Like many of you, I have had enough dreary moist days to last for a while. However, one major kingdom of life on this planet, fungi, finds this cool damp weather perfect for carrying out their mischievous manipulations of insects. Over the past week or two, inquiries have been pouring in regarding small dead flies adorning leaves, blossoms, and twigs on 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 flies, including the seedcorn maggot fly, when spring turns soggy and chilly.
The seedcorn maggot is a pest of many horticultural and food crops including soybeans, corn, peas, onions, potatoes and beans. Early in spring, adult flies emerge from small football-shaped pupal cases in the soil that have survived winter’s ravages. Like many early season pollinators, they feed on nectar from spring-blossoming plants and then lay eggs in rich soils. Eggs hatch and the translucent white larvae, called maggots, search for food. Normally, maggots consume decaying organic matter, but when a cool wet spring delays germination and development of crops, seedcorn maggots invade seeds and roots of seedlings, thereby creating significant injury. If temperatures are favorable, seedcorn 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 in spring until soils warm enough to ensure rapid germination and development of plants.
While the cool wet spring has spawned legions of seedcorn maggots, as temperatures warm up peril awaits the adult seedcorn 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, spores hatch and fungal hyphae penetrate the skin of the fly and establish a lethal infection. Once inside its host, the fungus takes control of the fly’s tiny mind and body transforming it into a fly zombie. Entomophthora takes control of the fly’s nervous system causing the doomed, but inherently fidgety fly to move ever more slowly upward and outward on a plant until it creeps to its 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 to distribute its spawn on vegetation where other flies will inadvertently become infected.
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 then unwittingly helps spread the infection – a remarkable example of a fly STD! As you encounter dead flies on the tips of leaves and branches, be glad that Entomophthora muscae attacks flies and not humans.
Bug of the Week thanks Chris Sargent, Paula Shrewsbury, Jerry Brust, and Galen Dively for inspiration and images used in this week’s episode. “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, “A fungus infecting domestic flies manipulates sexual behaviour of its host” by A. P. MøLLER, 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.