The short answer
Fish flies leave an acidic residue that etches siding permanently if left through a hot spell. Remove them with a low-pressure rinse and a cleaning solution within a week or two of the hatch — never by scraping or pressure blasting, which grinds the residue into the surface and can crack the panel.
If You Live Near the Water, You Already Know
Every June and early July, homes along Lake St. Clair get a visitor that residents further inland genuinely cannot picture. Fish flies — mayflies, technically — emerge from the lake bottom in numbers dense enough to show up on weather radar, and they coat everything.
Harrison Township, St. Clair Shores, New Baltimore, Grosse Pointe and the Selfridge corridor take the worst of it. Homeowners in Macomb Township or Washington Township twenty minutes inland may never see one.
The nuisance is obvious. The part most homeowners do not know is that fish flies do lasting damage to your home if you leave them.
Why Fish Flies Stain Siding Permanently
A fish fly's body is soft and full of fluid. When it dies against a warm wall — and they die in enormous numbers, usually within a day or two — that fluid breaks down into a mildly acidic residue.
Three things then happen fast:
- The residue etches the surface. Left on hot vinyl through a stretch of July sun, the acid works into the finish and leaves a shadow that remains after the body is gone.
- It bakes on. A vinyl wall in direct sun easily reaches 130°F. What was soft residue in the morning is bonded to the surface by evening, and it gets harder to remove every day.
- It feeds mold. Decomposing organic matter in gutters, window channels and behind shutters becomes a nutrient source for exactly the mildew and algae you spend money removing.
The window that matters: deal with a fish fly hatch within one to two weeks. After a hot month, what would have been a routine wash becomes a stain that may not fully come out.
What Not to Do
Homeowners reach for the two worst options almost every time.
Do not scrape or scrub dry. Fish fly bodies are gritty. Dragging them across dry vinyl with a brush or a putty knife grinds that grit into the finish and leaves fine scratches that hold dirt permanently.
Do not blast them with a pressure washer. This is the instinctive response and it causes real damage. High pressure drives residue further into the surface texture rather than lifting it, and forces water behind the panels — which on a wall already carrying decomposing organic matter is genuinely the worst possible outcome. It also cracks aged vinyl at the exact edges where fish flies cluster most heavily.
The Correct Removal Method
- Rinse first, low pressure. A generous low-pressure rinse floats off the bulk of the bodies without abrasion. Work top to bottom.
- Apply a cleaning solution and let it dwell. The solution breaks down the remaining bonded residue chemically. Give it several minutes. This is where the actual work happens.
- Soft-brush the heavy areas only. Under eaves, around porch lights, in window channels — a soft brush on an already-wet, already-treated surface, never dry.
- Rinse thoroughly again, including window frames, sills, screens and the gutter face where bodies accumulate.
- Check the gutters. After a heavy hatch, gutters along the lakeshore fill with a decomposing mat that blocks flow and smells. It needs pulling out, not just rinsing along.
This is exactly the process we run on lakeshore properties every summer, and it is one of the more common reasons Harrison Township homeowners book a professional house washing in late June.
Spider Webs: The Year-Round Version of the Same Problem
Waterfront homes have a spider population that inland homes do not, for a simple reason — the spiders are there for the insects, and near the lake there are always insects. Webs accumulate fastest under eaves, in soffit corners, around exterior lighting, behind shutters and along railings.
Two things make webs worse than they look:
- Webs are sticky and hold dirt. An abandoned web becomes a gray mat of trapped dust and debris that stains the surface underneath.
- Spider droppings stain. The small dark specks on siding under eaves are droppings, they are mildly acidic, and left long enough on vinyl or painted surfaces they leave marks that outlast the web.
A knocked-down web is also rebuilt within days. The reason a professional wash keeps webs away for months is that the treatment removes the residue and food source that made the location attractive, not just the web itself.
When to Schedule Around the Hatch
| Timing | What to expect |
|---|---|
| Before the hatch (late May) | Clean surface means less for residue to bond to, and gutters start the season clear |
| During the hatch (mid-June to early July) | Not worth it — new arrivals replace what was removed within days |
| Right after (2–3 weeks post-hatch) | The important one. Residue is removable and has not baked through a full month of sun |
| Autumn | Web and mildew cleanup, combined with gutter clearing before leaf drop |
Most of our lakeshore clients book the post-hatch wash and pair it with gutter cleaning, since both problems arrive on the same schedule.
The Bottom Line
Fish flies are a fact of life within a couple of miles of Lake St. Clair, and there is no preventing them. What you can control is how long the residue stays on your house. A wash within two weeks of the hatch is routine maintenance. The same wash in September is a stain-removal project with no guarantee it fully comes out.
Want This Handled Properly?
Macomb Power Washing Solutions holds a 4.9-star rating across 160+ verified reviews, and free estimates go out within 24 hours. We carry $2M general liability and full workers' comp on every technician.