The bot flies are in the family Oestridae. Within this family are four subfamilies, including the Gasterophilinae, the stomach bot flies. All subfamilies within Oestridae are related by their larval feeding characteristics. The larvae demonstrate obligatory myiasis because they require a living host to complete development (Mullen and Durden 2002). Completion of the bot flies' life cycle is dependent on the larvae consuming nutrients from tissues in the gastrointestinal tract of the horse (Zurek 2004).
(from ITIS 2007)
Gasterophilus intestinalis and G. nasalis are found in Florida. Although found throughout the entire state, geographical location determines seasonality of adult activity. In the southern portions of Florida, the adult fly will be more active throughout the year whereas fly activity in central and northern Florida may be limited to late spring through early winter (Kaufman et al. 2006).
Eggs: The female bot fly can oviposit between 150 and 1000 eggs on a horse's body (DuPonte and Larish 2003). This typically occurs during the early summer months. The female oviposits directly on single hairs of the horse's front legs (cannon bone area), abdomen, flanks, and shoulders. Ovipositing on the rear legs appears to be discriminated against by most flies, whereas age, breed, size, and sex do not appear to be a factor (Cogley and Cogley 2000).
The bot fly eggs are approximately 0.05 inches (0.127 cm) in length and are pale to grayish yellow (NDSU 1991). The eggs are essentially stalk-less and are attached near the tip of the hair. The eggs contain two regions on the lower half which surround the hair allowing for attachment and another region extending at a thirty degree angle from the hair (NDSU 1991).
Larvae: The eggs develop into first instar larvae within five days of being deposited by the female. Eggs hatch into a maggot within seven to 10 days of being laid. Larvae are stimulated to emerge by the horse licking or biting the attached, fully developed eggs. The larvae either crawl to the mouth or are ingested and subsequently bury themselves in the tongue, gums, or lining of the mouth and remain for approximately 28 days. After wandering in the mucosa of the mouth, the larvae molt to the second stage and move into the stomach (Merial 2001). The second and later third stage larvae typically attach to the lining of the stomach in the non-glandular portion near the junction of the esophageal and cardiac regions. The second and third instar larvae remain immobile for the following nine to 12 months (DuPonte and Larish 2003, Zurek 2004).
The third instar larvae are relatively large, between 1/2 to 3/4 inch (1.27 to 1.91 cm) long. They are adapted to life in the gastrointestinal tract with their rounded body, narrow, hooked mouthparts, and spines (Pfizer 2007). The hooked mouthparts (maxillae) enable the larvae to securely attach to the lining of the stomach and intestinal tract. The larvae use their flat mandibles to abrade the tissue of the stomach. The uniqueness of the spines is helpful in identifying the species. Gasterophilinae are all characterized with rows of smaller spines amongst rows of larger spines (Colwell et. al 2007). The third stage instar larva is distinguished by its yellowish color (NDSU 1991).
Pupae: After the third instar larvae have matured, they detach from the gastrointestinal tract and pass from the horse's body in the feces. The larvae burrow into the soil or dried manure where they pupate and remain for the next one to two months. This stage of the life cycle occurs between late winter and early spring. Because of horses' behavior to habitually defecate in the same location and the lack of larvae movement, the amount of pupae in fecal piles can become rather significant (Cogley and Cogley 2000).
Adult: The adult horse bot fly emerges after a three to 10 week period during the summer or fall season. After the fly emerges from the pupa, it quickly finds a mate. The mating activity typically occurs in the early afternoon during warm, sunny weather in relative proximity to horses or on hilltops. According to Cogley and Cogley (2000), mating may not solely rely on the presence of horses or hilltops. Mating is likely to occur around fecal piles where pupae numbers are large thereby greatly increasing the chances of male and female contact upon adult fly emergence. Once the male and female flies meet, they sink to the ground and copulation occurs within three to four minutes. Within hours, the female begins host seeking and oviposits. Dispersal of eggs by the female is not restricted to one horse but can occur on many horses within an area. The female increases the chance of larval survival by not limiting her eggs to one horse (Cogley and Cogley 2000). The adult female lifespan lasts seven to 10 days (Williams and Knapp 1999).
The adult fly is between 2/3 and 3/4 inch (1.67 to 1.91 cm) in length and resembles a bee with its black and yellow hairs. Because it is a fly, is has only one pair of wings. The adult has small, nonfunctional mouthparts and does not feed (DuPonte and Larish 2003, Kaufman et al. 2006). The female's abdomen is elongated, curled under and serves as an ovipositor (Zurek 2004).
The direct damage the bot fly causes occurs after the larvae enter the animal's mouth and gastrointestinal tract. When the first instar larvae burrow into the mouth, the horse may experience severe irritation, as well as the development of pus pockets and loosened teeth. Loss of appetite may develop due to the larva's inhabitance (Mullen and Durden 2002).
As the second and third instar larvae inhabit the gastrointestinal tract and attach to the stomach and intestine, multiple complications may arise. Larvae present in large numbers in the stomach can cause blockages and lead to colic. According to Mullen and Durden (2002), horses are capable of tolerating an infestation of 100 larvae. Large numbers of larvae impact the host by damaging the tissue of the stomach or the gut lining and consuming the nutrients that would otherwise be beneficial to the hosts' well-being. Other health issues that may develop due to a severe infestation of these larvae include: chronic gastritis, ulcerated stomach, esophageal paralysis, peritonitis, stomach rupture, squamous cell tumors, and anemia (Pfizer 2007, Williams and Knapp 1999).
horse bot fly larva inside eye
Chemical control. An insecticide can also be applied weekly during the peak egg laying season to the areas of the body covered with bot eggs. Oral medications can be used to reduce the numbers of larvae inside of the stomach. Commonly used medications include avermectins, which come in different formulations: liquids, gels, boluses, and feed additives. Avermectins work to control the adult and larval fly stages (Peter et al. 2005). The horse should be treated within one month after eggs are seen during the early summer months. A second treatment should be administered in the Fall to control the second and third stage larvae.
Insect Management Guide for horse bots
Authors: Morgan McLendon and Phillip E. Kaufman, University of Florida
Photographs: Morgan McLendon, Lyle Buss and Jerry F. Butler, University of Florida
Project Coordinator: Thomas R. Fasulo, University of Florida
Publication Number: EENY-406
Publication Date: December 2007
Copyright 2007 University of Florida
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Department of Entomology and Nematology
Division of Plant Industry
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adult G. intestinalis
egg case (hatched)
two egg casings (hatched)
larvae in stomach
third instar - dorsal view
third instar - ventral view
pupa
adult