This season has been delightful for devotees of spring blossoms as the cool damp weather has extended the period of flowering for many of the most beautiful annual and perennial plants in the Washington metropolitan area. Over the past week or two, inquiries have been poring in regarding small dead flies adorning leaves, blossoms, and twigs of a wide assortment of plants. Is this another harbinger of global change or a signal of pending environmental catastrophe from some heretofore unidentified pollutant? Nah, this is just a little surprise Mother Nature has for a fly called the seed corn maggot when spring turns soggy and chilly.
The seed corn maggot is a pest of many field and horticultural crops including soybeans, corn, peas, onions, potatoes and beans. Early in spring, adult flies emerge from football-shaped pupal cases in the soil that have survived winter’s ravages. They feed on nectar from spring-blossoming plants and lay eggs in rich soils. The eggs hatch and the translucent white larvae, called maggots, search for food. Normally, these maggots consume decaying organic matter, but when a cool wet spring delays germination and development of crops, seed corn maggots invade seeds and the roots of seedlings thereby creating significant injury.
If temperatures are favorable, seed corn maggots can complete a generation in about a month and several generations occur each year in Maryland. Growers can reduce damage caused by this pest by delaying planting until soils warm enough to ensure rapid germination and development of plants in spring. While the cool wet spring has spawned legions of larvae, as temperatures warm peril awaits adult seed corn maggot flies. Hiding on the springtime vegetation are infective spores of a fungus called Entomophthora muscae. As the fly alights on vegetation, unseen spores attach to the surface of its exoskeleton. When just the right combination of temperature and humidity conspire, the spores hatch and fungal hyphae penetrate the skin of the fly and establish a lethal infection. Once inside its host, the fungus manipulates the fly in several remarkable ways.
In a related species, the house fly, researchers found that infected female flies became highly attractive to randy males. In the process of wooing these moribund femme fatales, spores on the surface of the female fly infect the male who unwittingly helps spread the infection – a remarkable example of a fly STD! In a final act of subterfuge, Entomophthora causes the doomed, but inherently fidgety fly to move ever more slowly until it reaches a final resting spot at the tip of a leaf or branch. From this elevated perch, the fungus erupts from the skin of the fly and spews spores into the air, all the better distribute its spawn on vegetation where other flies will inadvertently become infected. As you encounter dead flies on the tips of leaves and branches, be glad that Entomophthora muscae attacks flies and not humans.
“Temperature, Moisture, and Habitat Effects on Entomophthora muscae (Entomophthorales: Entomophthoraceae) Conidial Germination and Survival in the Onion Agroecosystem” by R. L. Carruthers and D. L. Haynes and “Fungi as biocontrol Agents” by T. M. Butt, C. W. Jackson, and N. Morgan were used as references in preparation of this bug of the week. We thank Galen Dively, Jerry Brust and Paula Shrewsbury for providing images for this episode. To learn more about this curious insect, please visit the following web sites.