Traditionally, the most effective treatments for the control of horse bots have been applied at least 30 days after the first killing frost. Some authorities believe a second midwinter treatment is needed to "clean" the animal from bots possibly missed earlier. (Your veterinarian can best advise you on a workable program to fit your circumstances.)
Bots, not true worms, are the larval stage of bot flies. Bot flies are active from midsummer until the first killing frost. Adult bot flies, resembling honey bees in appearance, may cause animals to instinctively fear them. Females, unable to feed or bite, lay 150 to 300 eggs in a life span of 7 to 10 days. Eggs (pale yellow, pinhead sized) are deposited on the forelegs, chest, neck, belly, and occasionally flanks or hind legs of the animal.
Eggs accumulate until adults are killed by frost and can remain viable for several months. Eggs incubate in 1 to 2 weeks and hatch only if licked or eaten.
Sponging the animal with warm water (104 deg F to 120 deg F) on cool days (less than 60 deg F) will cause eggs to hatch and then die of exposure. (Firm rubbing is important.) Also, clipping hairs infested with bot eggs will aid in control. Some states recommend coumaphos (Co-Ral) 0.06 percent or tetrachlorvinphos (Rabon) 6.5 percent warm water solution to be sponged on the animal. (Use rubber gloves during applications and follow label directions and safety precautions.)
Currently available commercial products are very effective for control of bots. The use of sponging or mechanical/clipping removal of bot eggs should be viewed only as a supplement to your bot control program.