The tell-tale drone of their wings is enough to make the calmest person pick up a swatter and flail about. As they aimlessly spiral past your face, you lash out, ending up knocking yourself silly instead of knocking down the bug. If the flitting as your sitting is making you buggy, you might have a problem with flies in winter.
Cluster flies (Pollenia rudis) make your house a hotel during winter, coming out on warm days to press up to window panes and buzz up against light bulbs. Think twice before you flatten that flier – they’re not disease-riddled pests. They’re in your home looking for a little warmth, shelter, and understanding.
Cluster flies are what entomologists refer to as “nuisance insects,” which wander our homes in the off-season, waiting for spring. Found throughout the area, most homes don’t host more than a few, but occasionally tall buildings or houses near fields can be plagued by the winged invaders.
These medium sized, dark gray insects that look a lot like the common housefly, but with one distinction: these buggy intruders have a flair for fashion, sporting golden hairs on their thorax.
They’re harmless in the home, but in the wild, they’re an earthworm’s nightmare. Related to the blow fly, when cluster flies are young they’re parasites, feeding and growing within the body of the hapless worm. Once the adults emerge they buzz about the yard looking for love and laying eggs in soil. After hatching, the maggots move through the soil to find an earthworm to inhabit.
In fall, the flies look for shelter in mulch, rock crevices, and buildings, where they spend the winter like we do: semi-dormant and sluggish. Their search for sun takes them to the sun-exposed sides of buildings on warm days, following the heat upwards to the upper floors where they loiter near sunny windows.
If the thought of having parasites gather at your windows bothers you, relax. Keep in mind that they’re peering outside, where their prey rests in the winter soil. They never look at humans as an all-you-can-eat-buffet and they don’t develop in garbage, manures, or other oozing areas commonly associated with ‘filth flies’. Though they drone about the house, they don’t feed or reproduce indoors.
The best way to keep cluster flies from bothering you is to prevent them from entering your home. Make sure window weather stripping is tight, and doors, siding, pipes, chimney flashing, and soffit vents are sealed with good quality caulk. Check screens on doors and windows, repairing or replacing them if needed to form a full shield.
Because they spend time in the space between walls or fly about, indoor insecticides are not effective in ridding you of the bugs. If they’re bothering you now, pick up the swatter and patrol your sunny windows. In their sluggish state they’ll be easy to stun, and you can pick them up to return them outside.
This post previously appeared in the Longmont Ledger.