Moisture Ants

Order: Hymenoptera
Family: Formicidae
Species: FAcanthomyops spp., Lasius spp.

Description & Life History:

Moisture ants are comprised of multiple species of yellow ants and cornfield ants, in the genus Acanthomyops and Lasius, respectively. Both species can be described as “little yellow ants.” Using your handy WSU publication, “Identification and Habits of Key Ant Pests of Washington,” EB 0671, you can distinguish these genera from others by: the circular fringe of hairs around the terminal orifice (the hairy butt), the notch in its back (not convex in shape from the side) and the wide upper lip (wide clypeus). Yes, I know it sounds like a description of Quasimodo, but didn’t I previously warn you that identifying ants isn’t the most glamorous Master Gardener task? Becoming familiar with this key to ant pests will empower you to identify almost any ant genus that comes into the office and I strongly encourage that you learn how to use this key.

These ants, like all ants, are truly social insects that are altruistic; they sacrifice their own reproductive capabilities for the functioning of the colony. The have a cast system that works amazingly unified, just like bees and termites. Worker ants forage for sweet sugars and protein and are often found tending aphids for their honeydew. Workers bring back food for the developing larvae, other casts and the queen. Moisture ants are monomorphic meaning that all the casts look similar.

Ant colonies are started from a single pregnant female (queen) and can grow up to several thousand individuals over time. Some ant colonies can persist for over 20 years. During the summer and fall, reproductive ants (males and queens) are produced by the colony. The reproductives are winged and form mating swarms. Winged male ants are often brought into the clinic for identification; these ants are very difficult to identify and you will need the wingless workers or winged females for identification. Lasius species are most common type of moisture ant that is brought into our office.

Photo Caption: A Lasius spp. of ant. Note the notch in the dorsum (back) of the ant’s thorax (photo from http://www.acad.carleton.edu/curricular/BIOL/resources/ant/index.html).

Damage:

Moisture ants prefer to nest in water damaged, rotting wood. In nature, you can find colonies inhabiting fallen trees and tree stumps that are in the advanced stages of decay. Because moisture ants enjoy sweets, they can often come into our houses to feed on any sugar left unprotected; this can be annoying. And because they inhabit wood, they can cause accelerated decay in our homes. Colonies that develop in decaying wood of a house, like carpenter ants and termites, can weaken wooden structures.

Monitoring & Management:

If multiple winged moisture ants are found around the house, do not immediately assume that you have an ant problem. Many winged ants become trapped in houses and collect on the windowsill during swarming season. Instead of looking for a treatment for the problem, begin to inspect your house.

Since moisture ants prefer to nest in rotting wood, look for areas of your home that are susceptible to excess moisture. Check wood near gutters, leaky plumbing, windowsills and drains where wood can become damp. Also look for wood that comes into contact with soil, such as porch steps, support beams and low siding.

Investigate rotting wood for evidence of ant activity. Look for tunneling and sawdust. If damage is found, confirm that it is moisture ants causing the damage. Wood damaged by moisture ants is cardboard-like in appearance. Many other destructive wood-nesting pests, such as carpenter ants and termites, may be causing the damage and treatment of these pests may differ.

The presence of moisture ants nesting homes is an indication of another problem, water damage. Solving the cause of water damage will also solve the ant problem. Using insecticides to control moisture ants is a temporary fix to the real problem. Remove damaged wood and replace it with the proper materials that will not encourage rot.

If moisture ants are causing an annoyance around the house, reassure yourself that they aren’t nesting in your wood by investigating as described above. Often moisture ants will nest in your yard and come visit your house looking for sweets and other food items. An easy remedy is to tolerate them while you clean up any morsels of food that can be found lying around. WSU’s ex-entomologist and infamous ant expert, Dr. Rodger Akre assures us that they will soon leave, once peak season is over. “Here’s one case where you just simply get your vacuum out, put on your tennie runners and stomp them to death, or do whatever. Do your bit for the environment and just put up with them for two weeks and they will be gone. Do not spray, it is not necessary.”

Photo Caption: Water damaged wood in contact with the soil. This is a great place to look for moisture ant colonies (photo by Art Antonelli).