Most of the annoying ants we suffer with in California, especially here in San Diego, are actually an invader called Argentine Ants. They arrived via shipboard to Louisiana, and have spread throughout warmer climates. Argentine ants are so successful because they have multiple queens per colony and therefore recognize all other Argentine ants as family. They don’t fight among themselves. There is a colony that stretches from San Diego to near San Francisco.
Argentine ants have nudged out many of our native ants, which isn’t a good thing. We need our native ants for decomposition. Argentine ants harm arthropods and have a terrible effect on the ecosystem; here in Southern California their impact on horned lizards have been devastating. The only way you can tell them apart (unless you have very tiny ants or black or red ants) from natives of similar size is by studying them with a microscope. This blog post shows great photos of the difference between ants.
Argentine ants farm aphids on plants and trees, milking the bugs for their ‘honeydew’, a sweet excretion. They will bring aphids to your plants and farm them there. They will also farm scale underground around the trunks or stems of plants, especially natives such as California Lilac (ceanothus spp). By the time the plant show stress and the ants begin to farm aphids above ground, much of the damage has already been done.
As much against annihilation as I am, this ant does terrible harm to our environment and should be happily living back in its native South American river area. Not only is it directly harmful, but because it is everywhere it incites people to spray poisons that kill all the beneficial insects as well.
The best solution is a borax bait trap that you can make your own. Borax is a powerful killer and should not be used liberally. Yes, it is sold as a fertilizer and as a laundry additive, and that borax kills insects and beneficial flora and fauna as it enters the watershed and soil. It is toxic to pets and children. However, just a little solution used wisely can really help control these ants.
I use old spice containers that have the plastic shaker ends on them for the bait traps. The holes are small enough to prevent other insects or animals from entering the jar, but are big enough for the ants. Otherwise you can use butter tubs with small holes punched in the top. Put a cotton ball inside the containers.
It is recommended to make a 1% borax solution rather than a stronger one because you don’t want to kill the ants immediately. You want them to bring the bait back to the nest and feed it to the queen. I know that is horrible, but they would definitely do the same to us if they could.
This recipe is based on research done by entomologist John Klotz at UC Riverside. Dissolve 1 tsp. boric acid (borax) and 6 tablespoons sugar in two cups of warm -preferably distilled or dechlorinated – water. Soak cotton balls in the bait solution and place in spice shakers or plastic tubs with holes in the lid. The containers will also keep the cotton ball from drying out quickly. Place in a shady location in the path of Argentine ants. Clean the container and replace the cotton ball weekly (it will become moldy). At first the bait traps will attract more ants, which is fine because they are bringing the bait back to their nests. If you want to kill the ants immediately, add more boric acid. For long-term control, reduce the boric acid to 1/2% to allow worker ants to feed for a long time before they die and therefore bring more back to the nest.
Keep the excess boric acid solution capped and in the refrigerator well labeled, so no one drinks the sweet drink.
Be sure to keep an eye out for ant activity around the base of your native plants, and if you have aphids on the leaves of plants you no doubt have ants farming them there. Argentine ants are pests we really can eliminate without fear, and allow our native ants to reclaim their territory.