Managing Crapemyrtle Bark Scale – are we winning the fight?

Crapemyrtles are one of the most popular trees in the Southeast. They required minimum care before the recent introduction of a serious scale insect, the crapemyrtle bark scale (Acanthococcus lagerstroemiae, let’s called it CMBS). The first sight of an infestation is often the black sooty mold caused by the colonization of several fungi on the honeydew exudes by the scale (Figure 1). This can be misleading because a more common and easy to control pest, aphids, also cause sooty mold. However, aphids can only be found on leaves and tender growth while the CMBS is found on the bark tissue of mature branches. Colonies of white or gray felt-like bumps on branches that bleed pink ‘blood’ when crashed can confirm an infestation. In addition to the unsightly sooty mold, heavy infestation of CMBS may cause die back, stunted growth, few or no blooms, or death of young trees. Since its first report in Bossier City, LA in 2014, this pest has spread to almost every city in Louisiana where crapemyrtles are planted. Questions from landscape managers and homeowners on how to get rid of or prevent its infestation have become one of the top pest-related inquiries that the AgCenter extension personnel receives on a regular basis.

Soon after its first report, LSU AgCenter scientists began working on the biology and management of this pest. In 2017, with the support of a major USDA grant, a cross-disciplinary regional research team started to collaborate on the fight against this pest. As part of this effort, AgCenter scientists worked on the seasonal population dynamics of CMBS, residue of neonicotinoids in the crapemyrtle pollen, and the use of entomological fungi as low impact biopesticides, and the results generated from these studies have helped us understand the interactions among CMBS, its natural enemies, and the management practices. As summarized below, recommendations based on these studies reduce the reliance on pesticides, especially the neonicotinoids that can have a negative impact on beneficial insects and pollinators.

Crawler population monitoring and early spring treatments. The first instar nymph of CMBS is very mobile thus called crawlers. This is the life stage that they move around to find a suitable spot to settle and thus responsible for distribution of new infestations. This life stage is most vulnerable to insecticides – once crawlers develop their waxy coating and become sessile, they are particularly resistant to contact pesticides. Therefore, being able to better predict the emergence of the crawler stage is critical for effective management and prevention of new infestations. Degree day models, which are based on the concept that insect growth is closely tied to the temperature of its surroundings, can be used to time the dates of peak crawler activity. Currently, this model is still being developed, however, based on multiple years of monitoring crawler activities at multiple locations in the state with the help of local extension agents and Master Gardeners, we can summarize that, in Louisiana or likely in USDA zone 8, the first crawlers emergence is in mid-May, and then again in late July or early August in zone 8a (i.e., Shreveport) and possibly another peak of emergence in mid-October in zone 8b (i.e., New Orleans and Houma).

These peak crawler emergence periods, especially the first one, provide the best treatment windows for using low impact natural products or selective pesticides that need a good contact with the crawlers to be effective. For example, applying insect growth regulators such as pyriproxyfen during the crawler peaks, can reduce crawler density by 85 to 100% for up to 4 months after one application. Applying horticultural oil or other petroleum-based products can provide up to 50% control, which is still desirable when used in combination with other natural products.

The good and bad news with neonicotinoids. Data from several field trials indicate that systemic insecticides, particularly neonicotinoids that contain imidacloprid, clothianidin, dinotefuran, and thiamethoxam are the most effective control options against CMBS, especially when applied to the soil. However, this class of pesticides post potential risks to beneficial and non-target insects. Crapemyrtles do not have nectar but serve an abundant amount of pollen to bees and beneficial insects throughout the summer. A study by Auburn University revealed that ten families in two orders (Diptera and Hymenoptera) for a total of close to 200 taxa of pollinators visited the crapemyrtle flowers at suburban locations. Among them, the majorities are honeybees (Apis mellifera L.), bumble bees (Bombus spp), native bees (i.e., Lasioglossum spp), and several flies, wasps, and other insects. There is also a concern over the cost of these neonicotinoid treatments. On average, the cost of rotating between two neonicotinoid products is about $10 per 3-meter-tall tree. Unfortunately, this number increases quickly for a multi-trunk plant and insecticide applications need to be reapplied each year. It was also recommended to treat surrounding crapemyrtles, even if they are not apparently infested. This can post greater risk to beneficial and pollinators.

