Among the hundreds of mite species are some red varieties that can plague your garden and your skin. Arachnids instead of true insects, spider mites typically feed on plants or other insects, although the larval stage of the red chigger mite might take a bite of humans occasionally. These tiny pests are difficult to see in your garden, but the red dots are easier to spot on light-colored concrete.
You can hardly see a single clover mite, which measures about 1/30-inch long, but a group gathered on your concrete driveway is more noticeable. These mites typically feed on your plants, including grasses, vegetables and flowers. Because they tend to live in large colonies, they can do significant damage to your plants, but they aren't harmful to humans. When the weather cools, they can invade your home looking for places to hibernate and lay their eggs, coming in through windowsills, door frames and where the concrete foundation meets the house. They can leave red-colored stains on floors, carpets and curtains when crushed.
Chiggers also tend to congregate in groups, easily seen as they cross a concrete sidewalk or driveway, which they are likely to do several times a year. These red spider mites are slightly larger than clover mites, topping out at about 1/20-inch long as adults. The adults feed on small insects and insect eggs, but larval chiggers can bite humans. They tend to live around the base of briar patches or dense shrubs and climb up to a secure area of your body, typically near tight-fitting clothes such as the top of your socks. They bite and release after feasting on some skin cells, leaving an annoyingly itchy bite. However, chiggers aren't known to transmit disease.
Why They Gather on Concrete
Red mites appear in groups on concretes for a couple of main reasons. Clover mites might be searching for a new host plant, moving as a colony. Clover and chigger mites could be looking for a safe place to lay eggs, and concrete cracks make convenient spots. When a colony becomes too large, it usually splits into two parts, and one of the groups travels around the yard looking for a new home -- often crossing concrete during the search.
What to Do
Mites are notoriously hard to kill, and if you see a noticeable group of red mites on the concrete near your home, you likely have an infestation in your yard. Spraying the yard with a miticide is the best option for control. For large yards where spraying the entire area is cost-prohibitive, search for the "islands" of mites in your yard to spot treat. The Ohio State University Extension recommends placing 6-inch squares of black cardboard on the ground in different areas of your yard and looking for tiny dots of mites to climb on top. Treat the areas where mites appear on the cardboard. Removing dense growth can help prevent chiggers, while leaving patches of bare ground, perhaps covered in pea gravel or mulch, between your grass and your driveway or sidewalk can help keep clover mites contained; they prefer to avoid direct contact with soil or mulch when possible.