Forsythia and daffodils at last are giving us bursts of yellow to relieve the somber grays left over from winter. But another visitor is about to appear on the scene, a small black pest: the black fly.

Most Mainers know from bitter experience that these pesky bugs, said to be so vicious that they can drive moose out of the woods, can torment humans with their buzzing and bites.

They know, too, that they breed in running fresh water, so the offshore islands are mostly free of them. But the water must be clear, says Jim Dill, pest management specialist for the University of Maine’s Cooperative Extension in Orono. That’s why Maine has far more black flies now than it did 25 or 30 years ago, before Maine’s rivers and streams were cleaned up.

Black flies come out around mid-May, as soon as the air temperature reaches a steady 50 degrees. It’s common knowledge that they are gone by the Fourth of July. Mr. Dill calls that a myth. He is an avid hunter and has encountered them in the woods well into November. Their numbers may have decreased by that time, but their 40 different species appear at different times.

The females feed on animal blood. They swarm around the head, attracted by the carbon dioxide in breath, and can find their way into any gap in clothing and even under mosquito netting that is not completely tucked in. Their bites can cause anything from persistent itching to red quarter-sized swollen welts that can itch and hurt for days. People allergic to them can even suffer anaphylactic shock and require emergency medical treatment.

What to do about them? You can stay out of the woods, since they rarely attack in open areas, especially if a breeze makes it hard for them to zero in on a victim. If you do go where they are plentiful, wear long sleeves, tuck your pants in at the ankles, and apply on exposed flesh a good insecticide like one containing DEET or maybe Ole Time Woodsman’s Fly Dope Formula if you can find it.

Finally, isn’t there anything good about black flies? Some say that the males, which feed on the nectar of flowers, are effective in pollinating blueberries and other crops. Mr. Dill, the pest specialist, says they may do some incidental pollination but not on any large scale.

But he goes on to say that black flies are a major element in the food chain of Maine’s freshwater trout. The fish either eat the black flies directly or else eat other bugs that have eaten the black flies.

So there’s some good in them after all.