Elm Leaf Miner and Elm Leaf Beetle are having their way with Elm trees in the valley now. If you’ve noticed brown, falling leaves on the elms, it’s very possible it’s one of these insects. If you’ve noticed hundreds of small beige bugs congregating on the side of your house or trying to get inside, it’s probably the Elm Seed Bug.
Elm Leaf Miner
This is the main problem we’re seeing on elms this year. This adult insect is a small black sawfly. Eggs, laid on the leaves, hatch into larvae that feed between the leaf tissues causing a brown blistered look. Hold the leaf up to the light and you might see larvae between the tissues. Sand-like materials inside the leaf are droppings from the larvae. Towards the middle of June the larvae will drop to the ground where they pupate during the summer, fall and winter only to emerge next spring and begin the life cycle again. Ground beetles, braconid wasps and ichneumonids all prey on the leaf miner during various stages of development. Systemic insecticides are also effective against this insect. Read the label to ensure correct and safe use of chemicals.
Elm Leaf Beetle
This yellow and black stripped beetle and the newly hatched larvae feed on Elm leaves, leaving only a leaf skeleton. These skeletonized leaves are brown and may fall prematurely. The larvae feed for several weeks and then pupate for another two, usually at the base of the tree, before new adults emerge, fly into the tree to begin the process again. There are usually 2 generations a year. This insect overwinters as an adult and may be controlled with a systemic insecticide. That treatment may last two years. Carefully read chemical labels and follow instructions. Elm Leaf Beetle DO NOT transmit Dutch elm disease.
Elm Seed Bug
In 2012, the Elm seed bug (ESB), an insect new to the U.S., was found in Ada and Canyon counties. It was later found in Elmore, Gem, Owyhee, Payette, and Washington counties and in Malheur County, Oregon. The insect does NO damage to trees or buildings and does not present any human health threat. However, its habit of entering buildings in large numbers to escape summer heat, and later to overwinter, makes it a significant nuisance to homeowners. This insect resembles a small, beige Box Elder bug. Elm seed bugs spend the winter as adults, mate during the spring and lay eggs on elm trees. Once hatched, the ESB larvae feed on elm seeds from May through June, and grow into adults during the summer. A pesticide treatment can reduce the number of adult ESBs on your property, but do little to prevent future infestations. The best way to manage intrusions by ESB is by taking steps to block their entry. Seal around windows and doors and eliminate other entry points. When disturbed or crushed, the bugs produce an unpleasant odor.
Weather and predatory insects all play a role in the severity of the insect problems that are seen from year to year.