Michigan’s mix of forests, wetlands, Great Lakes shoreline, and cold winters creates habitat for a wide range of insects and arachnids. Some are seasonal nuisances that peak during a few summer weeks, others are invasive species reshaping the state’s forests, and a handful pose genuine health risks. Here’s what you’re most likely to encounter and what’s worth paying attention to.
Biting Flies: Black Flies, Deer Flies, and Stable Flies
If you’ve spent time outdoors in northern Michigan, you already know the biting fly rotation. Mosquitoes and black flies arrive first in spring. Black flies are tiny, gnat-like insects that swarm in large numbers, and their bites cause painful itching and swelling that can last days or even weeks. They breed in running water, and they’re most active through late spring, generally tapering off by June. Head nets are the most reliable defense during peak black fly season, especially in the Upper Peninsula.
By midsummer, deer flies and horse flies replace them. Deer flies have distinctive patterned wings and bright eyes. Horse flies are larger, drab brown or black, and notoriously aggressive. Females of both species circle their target in noisy loops before biting. Horse flies will even follow swimmers into the water.
Along Lake Superior and other shorelines, stable flies become the dominant pest in mid to late summer, particularly on warm, humid days with a south wind. They look like small house flies but deliver a sharp bite, typically targeting legs and ankles. Standard insect repellent doesn’t work on stable flies. Loose-fitting long pants and thick socks are the best protection. Their larvae develop in decaying vegetation that washes up on beaches, so they tend to concentrate along the shore.
Mosquitoes and the Diseases They Carry
Michigan mosquitoes are active from late spring through early fall, with peak populations in June and July. Beyond the obvious annoyance, two mosquito-borne diseases are worth knowing about.
West Nile virus is the more common of the two, with cases reported across the state most years. Eastern Equine Encephalitis (EEE) is rarer but far more dangerous. EEE is most commonly associated with swamp and bog habitats. Less than 1% of people infected develop serious neurological illness, but among those who do, about 30% die, and many survivors are left with permanent brain damage. Michigan falls within the Great Lakes region where EEE circulates, and cases tend to cluster in late summer when mosquito populations are high and the virus has had time to amplify in bird populations.
Ticks and Lyme Disease
The blacklegged tick (also called the deer tick) is Michigan’s primary tick of concern because it transmits Lyme disease. The bacterium that causes Lyme has been spreading steadily across the state. In the Upper Peninsula, multiple counties have well-established tick populations, partly due to proximity to Wisconsin’s heavy tick zones. In the Lower Peninsula, the first confirmed infected tick population was detected in 2002 along the west coast, and since then infected ticks have been spreading across several portions of the Lower Peninsula.
About 70 to 80 percent of people infected with Lyme disease develop the characteristic bull’s-eye rash, which makes early detection possible for most cases. Tick activity peaks in late spring and early summer when nymphs (the juvenile stage most likely to bite humans) are active, with a smaller peak in fall from adult ticks. Tall grass, leaf litter, and wooded trails are the highest-risk environments. Checking your skin thoroughly after time outdoors, especially around the hairline, armpits, and behind the knees, remains the most effective prevention.
Spiders Worth Knowing About
Michigan is home to roughly 500 native spider species. Only three of them are considered medically significant: the northern black widow, the yellow sac spider, and to a lesser degree the northern cobweb spider.
The brown recluse is not native to Michigan, but it has been found in at least 10 counties, including Genesee, Hillsdale, Lenawee, Ingham, Kent, Livingston, Oakland, Shiawassee, Washtenaw, and Wayne. Multiple finds have occurred in Genesee, Washtenaw, and Lenawee counties. At least one confirmed reproducing population, found in a home in Tecumseh (Lenawee County), was successfully eradicated. These are isolated introductions rather than an established statewide population. According to Michigan State University Extension, there’s no reason to panic over brown recluse sightings in the state, but it’s worth being aware they occasionally turn up, particularly in the southern Lower Peninsula.
Invasive Insects Reshaping Michigan’s Landscape
Several invasive insect species are causing serious ecological and agricultural damage across the state. The emerald ash borer is probably the most well-known. This bright, metallic green beetle, small enough to fit on the head of a penny, has devastated ash tree populations since its discovery in southeastern Michigan in 2002. Adults feed on ash foliage while larvae tunnel beneath the bark, cutting off the tree’s nutrient supply. The emerald ash borer is classified as prohibited in Michigan, meaning it’s illegal to possess or transport it as a live organism.
The spongy moth (formerly called the gypsy moth) is another major forest pest. Its caterpillars defoliate trees, stripping leaves and leaving trees vulnerable to secondary diseases and other pests that can ultimately kill them. Outbreaks tend to be cyclical, with heavy defoliation years followed by population crashes.
Several species are on Michigan’s watch list, meaning they’ve either been detected or are considered a high-probability future arrival:
- Spotted lanternfly: A colorful planthopper that feeds on over 70 plant species, including grapevines, fruit trees, and hardwoods. It hasn’t established widespread populations in Michigan yet, but neighboring states are dealing with major infestations.
- Asian longhorned beetle: Targets maple trees along with poplar, willow, sycamore, and horse chestnut. Larvae bore tunnels through branches and trunks, eventually killing the tree. Prohibited in Michigan.
- Hemlock woolly adelgid: Tiny insects that secrete white waxy tufts while feeding on hemlock sap. Infestations can kill needles, branches, and entire trees over time.
Japanese beetles, while less dramatic than some of these species, are a widespread nuisance. Their grubs damage lawns and turf, while the adults skeletonize leaves and flowers on ornamental plants, fruit trees, and crops.
Fall Home Invaders
As temperatures drop in September and October, a predictable group of insects starts looking for winter shelter. They’re drawn to wall voids, the space between your home’s exterior and interior walls, where temperatures stay in the 40 to 50 degree Fahrenheit range that lets them survive until spring. These insects don’t eat, reproduce, or cause structural damage once inside. The only real damage happens when you crush them into upholstery or lampshades, leaving stains that typically don’t come out.
The most common fall invaders in Michigan include boxelder bugs, which are flat, black, and red-orange insects associated with boxelder trees. The western conifer seed bug is another frequent visitor, shaped like a squash bug and found near evergreens large enough to produce cones. People often mistake it for the brown marmorated stink bug, which is a relatively recent arrival in Michigan and lives up to its name when smashed.
The best defense is late summer prevention. Inspect your home’s exterior for cracks and crevices, keeping in mind that insects can slip through gaps no wider than a credit card. Caulk around windows, doors, dryer vents, gas lines, and light fixtures. Check and replace worn door sweeps. Vinyl and aluminum siding offer many entry points that are harder to seal. Pesticides are often not very effective against these invaders, so physical exclusion is the better strategy.