ATA Turf Times – Amanda Vinson, Elijah Carroll, and David Held

Crape myrtle bark scale (Acanthococcus lagerstroemiae) is a non-native scale insect and economically important pest in the Southern United States, whose rapid spread is compelling researchers to find sustainable management solutions. Crape myrtle bark scale (CMBS) is native to East Asia and was discovered in the United States in 2004, in a Texas Nursery. This scale insect has quickly dispersed throughout the South and Southeast due to its high fecundity, or reproductive success, and ability to produce multiple generations per year. Crape myrtle bark scale threatens the aesthetic appeal of our beloved crape myrtle trees (Lagerstroemia spp.) in the landscape. For this reason, landscape managers should know the biology of this scale insect, management practices, and how those management practices can impact beneficial insects.

A Biological Understanding

CMBS are in the felt scale family (Eriococcidae) and can be found on and under the exfoliating bark of crape myrtle trees. These scale insects (Order Hemiptera) have piercing-sucking mouthparts that penetrate the bark and limbs to feed upon the sugar-rich phloem. Despite the common name, crape myrtle bark scale can survive on a few different woody plants, but populations are the largest on the namesake host. CMBS infestations are not easily detectable in early stages when population densities are low. If you were to observe an infested crape myrtle tree, you may see tiny white spots on the bark that resemble the end of a mini cotton swab. Adult stages can be more readily detected, appearing as small white spots and resemble the end of a cotton swab. CMBS are in a group called felt scales, referring to their felt-like hairs, or setae, covering the adults.

The CMBS life cycle begins as eggs inside an egg sac on the females. The eggs hatch, producing an immature “crawler” stage which lacks the felt-like covering of the adults. The mobile immatures are extremely small (smaller than a pin head) and pink in color. Crawlers, when alive, can be squished producing a pink-colored liquid. After feeding and furthering development, crawlers will settle on the trees, become immobile, and develop into either a winged (flying) male or an immobile female. The maturing female crawlers develop the white “felt” as they settle and feed, with the pink flesh still visible. The winged males disperse to other crape myrtles to find and mate with females. Otherwise, males do not feed or develop new infestations. As females feed they mature, producing hundreds of microscopic eggs.

The various life stages of CMBS are active at different times of the year. Our lab research in Fairhope, AL found a spike of mobile crawlers in October, followed by overwintering of all life stages through winter months. There is another spike of crawler activity again from April through June. We suspect that crawlers can disperse to adjacent trees by wind or perhaps by catching a ride on squirrels or birds. When present, these scale insects aggregate on limbs, branches, and tree trunks producing copious amounts of honeydew which can develop sooty mold, tree die-back, a loss of aesthetic quality, and even death of smaller trees.

Management

Currently, the most common treatment method for this pest is the application of systemic insecticides, namely dinotefuran and imidacloprid, to infested trees. Because crape myrtles are in flower most of the summer, these materials are commonly applied as labeled for drench applications to the soil. This management tactic is relatively expensive per tree and both active ingredients can move into pollen (no nectar in CM flowers) potentially placing non-target beneficial insects at risk. Our testing suggests new systemic insecticides are being developed. Insecticides that are chemically related to these newer products are less harmful to pollinators and natural enemies.

Naturally occurring lady beetles (AKA lady bugs) are aggressive predators of CMBS. Most people can recognize an adult lady beetle but may not recognize the larvae or pupal forms. Larvae of the mealybug destroyer, Cryptolaemus montrouzieri, can be mistaken for CMBS adult females. Even though they share a white, fuzzy, outer appearance, these larvae are larger in size and are mobile unlike the immobile adult female CMBS. The ladybeetle larvae are an effective biocontrol agent of CMBS and can be negatively impacted by insecticide applications. Monitor the trunk of the tree for lady beetle adults and larvae before applying insecticides to the trunk or branches of infested trees.

A Sticky Situation

As CMBS feeds on the phloem, they excrete a waste-product called honeydew. Honeydew is a clear, sticky liquid that will coat the plant leaves and bark. It is rich in sugar and carbohydrates and can act as a food source for other visiting insects. In our current research, we have found that honeydew production is seasonal in Alabama on crape myrtle trees. There is a large spike in production in October (significantly greater than every other month) and smaller amounts produced from June to September. Honeydew is not a direct indicator of CMBS because crape myrtle aphids found on the undersides of leaves also infest crape myrtle and produce honeydew. Ongoing research in our lab shows that trees with honeydew are more attractive to flies, lady beetles, and wasps, and bees.

A fungal pathogen called sooty mold colonizes the honeydew on leaves and bark of the tree. This is often the first symptom of a CMBS infestation. Sooty mold will cover the branches and leaves of crape myrtle trees in dark black film, which lowers the tree’s ability to photosynthesize. Sooty mold is difficult to remove once established and can result in a reduction in leaf flush and floral bloom. Trees with darkened branches and trunks that are slower to flush leaves than other crape myrtle trees should be inspected for CMBS.

A Concerning Approach

Before CMBS, crape myrtle was not commonly treated with insecticides. With CMBS present, insecticide use on crape myrtle trees has increased. Applications of these insecticides can be expensive and require trained personnel or specialized equipment to do soil applications. Beyond the economical limitations, neonicotinoid insecticide treatments to crape myrtles have high potential for effects on beneficial insects. Crape myrtles provide pollen resources to floral visitors and pollinators. A study we conducted in Auburn on landscape trees found many species of bees and flies visiting the flowers may be at risk from systemic treatments of neonicotinoid insecticides. Dinotefuran applied in the spring and imidacloprid applied in the fall before bloom can result in insecticides in pollen that can be harmful to bees through contact or ingestion.

A Way Forward

While systemic insecticides may pose hazards to floral visitors, they are still the most effective tools to combat CMBS. Landscape companies and homeowners may have other options to take the first steps to controlling crape myrtle bark scale in the landscape. Lady beetles can find CMBS and aphids naturally on trees. Our field research shows that 80-95% of CMBS on branches of heavily infested trees can be removed by lady beetles within 2 wk. Few attractants have been evaluated for lady beetles in urban landscapes. Our landscape tree experiments show nearly twice as many adult lady beetles attracted to infested trees with yellow panels compared to infested trees without yellow panels. However, attraction does not mean they are feeding. On those trees with yellow panels, there was only a 22% reduction in crawlers over 2 months. In areas with low numbers of lady beetles, there is limited impact of attractants.

There are still many steps to create an effective integrated pest management strategy for crape myrtle bark scale. Our on-going research now focuses on methods to preventive infestations, evaluations of other attractants for natural enemies, and testing the performance of newer insecticides and their compatible with pollinators and natural enemies.

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