The cottonwood leaf beetle, Chrysomela scripta Fabricius, is a pest I have written about in the past. Most years, I would normally let it pass since it’s usually not a big issue. However, I have been getting calls over the last couple of weeks and recently saw some big cottonwoods that have an incredible number of larvae and beetles on them which are causing some truly significant leaf damage as the beetles eat the leaves down to nubs.
These beetles are about one-quarter-inch long with a black head and thorax that often has yellowish or reddish margins. The wing covers, or back of the beetle, is an off-yellowish color with broken black strips. Many beetles, such as the ladybeetle and tamarisk beetle, are similar in shape but with different colorations and sizes. The larvae when young are black and as they grow older can be light to dark brown with four prominent white scent glands that excrete a bad-smelling fluid.
The female lays yellow eggs in clusters of 15 or more on the undersides of the leaves. These eggs will hatch in one to two weeks, with the young larvae feeding on the undersides of the leaves often leaving the leaf lacelike or skeletonized. Both the older larvae and the beetles can consume entire leaves except for the large veins. Adult beetles will also chew holes in leaves and eat on new shoots.
The larvae will begin pupating after feeding for a week or two with adult beetles emerging in five to 10 days. The pupating larvae hang off leaves, stems, fences and basically just about anything.Quite frankly, they are not a pretty sight – they look like something from another planet.
There are beneficial insects that will feed on both the cottonwood beetle larvae and eggs. Those include lady beetles, lacewings, spiders, and wasps. Any younger trees that are attacked by these beetles probably should have some control measures taken since the insect numbers I am starting to see suggests even the large trees could be stripped of leaves. Young larvae can be controlled with insecticidal soaps and horticultural oils and older larvae and adults can be controlled with commercial chemicals such as acephate, carbaryl and likely imidacloprid. There are also organic biorational or botanical pesticides that are recommended. Those include spinosad, Bacillus thuringiensis (Bt) variety tenebrionis or San Diego strain – not the strain used on caterpillars and azadirachtin – is also showing promise for control.
While here in Grand County we more often see the beetle feeding on cottonwoods, it also feeds on poplars, aspen and willows, so keep a watch out on those trees. We don’t see large numbers every year and it’s possible these might not be widespread, but since the beetle can have multiple generations a year it is something to keep a watch on.
Pictures of the beetle are available online at www.texasento.net/scripta.htm; dpughphoto.com/animals_symbol.html; and entnemdept.ufl.edu/creatures/trees/beetles/cottonwood_leaf_beetle.htm.
Thought for the day: “In summer the empire of insects spreads.” Adam Zagajewski.
If you would like to know more about these topics call the Utah State University Extension Grand County office at 435-259-7558 or email Mike Johnson at firstname.lastname@example.org.