A Staff Report from the Straight Dope Science Advisory Board

Do fire ants bite everything in their path?

October 21, 2003

Dear Straight Dope:

A week or so ago, I was helping my girlfriend's father lay mulch in his back yard when I was attacked by a ravenous horde of territorial ants, spurred on by their despotic queen. Having lived in Texas all of my short life, I am quite familiar with fire ants and their itchy, burning, stinging aftereffects. It was upon nursing my pockmarked feet that I wondered how ants knew to bite the surface of animal and human intruders. After all, it seems doubtful that ants would attack soil, bush branches, or other substances upholding what remains of the structural integrity of their feudal keep, or that they would turn their envenomed fangs upon innocent blades of grass. Do they attack foreign intruders based on texture, smell, or some other ingenious method, or do they instead bite everything in their path, hoping to get lucky and deliver a stinging message to a would-be conqueror? Also, what causes the skin outbreaks following a fire ant attack? From high school chemistry class, I know ants have formic acid, but some people have much more acute reactions to ant bites than others. Are inflammations and swelling based entirely on the acid's caustic effect on tissue, or is it caused by an allergic reaction, and if so, are some immune?

Fire ants, like most ants, have extremely sensitive chemical perception, and at least part of their recognition of potential stinging targets is based on chemosensory signals. However, given that they sting people's shoes, for example, they do clearly use other cues, such as texture and movement, when they decide whether or not to sting (as opposed to biting). If you were to watch a disturbed ant mound, you'd see that they bite many inanimate objects or surfaces nearby, but they generally reserve the sting for living targets. (Their jaws have no venom, only the stinger--biting and stinging are separate operations performed by different ends of the body.)

A single fire ant can sting repeatedly and will continue to do so even after it's out of venom; it clamps its jaws down and rotates in place, stinging as it pivots. The venom is complex, initiating histamine reactions and other adverse effects. About 95% of fire ant venom is non-protein and contains dialkylpiperidine hemolytic factors (not acid). These hemolytic factors induce the release of histamine and other vasoactive amines from mast cells. If a red imported fire ant (Solenopsis invicta) stings you, a sterile pustule appears at the sting site within 24-48 hours--no pustule forms from the stings of native species. The alkaloids aren't immunogenic (capable of inducing an immune response); their sheer toxicity to the skin is believed to cause the pustules to form. In other words, no one is immune to this effect. The pustules can easily become infected if not kept clean.

The venom also contains several allergenic proteins that can mean trouble for sensitive individuals. A minority of those stung are hyperallergic to the venom and can react quite strongly, with chest pains, nausea, dizziness, shock or even coma. A few deaths have been documented but these cases are rare. In short, much of the reaction to the sting is unavoidable, although allergic individuals will have more severe side effects.

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