[Tech Talk] Horse Flies and Deer Flies

Columns - Technically Speaking

Helpful information about these pests that can be communicated to your customers.

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June 25, 2010

Warm weather makes outdoor activities like picnics, beach-going, etc., pleasurable. It also brings many species of Tabanidae that can make those times a literal pain. In this article we’ll visit common genera of Tabanidae and discuss what we as pest management professionals need to know about them in order to speak intelligently with our customers.

Tabanidae is a family of true flies (Diptera) that go by several names including horse flies, deer flies, yellow flies, etc. They are nuisance pests due to their flying and buzzing around, but more importantly because of their painful bites. In fact, those of us that have experienced one of these bites may argue that referring to them as a nuisance is an understatement. Bites may exceed 10 per minute, making outdoor recreation and local tourism unpleasant or unfeasible. Tabanidae also may transmit some pathogens and parasites mechanically via contaminated blood on their mouthparts. This is rare, but those pertinent to human health include tularemia, anthrax, and Lyme disease.

Within the family, there are a few species that are notorious pests. What many people refer to as horse flies are usually one of two common species – Tabanus nigrovittatus and Tabanus simulans. They are relatively large species, with lengths between 10 and 30 millimeters. The antennae are short and the base of the flagellum is enlarged. The ocelli are either vestigial or non-existent. Their wings are either clear or spotted.

Deer fly species are mostly from two genera – Chrysops and Silvius. There are many species within these genera that will aggressively attack people for a blood meal. They are generally smaller than horse flies at a length between 6-10 millimeters. The antennae are long and the base of the flagellum in not enlarged. The ocelli are present. Their wings are banded.

Despite differences, both horse flies and deer flies deliver a painful bite. More specifically, the females bite. The reason for their voracity is to obtain nutrition in order to lay eggs. Males do not require blood meals (and do not even have the appropriate mouthparts to obtain them). Most species are diurnal, attacking during the day. They are drawn mostly to carbon dioxide let off from the potential host. Other chemical cues are 1-octen-3-ol, ammonia, and phenols. The reason the bite is so uncomfortable compared to other blood-sucking insects, like mosquitoes, is due to the way in which the blood is obtained. The mandibles and maxillary laciniae cut the skin and capillaries, without any anesthetic, in order to create a pool of blood to be consumed. This type of feeding is known as telmophagy.

Tabanidae are difficult to control. Pest management professionals can be an asset as educators. There are some cultural changes that one can make in order to get relief from these flies. Most species thrive in wet environments, so often water management on a property may help reduce a population. Also, appropriately timed removal of emergent vegetation may keep some species from being able to lay their eggs, thus disrupting the life cycle. Lastly, avoiding areas where forests meet open space (ecotones) will minimize bites, as many species wait for unsuspecting passers-by in these areas.

There are a of couple traps – the Manitoba and the Epps traps – that have been used to attract certain species. They tend to be big and bulky (designed more with livestock in mind) and only attract certain species. Many times the female has already laid a complement of eggs before being drawn to a trap. Even if large numbers are caught, relief may be temporary. One study reported catching more than 95,000 flies with only minimal relief.

Biological control is a potential control strategy that still requires more research. Each stage of the life cycle is susceptible to predators or parasatoids. Some potential biological control agents include "ladybugs" and parasitic or predatory wasps and flies.

Chemical control is generally not effective. The main strategy is aerial applications with pyrethroids, which offer only temporary relief. Repellents that are applied to the clothes and skin, such as DEET, may offer some relief.

These voracious and often beautiful flies deserve respect from us. They are highly specialized in order to obtain blood from animals including people, often resulting in pain and less frequently in disease. These are certainly pests that will not be controlled with a magic wave of the wand. Rather, as pest management professionals, we are most valuable at lending ideas on how to minimize contact with these blood-sucking pests.


The author is assistant branch manager for McCall Service, Jacksonville, Fla. He earned a B.S. in entomology and an M.S. in entomology specializing in urban pest management (specifically fly control) from the University of Florida. He is a member of the Copesan Technical Committee and can be reached at rwelch@giemedia.com.

Copesan is an alliance of premier pest management companies with locations throughout North America. Learn more at www.copesan.com.