Willow leaf beetles Skeletonize undersides of leaves

Holes in tree leaves with just the veins left is caused by beetles.

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Q. We have a Curly Willow tree that is being eaten by small, shiny black bugs. They are chewing holes in the tree leaves. What are they and what can I spray to control them?

A. Those shiny black bugs are likely imported willow leaf beetles (Plagiodera versicolora). Adults and larvae feed on the leaves of most willow (Salix) species, as well as Lombardy poplar (Populus nigra ‘Italica’) and cottonwood (Populus deltoides). Large populations of these beetles can cause significant damage to host trees, but they are not considered life threatening.

Life Cycle of the Beetles

Imported willow leaf beetles overwinter as adults under loose bark and in protected crevices of host trees. The shiny, black- to greenish-blue-colored beetles emerge in spring as host trees begin to leaf out. They feed by chewing holes in the leaves, consuming leaf veins as well as tissue. After feeding briefly, adult females lay masses of small, pale yellow eggs on the undersides of the leaves. The black larvae hatch in a few days, and begin to feed by skeletonizing the undersides of the leaves.

This means the insects consume the tissue on the underside of the leaves, but not the leaf veins. As the larvae mature, they also feed on the upper side of the leaves. Larvae pupate in three to four weeks. Insects undergo a complete transformation, from larvae to adult, during pupation. During that time, they do not actively feed, nor or they as susceptible to control as they are as larvae. We see two or three generations of imported willow leaf beetles annually.

Skeletonized leaf has veins left intact
‘Skeletonized’ leaf has veins left intact

As is often the case with insects that produce multiple generations a year, it is most beneficial to obtain good control of the first generation. That way the second and third generations are much smaller, easier to control, and cause less damage to the tree.

Using pesticides safely

BioNeem (azadirachtin); Sevin (carbaryl); horticultural oil (larvae only); pyrethrins and piperonyl butoxide; and Captain Jack’s Dead Bug Brew (spinosad) are labeled to control imported willow leaf beetle.

Spraying Trees

If your tree is very large, you should hire a certified arborist to spray it for you. They have the training and equipment to spray the tree thoroughly and safely. To find a certified arborist near you, log on to the International Society of Arboriculture (ISA) website.

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