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Volume 40 Issue 12 • July 22-28, 2010
now in our 40th season

Fighting Flies

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

There are so many wonderful things to love about summertime on Nantucket. Unfortunately, this week’s topic is not one of them. Right now we are inundated with deer flies and the occasional green fly at the Nantucket Field Station. As we’ll see, our standard arsenal of protection doesn’t do much to keep these persistent little guys off us, but there are a variety of measures, including environmentally responsible ones, of limiting our exposure to these pests.

So what are we up against?

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 species typically found near beaches 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 larger species of horse flies are less common, but inflict a painful bite. 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. The wings of deer flies are usually marked with dark patterns. 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 anecdotally 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 "hemi-hydrobionts." 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 time spent in the larval stage can last from a few months to a year. The larvae of Chrysops 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. The larvae, stage usually lasts from one to three years, depending on the species. 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 is ready to seek a host (looking for a mammal). The most common Nantucket 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.

We are fortunate to not have to deal with too many black flies which plague states to the north. A black fly is any member of the family Simuliidae of the Culicomorpha infraorder. There are over 1,800 known species of black flies (of which 11 are extinct). Black flies are much smaller than our deer flies and are notorious in New Hampshire and Maine where there are numerous freshwater streams to provide breeding habitat.

Biting flies in the family Tabanidae (particularly the deer fly, Chrysops discalis) can act as mechanical vectors for tularemia which is a zoonotic bacterial disease that can affect mammals. Individual flies can carry the organism for two weeks.

Now that we know what is biting us, or in the case of the deer flies, often landing on us to check us out before biting us; we can look at ways to reduce our quality time with them. The greatest horse and deer fly activity occurs on warm, sunny days when there is little or no wind. A slight drop in temperature or a sudden breeze reduces biting attacks. Horse and deer flies are visual insects, locating hosts by movement. Dark, moving objects and shapes are most attractive to the flies. They are also attracted to carbon dioxide that is released from their hosts. To reduce exposure to bites, it is best to wear light-colored clothing, including a light baseball cap.

There are no super effective long term methods to control horse and deer flies. A variety of trapping methods have emerged which can be useful for sampling a nearby population for entomologists. 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 ( as written by Dr. Russell F. Mizell, III, Professor of Entomology at the University of Florida Extension Office). Some people swear by this method and it sounds like a perfect field station experiment. 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. An even more embarrassing method is to attach a similar device on your head and walk around.  Another device is the Tred-not Deer Fly Patch, which is a new, 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. 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. 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.

There are some scents that repel deer flies although they are not necessarily the same smells that can keep mosquitoes and chiggers away. Lavender, garlic, peppermint, and eucalyptus oils are the most reliable scents to keep biting flies at bay. Citronella products are less effective and permethrin products in high enough concretions to repel deer and horseflies are only recommended for horses. 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-meta-toluamide) 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. Fortunately, our relatively dry early summer/late spring means that the number of these ravenous creatures is less than we endure during wetter years. I’ll see you on the trail soon with your sticky tape hats!


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