Pour a big pot of boiling water into the mound. Instant death but rarely kills the whole mound, which will be obvious the next day when you see the thousand dead ants that are carried out by the survivors. Stay at it and you get them all or they move. Either option is better than when you started
Years ago when I first went to Georgia, we would set anthills to fight with each other by exchanging shovelfuls between mounds. Like with the boiling water, you can rarely eradicate them by this method, but you can reduce their numbers significantly, and get to see the satisfying sight of the dead ants heaped up!
Because of the manner in which Fire Ants will make a new nest when the old nest is attacked, the best one can do, I think, is annoy them until they leave the area in which they are being a problem. They can't actually be eradicated, only reduced. The boiling water trick is an easy and cheap way to annoy them enough to make them eventually leave the area, but it takes persistence.
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I have a niece who is allergic to fire ants. We've found a once a year spraying of nematodes keeps the mound count low. We probably wouldn't need to respray if we irrigated the lawn, but as it is the fire ants start returning at the end of the summer drought. We sprayed again just a couple of weeks ago and the three mounds I knew of are already gone.
An unexpected benefit last year was my cat staying flea free. Flea larvae is another food source for the nematodes. The container lists somewhere around twenty different kinds of pest insects that the nematodes survive on. Since the fire ant allergy is our motivation, I didn't memorize the rest.
I picked them up at a local nursery, but if they aren't available in your area, I have seen advertisements for them online. If anyone knows of a why these wouldn't be organic, please let me know. I try to avoid inadvertently poisoning myself with my hobby.
More specifically, the lack of water. Most of our yard is still unaltered native grass mixes. We have long hot summers and have had long droughts during those summers for several years. Even if they aren't drying out and dying, the nematodes aren't able to move freely through the soil without at least some moisture and so probably starve out when everything gets bone dry.
Our native adapted plants (like those making up my 'lawn') have evolved to go dormant during the summer and green up again with the fall rains. I'd rather save our limited water resources for productive gardens. So far the fire ants only have a brief period each year to attempt to take back control of our yard.
I am developing a series of small swales across the backyard. My hope is that in time these might retain enough moisture to sustain small colonies of soil life (such as the nematodes) through the height of summer which can then recolonize the yard without needed my help each year. This is my first attempt of the kind, though. I don't want to raise my hopes to high.
Fire ants aren't frost proof, they are a tropical ant that goes underground at night to avoid freezing temperatures. So during the cold season I'll go out late on sunny winter afternoons when there is a hard frost expected that night and scatter the mound and ants downwind as far as I can toss them. Any ants that can't make it back underground before nightfall are dead. It usually takes 2 or 3 visits to completely depopulate the mound.
During the frost-free season I'll go around with a wheelbarrow filled with soapy water, shoveling each mound into the soapy water where the surfactant quickly kills the ants. After 2 hours I'll dump out the dead ants and soil. Once you've dug out the first couple of shovelfuls and the mound is disturbed, allow 5 to 10 seconds for a thick layer of ants to form on the soil's surface before skimming off the ants with a thin layer of soil, then repeat the process. After a number of cycles of this when the ants start getting thin on the ground, I'll stop, give the surviving ants a few days to regroup before making another visit.
I've been using this method for over 14 years to control fire ant mounds on my 26 acres of pastures. It's a continuing process since each summer alates start new colonies and mature colonies will relocate across the property line from adjacent pastures, but it has been very effective at controlling fire ants on my property.
Several yrs ago when I lived in south tx where fire ants are a huge problem. I was converting my garden over to a no till garden with a haymulch and the fire ants just loved living under the mulch. Because of the mulch It was pretty difficult to pinpoint where the mounds actually were. My feet would get bit up almost every time I worked in my garden. I did some online research and ran across a guy called Howard Garrett. He has a natural recipe for immediate control of fire ants, but what helped out me out more was his information on how the microbial health of the soil attracts or repels fire ants. When I followed his suggestions on building the beneficial microbes in the soil i noticed a drastic decline in fire ants in my garden over a couple years time. This was over 10 yrs ago and I moved out of the area shortly after that so was not able to monitor long terms affects. One of the main things that helped was scattering molasses granules over the garden. The good microbes love molasses. Google Howard Garrett.
Cinnamon. Whenever we have ant invasions, I just sprinkle cinnamon. Within less than 30 minutes, they are gone. Granted that may not help much aka they simply move, but you can put a perimeter/barrier with cinnamon and it not be crossed.