Ouch...Stupid Greenheads

That Pesky Greenhead Fly

It's a perfect early summer day. The sky is deep blue and cloudless, and there is a very light breeze. The sand under the afternoon sun is hot under your feet as you walk to the beach, and the incoming tide looks inviting as a slight perspiration has begun to form on your forehead. You are just about to set down your beach chair, when ouch...you are bitten on the calf by one of those pesky greenhead flies. Familiar to all of us who go to the beach in the summertime on Cape Cod, the greenhead fly can truly ruin an afternoon at the beach. While we can't keep them from biting, CapeCodWeather.Net thought it useful to supply some information about these annoying critters, so at least you know something about them.

The greenhead fly (Tabanus nigrovittatus) is a biting salt marsh fly which reaches peak population numbers on Cape Cod during mid to late July, with some years being worse than others. Similar to mosquitoes, only the female does the biting, as she is looking for a blood meal to provide protein for egg production. Greenheads mate on the open marsh, and after mating the female will lay an egg mass of 100 eggs or more on a leaf bade of the marsh grass (Spartina ssp.). Upon hatching, the larvae either drops or is washed to the surface of the marsh below, where it will eventually bury into the marsh and will remain there in this larval stage usually until the following spring, but some larvae will not emerge until the second year. Upon emerging, this larval stage is very predacious and even cannibalistic, eating a variety of other insect larvae and as noted even its own kind. A pupa stage follows this spring feeding frenzy, and the adult emerges a short time thereafter in late spring or early summer.

As we mentioned, adult mating occurs over the open marsh and the female will lay her first batch of eggs. This batch of eggs is produced without the need for a blood meal as the feeding during the larval stage provided the necessary proteins. However, given the female may lay several other batches of eggs, she now goes on the prowl looking for that blood meal, which is when we become welcome prey. The bright green eyes of the fly are used to see larger targets like ourselves, although they will take a blood meal from any mammal. It seems both movement and dark clothing attract the flies. Also, when one comes out of the water, the reflection of light off the wet skin seems to attract the flies.