Autumn is no time for an outdoor picnic, says entomologist Lynn Kimsey, chair of the University of California, Davis Department of Entomology and director of the Bohart Museum of Entomology. It's fly season.

The common house fly (Musca domestica Linnaeus) breeds in manure, compost piles, dumpsters, and, at this time of year, disced-over tomato fields and vegetable gardens, she said. It's commonly found on hog and poultry farms, in horse stables, and on ranches. But it also makes "house calls."

The housefly is known to transfer at least 100 different pathogens and carry about 6.6 million bacteria on its body, according to UC Davis forensic entomologist and fly expert Robert Kimsey. It's responsible for transmitting both parasitic and bacterial pathogens as well as viruses. Among them: typhoid, cholera and dysentery (bacterial diseases) and infective hepatitis (virus).

"In the early part of the last century," he said, "it was notoriously known as the 'typhoid fly' because of its propensity to transmit the pathogen of that frequently fatal disease."

Said Lynn Kimsey: "House flies are why we need indoor plumbing and window screens. Years ago, the high infant mortality was largely attributed to house flies that carried pathogens from the latrine into the kitchen, contaminating the baby bottles on the kitchen counter."

The female housefly typically lays 600 to 1,000 eggs during her two-month lifetime and can produce as many as 12 generations a year. The eggs mature in 10 to 12 days.

"Calculated over an entire summer season, a pair of house flies could produce 191 quintillion flies, enough to cover the earth 47 feet deep, if all their progeny were to survive," Robert Kimsey said.

Said Lynn Kimsey: "If the temperature gets high enough, flies can bang out a generation in a little over two weeks."

The fly problem seems particularly bad around tomato fields where wet rotting tomatoes provide food for their developing larvae. A mild summer, like the summer of 2008, exacerbates this problem. Fly reproduction won't diminish until the first cold snap and the start of cold and rainy days. The common housefly overwinters in its immature stages (larva and pupa).

"The best way to control flies is to exclude them from the material their larvae breed in--manure and household kitchen wastes, for example," Robert Kimsey said. To control flies, place boric acid in the bottom of dumpsters, microencapsulated or wettable powder formulations on walls near dumpsters and other breeding sides, and fly baits near adult feeding sources.

About the Author

Kathy Keatly Garvey

Kathy Keatly Garvey, UC Davis

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