Leaf Beetle Outbreaks are Generally Short-Lived

Willow leaf beetle damage
Willow leaf beetle damage. Photo: DNR

There have been quite a few observations of willow and alder leaf beetle activity this year and the activity has been covered on TV news and in newspapers. Readers of DNR’s Small Forest Landowner e-Newsletter were informed about the activity in the July edition.

Reports of willow defoliation, small maggoty larvae and brown leaves have come in from Lewis, Thurston, Mason, Kitsap, Pierce, King, Snohomish, and Skagit counties. Alder defoliation was more subtle and not reported beyond Thurston and Snohomish counties. Adult beetles showed up in high numbers in August, sometimes shifting their feeding to include the leaves of cottonwood, poplar and even dahlias. The damage alarmed many people and raised concerns that their trees were being destroyed.

I’m happy to reassure everyone that this is a normal, natural outbreak and that most affected trees will survive. Many insects have what are called “periodic population irruptions” (Note: volcanoes “erupt” but populations that rise suddenly are “irrupting”). While some irruptions occur on predictable cycles; some do not. I’ve seen abundant outbreaks many times over the last 20 years and while I’m not aware of a predictable cycle and don’t know what triggers them, it’s probably a combination of factors such as length of time since the prior irruption, weather factors, and the condition of the host foliage.

Predator satiation

These types of abrupt population increases are a common survival strategy called “predator satiation.” The strategy reduces the chances of an individual being eaten by providing more prey than the insects’ predators can consume, which allows more insects to survive and reproduce. As the weather has cooled and the rain came back, the adult beetles seemed to have disappeared into leaf litter and other sheltered places to overwinter. They’ll emerge in spring to lay their eggs on the newly growing tree leaves and continue the cycle.

Return to normal levels expected

Getting rid of the beetles is probably unnecessary because the natural host trees (willow, alder, cottonwood) are very resilient to defoliation and can tolerate high levels of leaf loss without long-term consequences. Homeowners who were disturbed or bothered by the beetles could rinse them off with the hose or squish them, but I don’t recommend pesticides. The beetles aren’t known for seeking shelter in people’s homes like some nuisance lady beetles or conifer seed bugs.

Alder leaf beetle damage
Alder leaf beetle damage. Photo: Daniela Lupastean, University of Suceava, Bugwood.org

Leaf beetle outbreaks are generally short-lived. Their collapse is likely a combination of factors such as natural winter mortality (our wet winters aren’t great conditions for adult insect survival); beetle diseases that are easily spread between beetles when numbers are high; and changes in food quality (trees actively respond when they are being defoliated and make their leaves less nutritious and less digestible). Plus, because the beetles predators had abundant food supplies, they’ve likely had bigger families which will be ready to eat the beetles they find next year. All those factors likely contribute to the beetle populations returning to normal levels within one to two years.

If you are concerned that your trees received serious damage from the defoliation, take a closer look at the twigs. Signs of survival include new, undamaged foliage being produced especially at the twig tips, buds ready for next year, and supple, succulent twigs. Break a twig or two off — if the interior bark is green and the internal tissue is moist, then that part of the tree is fine. If the twig is brittle and dry and its internal tissue is punky, then that branch tip has been killed.

It will be interesting to see whether high leaf beetle populations persist in 2014.

by Karen Ripley, Forest Health Program Manager
Washington State Department of Natural Resources