Be on the lookout for elm seed bugs

Elm seed bugs (Arocatus melanocephalus) are relatively new to Idaho. They first established in 2009
near Boise but have since been found throughout the state wherever elm trees grow. While similar to
box elder bugs, these insects are smaller, about 1/3 inch long. Adults have a dark, rusty-red and black
coloration. On the back, there is an upside-down black triangle set inside two rusty-red triangles. The
immature nymphs are only a quarter the size of adults. One of the most recognizable characteristics of
elm seed bugs is their ‘busy’ habit, running around much more than their box elder bug cousins.

Periods of extremely high temperatures may trigger mass migrations to seek more favorable climates
inside homes during the summer. When aggravated these bugs give off a foul odor sometimes described
as smelling like bitter almonds. Fortunately, there is only one generation produced each year.

As nymphs and adults feed on Siberian elm seeds and leaves, using their piercing and sucking
mouthparts, they cause little damage to overall tree health. After elm seeds mature and fall off the tree
the bugs become more mobile and can be found in large numbers congregating on tree trunks, the
ground and on nearby structures. These insects can congregate in large numbers in soffits, around
windows and under vinyl siding. With hot summer temperatures adults may congregate on the northern
side of buildings or in shaded areas attempting to escape the heat. They may also be seen in the fall
congregating on the southern side of structures seeking radiant heat.

During the fall insects will start to seek overwintering habitats and will often start invading homes. They may enter homes via cracks and crevices, but most commonly enter through windows and doorways
that have poor seals.

These are simply a nuisance pest but can overwhelm you with numbers. It is likely that these insects will
increase in our area. If you see these insects, take extra effort to bug proof your home, particularly if
elm trees are nearby. Weather-strip around loose-fitting doors, caulk around windows, and repair torn
screens. Use a shop vacuum to physically remove bugs; put a few inches of soapy water in the canister
to drown captured bugs. Rake and destroy elm seeds that fall from trees during late spring and early
summer to reduce nymph food sources.

Control can be obtained through methods similar to controlling box elder bugs. Products commonly sold
for this use contain active ingredients such as beta-cyfluthrin, bifenthrin, deltamethrin, cyfluthrin,
cypermethrin, lambda-cyhalothrin and permethrin. Only use products that explicitly allow for the
application sites where you intend to treat.

Don’t worry if you see one or two insects you think might be elm seed bugs. There are other insects that
may look similar. If you have elm seed bugs you will have LOTS of elm seed bugs. There must also be an
elm tree nearby.