A pollen residual study was conducted to evaluate the risk of translocation of imidacloprid and dinotefuran into pollen following a soil application at different timing – May 2019 (new leaves were budding out), November 2019 (trees were ready to go dormant), March 2020 (trees in dormant), and May 2020. Both insecticides were applied at label-recommended rates. Pollen samples were collected from treated trees in July 2020. Samples were analyzed for residuals of imidacloprid (and metabolites) and dinotefuran. Results showed that regardless of application timing, significant concentration levels of both active ingredients were found in crapemyrtle pollen (Figure 2). The highest concentrations were found at two months following May 2020 application. In general, dinotefuran was found in significantly higher concentrations than imidacloprid from the same application timing (i.e., 621.08 ng/g vs. 181.47 ng/g when applied in May 2020). Treatment concentrations were translated into the estimated mean oral amount of each active ingredient when consumed by honeybee workers based on their average daily consumption amount (Table 1). If we compare these numbers with the published LD₅₀ values for honeybees, such as EPA values, all the applications at 2, 4, and eight months before blooming have exceeded the chronic oral lowest observable adverse effect concentration for adult honeybees, which is 0.16 to 0.24 ng/bee. In addition, there might be contact exposure to contaminated pollen which poses direct risk to bees visiting neonicotinoid-treated trees.

Table 1. Estimated mean oral amount of active ingredient from crapemyrtle pollen per honeybee worker per day (ng/day/bee)

Entomological fungi as an allay for natural enemies (and us). Because CMBS is an exotic pest, there are not many natural enemies in the U.S. Five species of ladybeetles have been found feeding on this scale, especially the Chilocorus spp. A single fourth instar of C. cacti can consume up to 400 CMBS eggs in 24 hours. Because more specific biological control agents (such as parasitic wasps) are still being studied, it is very important to conserve the natural enemies we currently have in the landscapes. Fungal or bacterial biopesticides are developed from fungi or bacteria with shown efficacies against insect pests but with relatively lower impact on other organisms. A series of efficacy trials were conducted to evaluate four such biopesticides [Ancora® (Isaria fumosorosea), BioCeres® (Beauveria bassiana strain ANT-03), BotaniGard® (Beauveria bassiana strain GHA), Venerate® (Burkholderia spp.), and Grandevo® (Chromobacterium subtsugae)]. Products were applied as trunk sprays to trees infested with CMBS during winter, spring, and fall 2019. BioCeres® significantly increased mortality of CMBS only in the winter trial and BotaniGard® was more effective in a trial conducted in the fall as confirmed by field mortality and lab assay (Figure 3). However, all products failed to be effective in the late spring trial of which ambient temperatures were significantly higher than other trials. Apparently, temperature impacted product efficacy in the field.

Impact of these biopesticides on the natural enemies was assessed in laboratory conditions by dipping the insects in a biopesticide mixture and observing their survival over time. BotaniGard® significantly reduced the survival of larvae and adults of beneficial lady beetles (Hyperaspis bigeminata and Chilocorus spp.) and BioCeres® reduced the longevity of both ladybeetles by 40%. These results suggest that application of these biopesticides still post considerable risk to beneficial ladybeetles known to attack crapemyrtle bark scale.

So, are we winning this fight against CMBS?

It is a process – this is not a short-stay pest and it takes time to have what we need to save our crapemyrtles… and we are on the right track! With new findings from these research studies and other team projects, now we have a better understanding of this pest and its interactions with other players in this battle. Now we do not recommend the use of neonicotinoid pesticides as preventive measures on trees that are not infested or have very low infestation levels. They need to be considered as the last resort in the integrated pest management toolbox. We also do not recommend applying them in May because extremely high concentrations of these materials may be found in pollen when the trees are in peak bloom in June and July. We are hoping to develop the degree day models to enable to prediction of crawler peaks in other USDA coldness zones. Meanwhile, early- and mid-May is the best time for us in the zone 8 to detect the first crawler peak and apply horticultural oils and insect growth regulators that are less detrimental to natural enemies.

For more information, contact authors of this article, Yan Chen at yachen@agcenter.lsu.edu and Rodrigo Diaz at rdiaz@agcenter.lsu.edu