draw leaves together to construct overwintering structures called a “hibernaculum” (plural = hibernacula). The image below shows a hibernaculum. Note that the leaves must be pulled apart to expose the silk structure found within.
In the Shade
Early BTM leaf-feeding damage is commonly heavier on shaded foliage compared to foliage in full sun. This general pattern has been observed in multiple locations. Of course, eventually, entire plants are defoliated.
Skeletonized Leaves and Caterpillar "Nests"
Early instar BTM caterpillars commonly feed on the upper or lower leaf surfaces as skeletonizers. They may also feed along the edges of leaves to produce notches. The skeletonizing and leaf notching are subtle symptoms and may be difficult to detect in the early stages of an infestation. However, early instar caterpillars may also produce rudimentary “nests” which are much easier to spot.
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.
Bedraggled, See-Through, “Stick Shrubs”
Ultimately, 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 as shown below.
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 progressive damage by the opportunistic fungal disease known as Volutella Canker caused by two fungi, Pseudonectria buxi, and P. foliicola. This disease is not typically a boxwood killer unless infections are allowed to progress into the basal stem.
Stem dieback from Volutella is continuing and may be mistaken for BTM. However, boxwoods retain whole, browned leaves if the damage is from the cankering disease. Affected stems should be pruned to halt fungal cankering and support plant recovery.
On a side note, while Boxwood Dieback produced by the fungus, Colletotrichum theobromicola, has been confirmed in Ohio, it's rarely encountered, and symptoms appear much earlier in the season. Volutella remains the dominant boxwood disease in our state.
Still, if Boxwood Dieback 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. We’ve had reports of recommendations that boxwoods be removed and destroyed without the disease being confirmed. It would be a costly error if the boxwoods are disease-free, or suffering from Volutella canker which can be managed through targeted pruning.
Silk Webbing
As shown below, BTM caterpillars produce silk. They are not heavy silk producers like fall webworms (Hyphantria cunea); however, early instar caterpillars may produce small silk nests. Later instars produce wispy collections of silk entangling feeding debris.
Frass
Insect excrement is called “frass.” BTM frass looks like small, light green to greenish-brown pellets and may collect within the foliage or drop to the ground beneath infested plants.
Pupal Structures
Prior to pupation, the last instar caterpillars use silk to wrap themselves in boxwood leaves that are still on the plant. They then spin a loose silk cocoon and pupate. These pupation structures remain evident for some time and may consist of live or dead leaves as well as leaves the caterpillars have fed upon.
Gazing Into the Chrystal Ball, Not Standing Behind the Eight-Ball
With the latest BTM discovery in Ohio and the previous discoveries in Ontario, Canada, as well as in Massachusetts, Michigan, and New York, it’s becoming evident that it’s unlikely that BTM will be eradicated in North America. Fortunately, BTM can be effectively managed with insecticides. The caterpillars are not super cats; they’re easy to kill.
Other management options are also being pursued. As noted above, synthetic versions of the sex pheromone used by females to attract males have been developed. While the synthetic attractant is helpful for detecting and monitoring BTM, it may also be used in the future for “mating disruption.” The goal is to flood an area with wafting pheromone 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 here in southwest Ohio. The images below show a boxwood hedge with no caterpillar damage that is planted in close proximity to a sizable infestation.
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.”
BTM host preference trials are currently being planned. Thus, a highly effective, environmentally friendly 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 continue to detect new BTM infestations in Ohio as well as learn the geographical distribution of BTM within the state. This includes continuing to report caterpillar infestations even if they occur in an area where BTM has been confirmed. 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. Bt products are most effective on small caterpillars.
Professionals: A complete listing is available by clicking on this hotlink:
https://ir4.cals.ncsu.edu/EHC/InvasiveSpecies/BTM_FactSheet_VisualGuide.pdf
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