Island Science

Deer Fly, Don’t Bother Me

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

If you happen to see me walking around the field station maniacally slapping my head, just keep walking. We have had a beautiful, sunny, extremely dry summer. The ponds and groundwater are lower and the freshwater mosquitoes have much less area to breed. An added bonus and one of the most significant factors in year to year variations in tick populations is also the dry weather which greatly limits the moist, humid, tall grass environment they require. Who is happy besides the beach goers and sun seekers?

The deer flies……… and over the past week, the green heads are beginning to give the deer flies a run for their money. I wrote about these lovely flying Marquis de Sade imitators two years ago when the deer flies and green heads were especially ferocious, but I have learned a lot about them since then, and they are literally on my mind constantly, so I am hoping a revisit of the topic is all right with everyone.

A lot of people use the generic term “horse flies” to describe any slow moving big fly that is biting them. Horse flies and deer flies are members of the fly family Tabanidae whose members comprise about 4300 species distributed around the globe. Horse flies (genus Tabanus) are larger than deer flies (genus Chrysops). Both horse flies and deer flies have large heads and large eyes. Horse flies can range from 20 mm (3/4 inch) to more than 25 mm (1 inch) in length. A common Horse fly species typically found near wetlands and salt marshes is Tabanus nigrovittatus, or the “greenhead fly.” This species has brilliant green eyes which are sometimes crossed with red or purplish bands. Larger species of horse flies are brown to black and have varying stripes or triangles on their abdomens. The antennae of horse flies are thick and lengthen into 5 thinner segments; the antennae of deer flies are long and thin. The wings of horse flies are usually clear or completely dark, whereas deer fly wings have varying patterns. Deer flies are comparable in size to house flies and are mostly yellow, brown, or black with varying stripes and shapes on their abdomens. Deer flies are sometimes called yellow flies or pine flies. Deer flies also have brilliantly-colored eyes, ranging from gold to green, with large brightly-colored stripes. I have to admit I have not been lingering on the trails in order to check out their beautiful eyes.
This year on Nantucket, the deer flies so far appear to be outnumbering the greenheads.

Tabanids go through all four stages of metamorphosis which include egg, larva, pupa, and our favorites, the adults. Deer flies lay their eggs in moist vegetation such as old leaves and rotten logs and most species requires some type of wet area for their habitat although some paddock adaptive species can be found in drier areas. Deer flies may also lay their eggs in running water. Females lay batches of 100-1000 eggs (depending on the species) on vegetation that hang over water or wet sites. The females protect the eggs with a secretion that keeps them from drying out or being attacked by parasitizing wasps. Eggs are cylindrical in shape and measure from 1 to 2.5 mm in length. Once the egg shells have hardened, they turn black and look like tiny black spots on the underside of the vegetation. Eggs hatch within five to twelve days.

Larvae use a hatching spine to break out of the egg case. Chrysops spp. (deer flies) are termed “hydrobionts” and are found in areas with high water content. Tabanus spp. (horseflies) prefer dryer substrates and are “hemihydrobionts”. The larvae taper at each end and are usually whitish in color,

but also can be brownish or green depending on the species. Black bands are found around each segment of the body in many species. The larva breathes through a tracheal siphon located at their posterior end. Yes, you read that correctly, they basically have a breathing tube up their behinds.

The larva has a small head and 11 to 12 additional segments. The larvae of Chrysops spp. feed upon organic matter in the soil. Tabanus spp. feed upon insect larvae, crustaceans, and earthworms. Even though the Tabanus spp. are considered to be carnivorous and cannibalistic, reports of as many as
120 larvae per square yard have been found, so I guess they don’t have problems with crowds. Larvae pass through six to nine stadia. The time spent in the larval stage can last from a few months to a year.

The larva moves into the upper 2.5 to 5.0 cm of the soil, where it is drier, when it is ready to pupate. Within two days after moving to the surface the pupal stage is reached. The adult fly emerges from the pupal case via a slit located along the thorax of the case. In most species the males emerge before the females. After emergence of both sexes, the flies mate. The males are easily differentiated from female flies because eyes are contiguous in the males and widely separated in the females. I am assuming for the flies themselves, that they have this all figured out. Mating starts with the male pursuing the female.
Mating is initiated in the air and completed on the ground. The female then deposits an egg mass and begins to seek a large mammalian host in order to lay additional egg masses. The most common greenhead and deer fly species will live as adults for three to four weeks on average although some
species can live up to two months (lucky us!).

