Neozygites fresenii ("the aphid fungus")
Blake Layton (Mississippi State University) and Scott Stewart (University of Tennessee), Department of Entomology and Plant Pathology
This is the scientific name of the fungal disease that is often very effective in controlling cotton aphids. Although, it does not really have a common name, it is typically referred to as “the aphid fungus” in cotton. However, closely related fungi also affect species other than cotton aphids. This disease is much more effective than currently available insecticides, and it is doubtful whether we could effectively control aphids without this naturally occurring disease. This disease spreads quickly once it becomes active in a population of aphids, creating what scientists refer to as an epizootic. Once an aphid population crashes due to a fungal epizootic, populations tend to remain low for the remainder of the year. Occasionally aphid numbers will begin to increase again during the later part of the season, but these outbreaks are usually dampened by another epizootic.
Aphids that have died due to this disease often appear to be standing on their heads and still have their mouthparts inserted into the leaf. Aphids that have only recently died are light gray in color and are covered with a fine layer of fungal spores. After aphids have been dead for a day or more, they may be covered with a green to brown wooly fungal mass. Sometimes this mass is so thick and wooly that it is difficult to see the aphid that’s underneath. This is not the fungus that killed the aphid, but rather, is a secondary fungus that grows on the dead aphids. Still, the presence of fungal-infected wooly, gray to green aphids is an indication that the aphid fungus is present.
This fungal disease belongs to a group of approximately 200 fungi, most of which attack insects or mites. The seed-like primary spores of this fungus germinate on the leaf surface and grow into thin stalks that are about “ankle high to an aphid”. These stalks then produce secondary spores that are sticky on the end. When an aphid brushes against these secondary spores they stick to the aphid, germinate and penetrate the aphid’s body. The fungus then grows in the blood of the aphid, killing it and producing fungal spores that are discharged into the air to repeat the cycle. It takes about three days for an aphid to die after it becomes infected with the fungus.
Interestingly aphids will typically die shortly after dark, which allows the spores to be discharged at night when humidity is high. Some of the discharged spores land back on the leaf near the aphid, but most are carried away by air currents to help spread the disease. The disease may also be spread by migration of winged aphids which have secondary spores stuck to their bodies, and parasitic wasps may also help spread the spores.