By Greg Rubin, CNPS-San Diego Garden Committee member
Many people have been experiencing problems with many native species, such as Ceanothus, manzanita, mallow-like plants, and mounding perennials. One of the primary causes, surprisingly, appears to be invasion by Argentine ants! The increasingly hot, monsoonal weather of recent years greatly promotes them. What these ants are doing is placing insects like scale and aphids all over the ROOTS, which literally suck the life out of the plant from below, often undetected to those without the experience to pick up on the subtle clues (Figure 1).
Argentine ants appear to be responsible for other horticultural threats. They plant innumerable types of weeds, including Veldt grass, spotted spurge, petty spurge, purslane, scarlet pimpernel, chickweed, brass buttons, and dandelion, as well as natives like Miner’s lettuce and Purple three awn grass (they’re not very picky). Often massive infestations of weeds can be associated with ant activity; they give themselves away by the weeds they plant.
An additional concern, of grave consequence, is that these same ants may be spreading pathogens like Phytophthora. This is especially significant as there are virtually no treatments available for these water molds (Sudden Oak Death is just one example of this devastating group of pathogens).
Indications of Argentine ant presence may include:
- Lots of tiny black/brown non-biting ants (obviously!). They are the same type that get into our kitchens and trash.
- If pulling back the mulch from the base of the plant reveals ants, it's probably infested.
- Sometimes the entrance to the nest is not at the immediate base of the plant. If you suspect an ant infestation, pull the mulch back here and there a couple feet away.
- Also look for little bumpy scale insects at the base of the trunk or higher. May be white, gray, or brown, and can be hard to detect because they look like rough bark
- Look for previously mentioned weeds around the base or they may even mark their trails. The ants love to eat the small fleshy seed attachments and end up planting the seed as they go (see Fig. 2).
- As the ants remove soil from the roots, the plant may become "loose" (destabilized) in the soil and can be easily rocked back and forth.
- Many times when we remove a recently dead manzanita or ceanothus, the dirt just falls away from the roots, leaving behind almost polished wood. Mallows and mallow-like plants, including Fremontodendron, appear to be especially vulnerable. This soil removal from around roots may be partly responsible for downing many trees in powerful storms.
The way it works is that the ants thrive on the honeydew secreted by sucking insects. They actually farm them! The ants loosen and remove soil from the roots, then apparently place the scale all over them in large galleries. This alone can be tremendously damaging to the plants. The ants will then defend them with their lives, even attacking lady bugs and lacewings that try to eat above-ground scale or aphids. Ants can also attack pollinators as they compete for the nectar. Because most of the scale is below ground, they are much easier for ants to defend. However, it appears that in the absence of their ant protectors, sucking insects are devoured quickly by natural predators or suffocate from an accumulation of honeydew. Hence, we target the ants.
Dealing with infestations usually requires 2 approaches: Part 1 involves localized treatment of individual plants (only those threatened with imminent collapse if nothing is done) to kill the scale that is currently damaging the roots, and Part 2 is to set up a bait program to eradicate ant nests in the long term.
We recommend that all treatments begin with spraying an unbroken line of pyrethrin or synthetic pyrethroid around the perimeter and/or discrete sections of your yard to prevent additional trails coming into your property from the neighbors or other areas, thus isolating the treatments. A good product for this is Home Defense, available at most home improvement stores.
Treating an active infestation involves spraying the plant and the soil underneath it (approximately 1.5 times the diameter of the plant) to kill the scale and ants. This is typically done for plants that have lost a lot of leaves and appear in danger of imminent collapse. We have been using a product like Bayer Advanced Complete Insect Killer, in a hose-end spray form (see Figure 3), to treat the foliage and to drench the soil. It works in two ways: A Pyrethrin-based insecticide to kill the scale/ants on contact, and a systemic insecticide to protect the plant for around 3-6 months or so. Pyrethrin, although powerful, is considered somewhat organic in that it is derived from Chrysanthemums. The other constituent is a neonicotinoid that may be linked to colony collapse disorder in bees. It is essential that this product be applied AFTER flowering so bees will not be affected. Fortunately, since most native plants flower during the cooler weather of winter and spring, when ants are least active, most pollinators are long gone by the time things heat up in summer. This product is available in most Home Depot centers, farm & garden centers (like Grangettos), or on-line. There may be more organic approaches, like beneficial nematodes, that we have not tested yet.
You want to spray just the plant and immediate understory, as described above, because you do not want to kill the rest of the ant colony, so that the workers are available for Part 2, bringing the bait back to the colonies to kill the queens, thus eradicating the nests.
For Part 2 we recommend a bait like Advion, approved by several UC Riverside entomologists, which comes in either bait station or gel form (see Figure 4). Advion arena bait stations are somewhat protein based, in that the bait is peanut butter. Ants seem to go back and forth between sugar and protein in preference. It has been extremely effective, especially during the warmer months when the ants are most active, knocking out most of the colonies within a week. The bait is highly targeted to just the ants and should have no effect on beneficials, other than increasing the number of good bugs in their absence The key is that it acts slowly enough to be brought back to the nests and fed to the queen, which ends up dying from the accumulation. Unfortunately, most baits kill the workers too quickly and it doesn't make it back to the queens and larva. Workers only live about 40 days; queens can live upwards of 15 years, pumping out up to a thousand babies per month, and there may be hundreds of queens on any given property.
Recent tests with Advion have achieved about 80% eradication in just 1-2 weeks. What is significant is that in the absence of ants, most of the problems start to disappear. It remains to be seen if this is a "silver bullet", and whether it may turn out to be the main issue with higher native mortality in landscape situations. This is all fairly new insight gained within the last 2-3 years, but it is becoming apparent that the scale of the problem is enormous
There are occasionally other issues, like pathogens un-related to ants (if there are any), that can lead to native plant mortality, but it looks like ants may be responsible as much as 80% of the time. Ant baits are available at farm and garden supply stores or on the internet. Here are a few links where the treatments listed above can be purchased:
There is another treatment out there that has at times been quite effective but can also be a bit spotty. It is called Gourmet Liquid Ant Bait (see Figure 5), which is a glucose/protein based bait with 1% boric acid content (we usually dilute this by 50% with distilled water). The problem with the Terro baits available at Home Depot is that they are way too strong, at 5.4% concentration. They kill the workers too quickly so the bait never makes it back to the nest.
The Gourmet Liquid Ant Bait is loaded into KM Antpro dispensers and placed in shady areas generally in a 50 to 75’ grid on your property. They should be located at least a few feet away from the ant trails in order to entice scouts to set up scent trails that bring in the workers. Follow the instructions as precisely as you can. If you have no choice but to locate the Antpro dispensers out in the open, you can place a tilted bucket (or other object) over the dispenser to shade it. Shim it slightly underneath so that the ants can get under it.