Tree Facts: Tree diseases – elm leaf beetles

Shauna Kopren
Northwest Area Conservation District

In this tree disease feature, we will be discussing elm leaf beetles, a common insect that chews on the leaves of elm trees. The presence of round irregular shaped holes in the center of elm leaves is an indicator of elm leaf beetles, the holes being caused by adult elm leaf beetles chewing on the leaves. The dark grub-like larvae also chew on the undersides of the elm leaves, often avoiding the larger leaf veins producing a skeletonizing injury pattern. Damaged leaves from larvae will have a lacy appearance, and may turn brown and fall from the tree prematurely.

Chemical controls can effectively control elm leaf beetles and are best used in areas where outbreaks occur regularly or where there have been sightings of beetles laying eggs. There are three ways to manage elm leaf beetles using insecticides:

1. Systemic insecticides that are applied to the soil and then move into the leaves where elm leaf beetles feed. It is the most useful control however there is a lag period of two to four weeks before there is enough concentration in the leaf to kill the beetles. The more moisture there is in the soil the faster the insecticide will move so supplemental irrigation may be required.

2. Insecticide sprays that are applied to leaves and directly contact and kill feeding beetle larvae and adults. This is best applied after majority of the eggs have been laid but before larvae cause significant damage to the leaves.

3. Trunk banding with insecticides can be used to kill larvae as they craw down the trunk after the first cycle of feeding in early summer. The bands should be placed just below where the major branches join the trunk and be at least one foot wide.

Natural control options are more challenging since elm leaf beetles have few natural enemies however some insects that might feed on them during various life stages include predaceous stink bugs and plant bugs, and some small parasitic wasps. An important factor that affects elm leaf beetle populations naturally is weather. Long winters or late spring freezes may reduce large numbers of overwintering beetles and heavy winds and wind storms can blow small larvae off trees.

Before the treatment of elm leaf beetles, consultations should be considered to insure the correct application of the insecticide. Please contact Shauna Kopren with questions on elm leaf beetles or other related topics at 605.244.5222 or shaunakoprennrs@gmail.com.