HORSE BOTS 
How To Control Them 
Three species of horse bot flies are 
serious pests in this country. 
The common bot fly and the 
throat bot fly occur throughout the 
United States. The nose bot fly is 
found in Northwestern and Mid- 
western States. 
Bot flies can be controlled by 
treating infested animals to destroy 
larvae and eggs. 
HOW BOT FLIES LIVE 
Bot flies have four life stages— 
adult, egg, larva (bot), and pupa. 
These stages are described on this 
page and on pages 4 and 5. 
Adult flies do not feed. Their sole 
purpose in life is to reproduce. They 
live from a few days to 3 weeks. 
In Northern States, flies appear 
about the middle of June; they live 
until there is a heavy freeze. In 
Southern States, they appear as 
early as March and may be seen 
until December. 
Eggs usually can be found on 
horses as soon as flies are seen. Egg 
laying continues as long as adults 
are present. Eggs of the common 
bot fly that can hatch may be found 
on the animals as late as December 
in Northern States, and as late as 
February in Southern States. 
During the larval stage, the bots 
develop inside the animal; they 
grow from minute size to about two- 
thirds of an inch in length. The 
exact nature of the food they take 
is not known, but bots feed on body 
fluids. 
Mature bots may be found in the 
digestive tracts of horses and other 
equines at any time of the year, but 
they usually pass out of the animals 
by October. Practically all the bots 
found in animals during the early 
part of the winter are young. 
When fully developed, bots pass 
out of the animal to the ground, 
where they seek protection. They 
crawl very little. Usually they bur- 
row into the ground near the place 
where they were dropped. They 
burrow only deep enough to escape 
the direct rays of the sun. 
COMPARISON OF THE 
THREE SPECIES 
Common Bot Fly 
(Gasterophilus intestinalis ) 
Adult— Largest of the three 
species; about the size of a honey 
bee. Body is covered with black and 
yellow hair. Wings are mottled. 
Female hovers about animal; 
darts from place to place as she 
cements egg after egg to hairs on 
the forelegs (a favorite place), the 
3 
