Box Tree Moth

Summary

Box tree moths are invasive moths that feed on boxwood (Buxus sp.) as caterpillars. They were detected in Toronto, Canada in 2018 but have not yet been found in the United States.

Classification

Common name: Box tree moth

Scientific name: Cydalima perspectalis (Walker, 1859)

Order: Lepidoptera (moths and butterflies)

Family: Crambidae (crambid snout moths)

Distribution and spread

Box tree moths (Figures 1, 2) are native to eastern Asia, including Japan, China, the Russian Far East, Korea, and India. They were introduced into Germany in 2006 and have subsequently spread throughout Europe (Figure 3). Box tree moths were first detected in North America in Toronto, Canada in November 2018 and as of this writing (spring 2020) do not appear to have spread beyond the city. They have not yet been found in the United States but given the proximity of the infestation will likely arrive in the near future if they are not eradicated.

Figure 1. Box tree moth caterpillar. Photograph by  Christophe Quintin via Flickr, used under a CC BY-NC 2.0 license. Cropped from original.

Figure 2. Box tree moth adult. Photograph by Didier Descouens via Wikimedia, used under a CC BY-SA 3.0 license.

Figure 3. Box tree moth invasion of Europe. Modified from Bras et al. (2019) and EBTS (2020).

Box tree moths were able to spread so quickly through Europe due to a few factors. First, two species of boxwood are native to Europe, while none are found in the wild in continental North America. Once the moths got onto wild European boxwoods, they were able to spread relatively unchecked. While boxwoods are commonly planted in North America as ornamentals, the largest plantings occur in urban areas. The relative lack of boxwoods in more rural areas may help slow the spread of the moths between urban and suburban areas. Second, based on DNA evidence, box tree moths were introduced into Europe from Asia multiple times and moved within Europe via nursery trade after they were introduced. Given the European example, it may be easier for the United States and Canada to support initiatives to limit the spread and quarantine regions if the moth becomes more widely established.

Damage

Box tree moth caterpillars feed exclusively on boxwoods. The young caterpillars feed on the undersides of leaves, which give them a "peeled" appearance from the top. Older caterpillars consume the entire leaf except for the midrib. In addition to feeding damage, caterpillars web together leaves and construct silken retreats. Because they lack natural enemies in Europe, box tree moths decimate boxwood plantings. Extensive feeding kills individual plants and entire plantings.

Description

Box tree moth eggs are pale yellow (Figure 4); they are laid in groups of 5–20 and overlap like shingles. Eggs take three days to develop and when the young caterpillars are close to hatching, the head capsule becomes visible through the egg. Box tree moth caterpillars (Figures 1, 5) are green and yellow with white, yellow, and black stripes and black spots; they are only caterpillars in the region that feed on boxwood, so finding them on the host plant is distinctive. Box tree moth caterpillars take about 14 days to mature. Pupae are found amongst webbing and damaged leaves (Figure 6). Young pupae are green with brown stripes; when pupae are ready to turn into moths, the pattern of the wings becomes visible through the pupal skin. Pupae take about 14 days to develop. Most adult box tree moths are white with a brown border (Figure 2). Some adult moths may have an additional brown border on the edge of the forewing. A minority of specimens (5–10%) are entirely brown except for the white commas on the forewings (Figure 7). White box tree moths look superficially similar to melon worm moths (Diaphania hyalinata), which are common in the southeastern United States and sometimes migrate into Pennsylvania in the fall, but can be distinguished by the presence of white commas on the forewings (orange arrow) and white prothorax which interrupts the brown border (red arrow) (Figure 8). Adult box tree moths can survive for about a month. They are strong fliers and can disperse 4–6 miles.

Figure 4. Box tree moth eggs. Photograph by Cosmi O. Manci, used with permission.

Figure 5. Box tree moth caterpillar. Photograph by Cosmi O. Manci, used with permission.

Figure 6. Box tree moth pupae. Photographs by Cosmi O. Manci, used with permission.

Figure. 7. Dark (melanic) phase box tree moth. Photograph by George Partsinevelos (Benaki Phytopathological Institute Dept. of Entomology and Agricultural Zoology), in Strachinis et al. (2015). Used with permission.

Figure 8. Comparison of adult box tree moth and melonworm moth. Photograph of box tree moth by Didier Descouens via Wikimedia, used under a CC BY-SA 3.0 license. Photograph of melonworm moth by Mark Dreiling via Bugguide, used under a CC BY-ND-NC 1.0 license.

Life history and behavior

Box tree moths have 1–5 generations per year depending on the latitude and local climate. Based on information from Europe, they will likely have two or perhaps three generations in Pennsylvania. Box tree moths overwinter as 2^nd to 5^th instar larvae and can survive temperatures to at least -22°F (-30°C). Overwintering caterpillars have a development threshold of 46–53°F, so they will begin feeding in early to mid-spring. They have an obligatory diapause (when the caterpillars stop eating and rest) of 6–8 weeks when day lengths reach 13.5 hours; this occurs between 15–20 April in Pennsylvania, so feeding in our area will begin again in late May to early June.

The few photographs of adult moths in Toronto that have been posted to the community science website iNaturalist occur in two clusters from late June and July and again in late August through September, which suggests there may only be two generations per year in our region.

Control

Because it is not established in the United States at this time, there are no official recommendations for controlling box tree moth. When infestations are small, hand-picking caterpillars and disposing of them in soapy water is likely going to be an effective solution. When caterpillars are small, it is possible to knock them off plants with a strong jet of water, which kills many of them as they are unable to climb back up the plant before starving.

A sex pheromone is commercially available and pheromone traps are used to monitor plantings for box tree moth. While it doesn't seem to have been done in Europe, there may be an opportunity to use the pheromones for mating disruption.

There are a number of natural enemies of box tree moth that have been recorded from their native range, including a variety of parasitoid wasps and flies. However, all of the species have broad host ranges and also attack other caterpillars and are not suitable for release in invaded areas. A handful of studies in Europe found low levels of parasitism by native parasitoid wasps and flies, but never enough to exert control over the moths.

Horticultural oil and insecticidal soaps will likely be able to control young caterpillars and biopesticides that work on other caterpillars, such as Bt and Spinosad, have been used to kill box tree moths in Europe. Broad spectrum pesticides that are labeled for caterpillar control on ornamental plants will also likely provide acceptable control. However, it should be noted that adequate coverage can be difficult to achieve as the young caterpillars feed only on the undersides of leaves and older caterpillars are protected by silken retreats.

References

Bras, A., D. N. Avtzis, M. Kenis, H. Li, G. Vétek, A. Bernard, C. Courtin, J. Rousselet, A. Roques, and M.-A. Auger-Rozenberg. 2019. A complex invasion story underlies the fast spread of the invasive box tree moth (Cydalima perspectalis) across Europe. Journal of Pest Science, 92: 1187–1202.

(EBTS) European Boxwood & Topiary Society. 2020. Box tree moth & caterpillar. Available online.

Strachinis, I., C. Kazilas, F. Karamaouna, N. E. Papanikolaou, G. K. Partsinevelos, and P. G. Milonas. 2015. First record of Cydalima perspectalis (Walker, 1859) (Lepidoptera: Crambidae) in Greece. Hellenic Plant Protection Journal, 8: 66–72.

Wan, H. T. Hays, M. Kenis, S. Nacambo, H. Xu, F. Zhang, and H. Li. 2014. Biology and natural enemies of Cydalima perspectalis in Asia: Is there biological control potential in Europe? Journal of Applied Entomology, 138: 715–722.