Most deer flies attack people and pets around the head, neck and shoulders. And like their popular brethren, the mosquitoes, it is the females that bite humans and animals in order to obtain a blood meal to feed their eggs, males are content with pollen and nectar. Female horse flies and deer flies are active during the day. And they are more likely to attack you on warm sunny days. Many species will completely leave you alone at night or in shady areas. These flies apparently are attracted to such things as movement, certain colors (primarily dark colors), shiny surfaces, carbon dioxide, and warmth. Once on a host, they use their knife-like mouthparts to slice the skin and feed on the blood pool that is created (a lovely image). Females use scissor-like mandibles to pierce the flesh of bite victims and, like  many other blood sucking insects, inject an anti-coagulant into the blood stream while feeding. Bites can be very painful and there may be an allergic reaction to the salivary secretions released by the insects as they feed. The irritation and swelling from bites usually disappears in a day or so. However, secondary infections may occur when bites are scratched. General first aid-type skin creams may help to relieve the pain from bites.

Deer flies can make outdoor research at the field station a pain, almost literally. Fortunately, we have a tough group of junior rangers and older interns, and they have been game to try out a variety of things to protect their heads and ears from the buzzing menaces.

A variety of trapping methods have emerged which can be useful for sampling a nearby population for entomologists and these may even be effective in controlling populations.

Do you like the color blue?? Apparently, so do deer flies. In fact, wily park rangers and land managers have found a way to troll for deer flies that capitalizes on their penchant for bright blue colors and moving targets. The trolling method and variations of sticky substance covered blue buckets are described at by Dr. Russell F. Mizell, III, Professor of Entomology at the University of Florida Extension Office. Some people swear by this method (the testimonials on the site are some of the funniest I have ever read) and his recommended methods have led to a variety of excellent field station experiments. The trolling trap device consists of a blue cup or bucket coated with sticky material (such as Tangle-trap organic insect trap coating); the cup is placed outside of a moving vehicle, usually stuck on the end of a pole sticking out away from the car. The cup or bucket attracts horse and deer flies with its movement and color. They recommend driving around at speeds less than seven miles per hour to catch flies and reduce the population. We have done this at the field station and collected between 20-50 flies in less than 30 minutes. I can’t say the carbon footprint of this method is zero though. An even more embarrassing method is to attach a similar device on your head and walk around collecting the insects on your head. Deer flies and greenheads travel to the highest point on your body. We use “Solo-like” plastic 12-16 ounce cups in blue and red covered with Tanglefoot and strapped to intrepid junior ranger team leader’s heads. Another device is the Tred-not Deer Fly Patch, which is a non-chemical sticky patch for controlling horse and deer flies. Some testers have reported good results from these odorless adhesive patches. The patches are 7.5 cm (3 inches) wide by 15 cm (6 inches) long, and are worn on the back of a baseball cap to trap and hold biting deer flies. Some folks make sure to buy a dark blue or dark colored hat and coat the entire hat in Tanglefoot. So when the little buggers come get you, they get stuck there and go along for the ride. It can be a little gruesome.

Closer to home, on Cape Cod, entomologists have been using box traps for many years on salt marshes to reduce greenhead populations. For “do it yourselfers” out there, go to for detailed plans to build your own box trap. We have two at the field station that have been very effective; one of which collected 40 flies the first 48 hours. And if you want to buy a trap and are too lazy or fly bitten to make one; just go to this web site and for $60.00 you’ll have a contraption that lowers the population, without poison and with some entertainment value. Traps should be placed on the marsh edge near the upland or along the open edge of wooded or shrubby areas.

The best locations are at breaks or openings of low vegetation in screening stands of trees or tall brush near the marsh. These areas are called fly-paths because they lie in a direct line for the normal flight path of flies leaving their larval home. Another method is called the “malaise trap” (I love that name) which can catch large numbers of flies by simply being in their flight paths or by the use of attractants, such as carbon dioxide and octenol. These are given off by respiring mammals like us and act as an attractant for flies and mosquitoes. The best personal protection is a wide brimmed hat, long sleeves, and long pants. Other methods used by campers include carrying a pole and

holding it over your head (did not work for me last week), or wearing protective gear like mosquito mesh or a bandana to protect your head and neck. Deer flies and green heads are not deterred by the same smells and repellents as other insects like mosquitoes and ticks. Citronella products are less effective and permethrin products in high enough concentrations to repel deer and horseflies are only recommended for horses. In many states these are a serious pest for livestock.

Unfortunately, there is not a lot one can do to control these flies. The wetlands they use for reproduction are precious and necessary habitat. Repellants containing DEET (N-diethyl-metatoluamide) do not prevent flies from landing, although they may inhibit the flies from biting. Surveys undertaken of park rangers, landscapers, and outdoor enthusiasts indicate that they have had little success protecting themselves with DEET. Long sleeve shirts, a hat, and long pants are your best and safest offense. The prime season on island for both the deer flies and the greenheads is July and August. As bad as it seems, we are actually fortunate in that our relatively dry early summer and late spring means that the number of these ravenous creatures is less than we endure during wetter years.
When you come out to the field station, join one of our junior ranger walks to see these ridiculous looking sticky hats in action.