Crapemyrtle Bark Scale Update

Posted on Categories General, Growers

The crapemyrtle bark scale is an invasive pest of crapemyrtles, first introduced in the US in 2004. For a summary the scale, check out the page on crapemyrtle bark scale.

Earlier this year (2017), crapemyrtle bark scale has been found on landscape beautyberry in East Texas.

Crapemyrtle bark scale on beautyberry
Photo credit: Janet Creech.

This post with provide an update on four main objectives:

  1. Crapemyrtle cultivar susceptibility to crapemyrtle bark scale
  2. Population cycles of crapemyrtle bark scale
  3. Insecticide trial for management

1. Crapemyrtle cultivar susceptibility to crapemyrtle bark scale

In the summer of 2015, several researchers and collaborators investigated crapemyrtles in McKinney, TX, at The Crape Myrtle Trails of McKinney.

Three branches were removed from three trees of each cultivar, and number of crapemyrtle bark scale were counted on the terminal 30 cm.

Figure 1. Average number of crapemyrtle bark scale from 30 cm branch (3 branches per tree, 3 trees per cultivar) from different cultivars of crapemyrtles.

The data above should be taken with a grain of salt, as the larger bars don’t necessarily mean that those cultivars are more susceptible. Since these were trees in the landscape and there was no controlled infestation done, several factors could have resulted in the differences in the numbers of scale that we saw on the different cultivars. In Tyler (TX), there are several Natchez in the landscape and they are covered in scale, so we can’t look at the numbers above. Rather, Figure 1 above demonstrates that the scale was found on all the cultivars of crapemyrtles that were monitored.

2. Population Cycles of Crapemyrtle Bark Scale

To know when to apply contact insecticides, we need to know when the vulnerable stage of the scale is out. When scales first emerge, they have no “wax” layer on top of them and crawl around looking for a place to feed. Once they settle, they start forming strands of wax, that will cover the bodies and make them more difficult to control with a contact insecticide. The white spots on the tree are either adult female egg sacs or male pupae, meaning that they are not feeding and well protected, thus contact and systemic insecticides will be less effective against them at this stage.

Different stages of crapemyrtle bark scale
Figure 2. The far left pane shows an example of highly infested crapemyrtles late in the season. The middle pane shows the difference between male and female adult crapemyrtle bark scale. This stage will be hard to control and have already done “their damage” by making the tree unsightly. The right pane shows the relative size of the immature nymphs next to the adults, giving an idea of the size of the immatures. This is the vulnerable stage we want to target with insecticides.

The scale populations were monitored across several locations in 2015 and 2016.

Figure 3. Population cycle of crapemyrtle bark scale in 2015. The crawler numbers increase towards a large peak between mid-April to beginning of May for all locations. Note: Dallas monitoring data is missing for that time period.
Figure 4. Population cycle of crapemyrtle bark scale in 2016. The crawler numbers increase towards a large peak between mid-April to beginning of May for all locations in Texas and Louisiana. Arkansas scale populations appear to follow a different pattern which needs further investigation. Note: Dallas monitoring data is missing for that time period.

In both 2015 (Figure 3) and 2016 (Figure 4), it was found that the scale crawler populations increased towards a peak between mid-April to beginning of May. Dallas/McKinney data was missing during that period. Also notice that Arkansas appears to follow a different pattern than Texas and Louisiana. More data across different climatic regions need to be collected to understand population cycles beyond the states of Texas and Louisiana.

3. Insecticidal Trial for Management

In 2016, 42 infested crapemyrtles in the landscape were treated with different insecticides to determine efficacy against crapemyrtle bark scale.

Figure 5. Efficacy of insecticides against crapemyrtle bark scale on landscaped crapemyrtles.

All contact insecticides (acephate, horticultural oil + azadirachtin, and bifenthrin) were applied twice to the bark of the crapemyrtles in a two-week interval. Dinotefuran and imidacloprid were applied once (Spray 1) as a soil drench.

Acephate appeared to have no efficacy against the crapemyrtle bark scale throughout the length of the trial. Horticultural oil + azadirachtin appeared to offer suppression, but less than 50% control of the crapemyrlte bark scale when applied twice. For horticultural oil + azadirachtin to provide better control, they may need to be applied more frequently.

Bifenthrin appeared to provide very effective quick contact control against crapemyrtle bark scale, however, scale populations increased again later in the season (although scale populations appear suppressed).

Dinotefuran and imidacloprid did not provide control for the first 45 – 50 days, but provided 100% control after that.

As a result, this data suggests that applying soil drenches of dinotefuran or imidacloprid when the crapemyrtles first start flushing out leaves and taking up plant nutrients (March/April in Texas), and applying bifenthrin twice on a two-week interval starting mid-April.

Our observations and work of other collaborators suggest that spraying insecticides for control of crapemyrtle bark scale in the landscape might not be necessary. Some preliminary work by Dr. Mike Merchant (Texas A&M AgriLife Extension) has demonstrated that lady beetles can provide about 75% control of scale insects. Insecticide treatments may be more necessary for growers that require clean pest-free plants for sale and distribution.

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