Crapemyrtle Bark Scale

Summary

Crapemyrtle bark scales (Figure 1) are an invasive insect that feed primarily on ornamental crapemyrtle. While they rarely kill crapemyrtles outright, crapemyrtle bark scales negatively affect the aesthetic value of infested plants by decreasing the size and abundance and of flowers, causing stunting and branch die back, and producing honeydew that grows black sooty mold.

Classification

Common name: Crapemyrtle bark scale

Scientific name: Acanthococcus lagerstroemiae (Kuwana, 1907)

Order: Hemiptera (true bugs)

Family: Eriococcidae (bark scales)

Description

Crapemyrtle bark scales are the only bark scales that infest crapemyrtle in North America, so identification is relatively straightforward. The most obvious life stages are mature females and males, which spin white coverings under which they lay eggs or metamorph into adults, respectively (Figures 2, 3). Once adult males emerge, they appear as small, pink winged insects with two long filaments on the end of the abdomen. Males lack mouthparts so do not live long, but can be obvious when an infestation is large. Crawlers (the first, highly mobile life stage) and adult females (before they lay eggs) are slow-moving, spiky insects that are best observed under magnification (e.g., a hand lens or stereomicroscope) (Figure 4).

Crapemyrtle bark scale eggs are pink and exude a pink liquid when crushed. One field technique to identify crapemyrtle bark scales is to crush suspected female shelters with a fingernail, which should produce a pink streak (Figure 5).

Distribution

Crapemyrtle bark scales were first found in Richardson, Texas in 2004. Since then, they have spread across 14 states and Washington, D.C. (Figure 6). Infestations appear scattered across the southeast rather than moving in a distinct wave across the region, which suggests they are likely being transported primarily through infested nursery stock. Modeling studies suggest that crapemyrtle bark scales will be able to persist throughout the potential range of crapemyrtle in North America, including most or all of Pennsylvania (Figure 7).

Hosts

Crapemyrtle bark scales primarily infest crapemyrtle species. Crapemyrtles are one of the most popular ornamental shrubs and trees, especially in the Southeast, due to their low maintenance and showy summer flowers that are produced on new growth (unlike many other flowers woody plants that only produce flowers on older growth).

Most crapemyrtle cultivars that are planted as ornamentals are L. indica, which has been planted in the United States since 1786, and L. indica hybrids. A number of popular cultivars are based on or include Japanese crapemyrtle (L. subcostata var. fauriei), which was introduced in the 1950s as a source of resistance for powdery mildew. A number of other species, including L. speciosa., L. limii., and L. ovalifolia., are also sometimes used in hybrids and occasionally as straight species.

Crapemyrtles have historically been restricted to USDA Hardiness Zones 6–10, which excludes the northern tier of Pennsylvania and high-altitude areas of the Appalachians. However, newer “cold hardy" varieties are advertised as being hardy to USDA Hardiness Zone 5, which would then include all of the state.

Besides crapemyrtle, crapemyrtle bark scales have been reported on St. Johnswort (Hypericum kalmianum) (native to Pennsylvania) and American beautyberry (Callicarpa americana) (not native to Pennsylvania but hardy in USDA Hardiness Zones 7–10, so may be planted as an ornamental in the southeast corner of the state). It's unclear if these were incidental hosts that just happened to be infested because they occurred near heavily infested crapemyrtle or if they are important secondary hosts.

In Asia, crapemyrtle bark scale has been reported from crop species such as apple (Malus domestica) and soybean (Glycine max), ornamentals including Korean boxwood (Buxus microphylla), brambles (Rubus sp.) and border privet (Ligustrum obtusifolium), which is invasive in North America. Crapemyrtle bark scale has not been reported from these hosts in the United States and an experimental study failed to reproduce some host associations (e.g., Korean boxwood), so it's unclear if crapemyrtle bark scales will infest them or, if they do, if they will  be an important pest.

Life History and Behavior

Female crapemyrtle bark scales construct white, felt-like sacs in which they lay 100–300 pink eggs (Figure 2). Once the eggs hatch, the crawlers move to new areas of the host to feed (Figure 3). They molt into sessile nymphs that lack legs and feed in place without moving. Mature nymphs either molt into females or spin a white sac and molt into “prepupae", then into “pupae", then into adult males (Figure 4). Nymphs and females feed and secrete honeydew. In southern areas, there are 2–4 generations per year and all stages are capable of overwintering. It's unclear how many generations may occur in Pennsylvania.

Crapemyrtle bark scales can be found anywhere on the host. However, infestations are often densest on the underside of upper branches, so these should be examined in addition to the trunk(s) when determining if an infestation is present (Figure 1).

