Jeffrey Hahn, Asst. Extension Entomologist
Many people in Minnesota, including the Twin Cities, that spent time outdoors during mid-September encountered large numbers of small dark-colored 'gnats'. Upon closer examination, these insects turn out to be winged aphids (examples include soybean aphids, basswood aphids,and oat bird cherry aphids). This has been a favorable summer for aphids and large numbers were produced.
Photo (left): 'Winged and wingless soybean aphids on
buckthorn in spring'. Christina DiFonzo,
Michigan State University, Bugwood.org.
While the relatively cool summer had only a minor impact on the development and reproduction of aphids, these conditions had a more significant effect on aphids' natural enemies, such as lady beetles. The cooler weather slowed down their rate of reproduction which ultimately allowed aphid numbers to thrive.
Aphids have a very unusual and complicated life cycle. Many typically live on two different host plants. They spend the summer on their primary host (e.g. soybeans for soybean aphids) feeding on sap and producing many generations. Aphids reproduce parthenogenetically, i.e.eggs are not fertilized and only females are produced. Females give
birth to live young.
The end of summer brings shorter day length. When combined with a few days of below average temperatures, these events trigger the production of a generation of winged females aphids (winged males are produced about a week later). This year, that cold weather occurred at the end of August. These winged aphids take flight and look for their alternative host plant (e.g. buckthorn for soybean aphids). It is this migration that people have been seeing recently.
Once aphids land on their second host, they produce several generations of wingless aphids. When the males arrive, they mate and then the females lay fertilized eggs. Most aphids overwinter in the egg stage. When spring arrives, the eggs hatch into wingless females which produces two or three generations. Eventually a winged generation is produced, and these aphids fly back to their original host plants to start the cycle all over.
A common question people have asked is whether these insects will bite. Even though they are gnat-like, the answer is no. They are harmless to us and do not bite like a mosquito or black fly. However, it is possible that they may taste test people by trying to insert their
mouthparts into us which can result in a mild prick. Fortunately,aphids then realize that we are not food.
People have also wondered whether using a mosquito repellent will help keep these aphids off of us. Repellents are designed to hide our chemical scent from blood-feeding insects, especially mosquitoes. Since aphids are not seeking us out to feed on us, but just encounter us randomly, repellents will have no effect against them. However, they are attracted to the color yellow so one step you can take to reduce the aphids that land on you is to avoid wearing any clothes of yellow color.
There have been a few reports of people applying insecticides into the air to try to kill the aphids around their homes. This, of course, is an entirely futile gesture and just puts the individual at more potential risk from an accident with pesticides. People should not
attempt to spray the aphids in their homes.
Fortunately, this migration of aphids is a short-lived problem and goes away on its own. The greatest numbers of aphids were out for about a week during mid-September and have noticeably declined since then. By October, there should be very few still actively flying.