Box Tree Moth (BTM) (Cydalima perspectalis, family Crambidae) is a non-native pest of boxwoods (Buxus spp.). BTM is native to a wide geographical range from the Koreas across China and the Indochina peninsula into eastern India. Not coincidently, this is the same geographical range for the so-called “Asian boxwoods.”
BTM is not considered a significant pest of native boxwoods in its Asian range. Presumably, this is due to the natural development of a range of 3-Ps (Predators, Parasitoids, and Pathogens) that target BTM.
Also, the relatively narrow BTM host range meant that the moths and their primary host co-evolved. Thus, it's likely that natural selection imbued Asian boxwoods with some form of defense(s) against its lepidopteran nemesis.
BTM was discovered in Europe in Germany in 2007 and has spread quickly across the continent ravaging the native European boxwoods. The rapid spread of BTM was mostly likely abetted by several factors including the inability of the native boxwoods to defend against the non-native BTM. This is common when a native host appears on the dinner plate of a non-native pest.
It's also common for non-native pests to rapidly develop highly damaging populations once they are free of the population suppression effects exerted by natural enemies (the 3-Ps). This “pest release” has been observed and documented with other non-natives.
Finally, native European boxwoods are commonly found in wooded areas between urban sites where boxwoods were planted in landscapes. The wild boxwoods served as a “bridge” between urban centers and vice versa.
BTM in Ohio
BTM was first confirmed in Ohio by the Ohio Department of Agriculture (ODA) in late June 2023 based on an adult captured in the southwest part of the state in Loveland near the conjunction of Hamilton, Clermont, Warren, and Butler Counties. Adult moths were subsequently trapped in Montgomery and Greene Counties in September 2023. The southwest Ohio infestations are the southernmost BTM populations currently known to occur in North America.
As noted in last week’s BYGL Alert titled, “ODA Issues Quarantine in Southwest Ohio for Invasive Box Tree Moth,” a BTM quarantine is now in effect for four Ohio counties (Butler, Clermont, Hamilton, Warren) and parts of two other counties (Montgomery and Greene). 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
The overarching purpose of a plant pest quarantine is to prevent the further spread of the regulated pest through the movement of infested plant material. Thus, boxwoods are not allowed to be moved outside the Ohio BTM quarantine zone. Boxwoods can be moved within the zone.
However, to reduce the business impacts for nursery stock producers located within the BTM-regulated area, the ODA, in partnership with the United States Department of Agriculture (USDA), has developed a Box Tree Moth Compliance Agreement. The agreement requires nursery stock producers to work closely with their ODA Nursery Inspector to implement a multi-step BTM pest management plan developed by the ODA/USDA in conjunction with research scientists.
Strict adherence to the agreement allows shipments to be made outside of the quarantine zone. Nursery producers can learn more about the agreement by contacting their ODA Nursery Inspector.
Potential BTM Impact
BTM feeding damage is confined to boxwoods. Although the scientific literature lists a few other hosts in Asia, BTM has not been found feeding on any other host in North America besides boxwoods.
BTM is a potential boxwood killer if infestations are allowed to develop and expand unchecked. 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.” Of course, BTM caterpillars are relatively easy to kill, so their impact can be mitigated if suppressed before they cause significant harm.
A BTM Life Cycle Primer
Like other moths and butterflies (order Lepidoptera), BTM develops from eggs to adults through complete metamorphosis. 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 instar stages although some published scientific papers report that the caterpillars complete their development after 6 instars. Of course, as the caterpillars become larger, they consume more foliage to 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, throughout the growing season, 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.
BTM produces a “split generation” with eggs hatching in the fall and caterpillars feeding before overwintering in a specialized structure called a hibernaculum (pl. hibernacula). The caterpillars aren’t hibernating like bears, they are in a physiological “deep sleep” called diapause. The literature consistently notes that the overwintering caterpillars are in the 3rd or 4th instar stages. However, caterpillars in several locations in southwest Ohio appeared to be 2nd instars. The apparent discrepancy deserves further investigation.
BTM early instar caterpillars have emerged from diapause and are currently resuming their feeding in S.W. Ohio. The adult moths produced from this split generation will lay eggs to initiate the first full generation later this season.
The scientific literature notes that BTM may have as many as 5 generations per season depending on the climatic conditions in its native Asian range and where it has been introduced into Europe. BTM produces 2 overlapping generations in Toronto, Ontario, and in Niagara County (Buffalo), NY.
The number of generations in S.W. Ohio may be the same, or different, owing to our warmer temperatures. It’s important to note that like all insects, BTM is “cold-blooded” (ectotherms) meaning that the pace of their metabolism, and thus the speed of their development, is influenced by temperature.
The figure below shows the potential for hot summer temperatures to significantly influence caterpillar development. All of the caterpillars in a BTM population in S.W. Ohio last season were in the early-instar caterpillar stage shown in the picture on the left. Only nine days later, all of the caterpillars were in a late instar stage shown on the right.
While anecdotal, the photographic evidence indicates that we may experience something different in S.W. Ohio compared to elsewhere. The number of BTM generations will be investigated during the upcoming season by the ODA using traps baited with a synthetic version of the pheromone used by BTM females to attract the male moths. Significant “spikes” in trap numbers indicate the appearance of a new generation.
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