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July 11, 2001

Each of the past few years, we have received increased reports of mayflies as nuisance pests in landscapes of homes and other buildings along ponds, streams, and other bodies of water. Although not directly associated with the landscape, this article is included to help landscapers assist their clientele.

Mayfly, also called shadfly, adults have elongated, slender bodies, with two filaments or “tails” that are one-half or more of the body length. Some species also have a central abdominal filament, causing those species to have three tails. When the insect is at rest, these tails may be held together and look like a slender extension of the abdomen. Mayflies have short antennae but, when sitting, typically extend their long front legs out in front of the body. They have two pairs of clear, triangular wings with many longitudinal and cross veins. When at rest, they hold their wings together and vertically above the back. There are many species of mayflies in Illinois, with adult lengths (including tails) ranging from 1/4 to 2 inches.

One of the most obvious characteristics of the adults is their large numbers. They can emerge in huge numbers from a body of water. They tend to sit on upright objects and can completely cover the surfaces of posts, sheds, and light poles. They are also attracted to lights at night.

Adult mayflies do not feed. Adult males may live only a few hours, whereas females tend to live several days. Eggs are laid into the water, where they hatch into nymphs. These nymphs have external gills and feed on algae and other plant material. Most live for a year as nymphs before climbing out on emerged plant stems or the water surface. The nymph molts into a subimago, a heavy-bodied, thick-winged stage that flies to the shore. There, it molts again into the slender, thin-winged, reproductive adult. For those interested in entomological trivia, this is the only group of insects to have two life stages with wings. The shed subimago skin looks very similar to the adult, continues to hang onto upright objects, and doubles the perceived number of mayflies. These cast skins have been linked to human allergies.

Mayflies are used as indicator species when testing for environmental quality. The nymphs’ external gills make them very vulnerable to silting and pollution, and they are rarely found in degraded bodies of water. The continued interest in environmental quality over the past 25 years, along with the federal Clean Water Act and similar legislation, has caused lakes, ponds, streams, and rivers in Illinois to become considerably cleaner. These cleaner waters have allowed mayflies to make a comeback.

It is common for long-term residents to observe large mayfly emergences for the first time in 30 to 50 years. As a water body continues to improve in quality, the mayfly species are likely to change, so that the time of year that large numbers of adults occur can change. Also, as the water continues to improve, higher numbers of mayflies are likely. Until the late 1930s, Chicago experienced mayfly emer-gences from Lake Michigan that caused mayflies to pile up several inches deep on Lake Shore Drive. We may be returning to those numbers.

Reducing outdoor lighting, particularly on or near buildings, helps reduce the numbers of mayflies on lakeside and riverside residences. Replacing mercury vapor lights with sodium vapor lights and replacing incandescent lights with dichrom yellow “bug” lights reduces the attractiveness of the lights. Shields that restrict light to the immediate area also reduce their attractiveness. Clear mercury-vapor lights placed at least 200 feet from buildings help attract mayflies away from them.

Realizing that the mayflies are indications of a healthier body of water helps some residents cope with the nuisance. Once the cleanliness of a water body stabilizes, residents should be able to record heavy emergences on the calendar and plan outdoor parties and other gatherings to avoid them. Insecticide sprays and fogging are likely to have little effect on the adult numbers. Application of insecticides to the source water body not only would be highly illegal and severely punishable but also would be unlikely to result in anything other than a fish kill.

Author: Phil Nixon


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