County Broadband Internet Survey

Harnett County,
North Carolina

Cooperative Extension 

What is the safest pesticide I can use to kill fire ants?


Question What is the safest pesticide I can use to kill fire ants?

Answer While pesticides are designed to kill, there are differences in their “safeness” (or toxicity). If an insecticide targets a pest like the fire ant, then the toxicity to the ant is 100%. However, the toxicity to the applicator or other non-target organisms may vary.

Researchers developed a way to measure and compare the acute toxicity of any substance. This measurement is called the LD50. Stated in milligrams (mg) of pesticide per kilogram (kg) of body weight, an LD50 represents the individual dose required to kill 50 percent of a population of test animals (e.g., rat, fish, duck ). The lower the LD50 dose, the more toxic the pesticide.

LD50s are determined for dermal application, ingestion and inhalation. Each pesiticide has a different toxicity for each of these situations. In other words, pesticide toxcity varies depending on whether you get it on you, eat it or breathe it.

While LD50s for every pesticide have been determined, they are not listed on pesticide labels. It is too difficult for the average person to apply the LD50 concept. The Federal government has simplified this comparison method down to three words – Danger, Warning and Caution. One of these signal words is located on every pesticide sold in the US.

Danger means the pesticide is somewhere between extremely and highly toxic. A drop to a tablespoon will cause an acute reaction (death is an acute reation). Warning means the product is moderately toxic. Exposure to an ounce of this product may be fatal. The safest signal word is Caution. Pesticides marked caution will range between slightly toxic to harmless.

Since all of the pesticides result in death for the fire ant, why not use only the products which are minimally toxic? If safety is the most important factor to you as an applicator, then shop for insecticides which are labeled “Caution.” Sometimes speed of control is the primary factor.

The safest fire ant killers are baits. The tradeoff for safety is the length of time it takes for these “safe” insecticides to work. For example, the active ingredient methoprene (trade name Extinguish) has the highest LD50 of any fire ant killer on the market (34,600). Remember that the higher the number the safer it is (for comparison table salt has a LD50 of 3,000). The only thing safer than methoprene is kindness, which is primarily antidotal and not an approved pesticide. While methoprene is super safe, it takes 8 to 12 weeks to control the fire ants. The next four “safe” active ingredients are spinosad,fenoxycarb, pryiproxyfen and hydramethylnon. Visit this website for quick answers to questions about these pesticides If you do not have internet access, then call me at 893-7533 or email me at

Do not confuse speed of killing the target pest with degree of death. Pesticides may vary in their speed of killing, but there is only one degree of death. Dead is dead. Super toxic pesticides (and/or fire) do not kill the ants any “deader.”

Gary L. Pierce

Horticulture Extension Agent

Harnett County

Board of Commissioners
GIS Portal
Register of Deeds Search
Employee Information