Differences of Southern and red imported fire ants
The southern fire ant (Solenopsis xyloni) and red imported fire ant (Solenopsis invicta, RIFA) both inhabit lawns and can bite and sting people and pets. In California, we have both species,, and they can coexist in proximity.
Both species are reddish in color and show worker size variation, meaning that workers range from small to large within the same colony. Also, members of both species can sting people and animals.
Despite the similarities, there are a few differences that can differentiate these two ants. Here you will read a summary of such differences:
Color
Although not always reliable, color can help us differentiate them in the field. Southern fire ants usually have a lighter red head and thorax, as opposed to the darker red or reddish-brown color of RIFAs. Such color difference can be seen with the naked eye.
Shape of head
The front part of the head, or clypeus, provides a very useful feature for ant identification. However, this character is only visible under higher magnification under a microscope. The clypeus in RIFA has three (3) projections, whereas in southern fire ants it has only two. Now, one problem when identifying RIFA using this feature is that not all RIFA workers have a conspicuous middle projection on their heads, and only larger workers tend to have it. As a result, it's best to check larger workers for this character.
Invasiveness
The southern fire ant is native to North America, including California. However, RIFA is an invasive species from South America. Both can inhabit lawns, but RIFA tends to be more aggressive when invading and spreading in an area. RIFA tends to take over the entire lawn and often competes with other ants, especially Argentine ants.
RIFA is generally the more serious pest. RIFA Colonies can reach high densities, workers aggressively defend their nests, and their painful stings can pose risks to people, pets, livestock, and wildlife. RIFA can also interfere with agriculture and damage electrical equipment. Southern fire ants can also sting and become a nuisance on lawns; however, they generally do not produce the same large-scale ecological, agricultural, and public health impacts associated with RIFA.
Management
Both Southern fire ant and RIFA can be managed using insecticides. When doing so, the best practice is to use granular baits specifically made for fire ants. All these baits have oil-impregnated granules made of corn grits that are highly attractive to fire ants. While fire ants may feed on sugar-based lures and baits, they are more attracted to oils and grease. Also, the insecticides in fire ant baits tend to have a more delayed action, giving foraging ants enough time to feed, transport, and share the toxicant with their colony mates. The speed of action depends on the active ingredient, ranging from a few days to a few months until colony elimination. The delayed action of baits creates an ideal situation for maximum ant control, as the whole colony can be affected by the bait rather than only foragers. As a reminder, in fire ants and many other ants, only 10-20% of the colony forages outside the nest, so a fast-acting bait or insecticide spray can kill only 10-20% of the colony at best. Slow-acting baits, on the other hand, can slowly reach all members of the colony, including queen(s), and cause whole-colony collapse.
Liquid insecticides can also control fire ants when applied as a soil drench; however, it is best not to use them unless in emergency situations. In a nutshell, liquid insecticides have limited effect on the colony and can contribute to soil and surface water pollution. Granular baits, however, are much more effective in colony elimination and are less likely to cause environmental hazards. Read more about all the options available for fire ant management