Monday, July 03, 2006

Deer fly defense

During their relatively short breeding season, four to five weeks in June and July, the deer fly (Chrysops discalis) can make life miserable for the dedicated woods walker. These flies feed by dive-bombing the victim, quickly slashing a hole in the skin, secreting saliva with irritating chemicals that increase blood flow, then eating blood from the wound.

These flies typically feed on large mammals. They are attracted to the plume of carbon dioxide that typically emanates from such mammals, as well as to the movement of large dark shapes. Their brute-force feeding style does not work well on fur-covered surfaces, so the flies tend to seek out parts of the body without protection, such as the eyes and ears.

In my experience, the use of DEET-containing repellent has absolutely no effect on deer flies. What does work is to wear a broad brimmed felt hat. The flies lock on to your CO2 plume, then home in on the hat, then dive in and attempt to feed madly on the brim of the hat. Their behavior pattern is fixed. They do not attempt to move around on the victim prior to feeding; the dive bomb and instantly start chewing.

Discovering this quirk of deer fly behavior has really improved my experience of the woods this time of year!

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