Damage to Crapemyrtle

Crapemyrtle bark scales feed on sap. They do not generally kill crapemyrtle outright but can reduce the aesthetic value by decreasing the size and abundance and of flowers, stunting growth and causing branch die back, and producing honeydew that grows sooty mold (Figures 8–10). Honeydew production can be so copious under large infestations that the trunks and surrounding areas are blackened by smelly, sticky sooty mold (aphids such as crapemyrtle aphid can also produce honeydew and associated sooty mold, so the presence of either may not, on their own, indicate a bark scale infestation). These issues negate many of the main reasons that crapemyrtle is such a popular ornamental.

Control

Heavily infested branches can be pruned out and destroyed. In colder areas where plants are trimmed back to the roots, this may be enough to eliminate infestations as crapemyrtle bark scales are not known to overwinter on the roots or other host plants.

Scales can be manually removed by scrubbing them off with soapy water and a stiff-bristled brush. While this will likely not be enough to completely eliminate an infestation, it will help reduce the population and expose the remaining scales to other control options.

Lady beetles, and especially twice-stabbed lady beetles (Chilocorus species), have been observed to feed on crapemyrtle bark scale in the United States (Figure 11). Unfortunately, these predators generally arrive too late in the season to adequately control the scales. Sixteen species of parasitoid wasp have been reported from crapemyrtle bark scale worldwide. Some of these species occur in the United States but have not apparently been recorded from crapemyrtle bark scale yet. Additionally, two species of green lacewing have been reported to feed on crapemyrtle bark scales in Asia. While all of these natural enemies may attack crapemyrtle bark scales, the fact that they continue to spread and be problematic suggests that natural enemies alone will not be enough to control them, at least at the present time.

Horticultural oil or insecticidal soap can be applied to control crawlers and nymphs. Because it is unclear how many generations may occur in Pennsylvania and generations appear to overlap elsewhere, it is difficult to provide precise timings for oil or soap sprays. However, it is likely that sprays throughout the warm months will be effective and multiple applications may be needed. Manually removing bark scales from heavily infested areas using a brush will help oil or soap be more effective.

Systemic insecticides can be allied as a soil drench around the infested plant. The most effective active ingredient appears to be Dinotefuran, which is included in a number of different commercially available formulations.

Because natural enemies attack crapemyrtle bark scales and likely exert some control (even though it’s not enough to totally eliminate the scales), broad-spectrum insecticide sprays should be avoided in favor of more biorational options such as those listed above.

Some studies have found that crapemyrtle bark scales infestations may be correlated with shade levels, so new crapemyrtle plantings should be made in full sun to reduce potential future infestations. Crapemyrtle planted in urban settings may be at greater risk compared to surrounding rural areas due to increased temperature and humidity and decreased natural enemy populations.

A handful of studies have found that susceptibility or resistance to crapemyrtle bark scale varies between of different crapemyrtle species and cultivars. Unfortunately, most cultivars have not been tested in a systematic way, but in the future it may be possible to plant cultivars that are resistant to crapemyrtle bark scales.

References

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Merchant, M. E., M. Gu, J. Robbins, E. Vafaie, N. Barr, A. D. Tripodi, and A. L. Szalanski. 2014. Discovery and spread of Eriococcus lagerstroemiae Kuwana (Hemiptera: Eriococcidae), a new invasive pest of crape myrtle, Lagerstroemia spp. Available online: accessed on 16 May 2016.

Schultz, P. B., and A. L. Szalanski. 2019. Hypericum kalmianum (St. Johnswort) confirmed as a new host of the crapemyrtle bark scale in Virginia, U.S.A. Journal of Agricultural and Urban Entomology 35: 12–14.

Suh, S.-J. 2019. Notes on some parasitoids (Hymenoptera: Chalcidoidea) associated with Acanthococcus lagerstroemiae (Kuwana) (Hemiptera: Eriococcidae) in the Republic of Korea. Insecta Mundi 0690: 1–5.

Wang, Z., Y. Chen, and R. Diaz. Thermal tolerance and prediction of northern distribution of the crapemyrtle bark scale (Hemiptera: Eriococcidae). Environmental Entomology, 48(3): 641–648.

Williamson, J. 2021. Crapemyrtle bark scale. Home & Garden Information Center, Clemson Cooperative Extension. 6 pp.

Wu, B., R. Xie., G. W. Knox, H. Qin, and M. Gu. 2021. Host suitability for crapemyrtle bark scale (Anthococcus lagerstroemiae) differed significantly among crapemyrtle species. Insects 12(6).