Natural History Survey
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The soybean aphid, Aphis glycines, is now in its fourth summer in the United States. Each growing season has seen range expansion and different population dynamics across that range. Colonies have been found in Georgia, Mississippi, Maryland, Virginia, and North and South Dakota, as well as from Quebec to Manitoba in Canada. The soybean aphid survives the winter as eggs deposited on buckthorn, Rhamnus cathartica, a widespread invasive shrub of European origin. The continued spread and presence of spring colonies in much of the observed summer range suggests the aphid has successfully overwintered in these areas.
The abundance of buckthorn is considerably greater in the northern part of the U. S. as well as in Illinois, where it is much more abundant north of I- 80. Only scattered patches/ plants can be found in the southern half of Illinois, and these are often found in urban settings. To survive in the southern part of the soybean growing areas in the U. S., the soybean aphid must find these widely scattered hosts or have another suitable winter host.
|To see if there are additional satisfactory winter hosts, an experiment was begun in the fall of 2002. In cooperation with a horticulturalist from Iowa state who grew the experimental plants, eight cages, each containing 11 species of Rhamnus and related genera, were set up in September, and into each was placed four soybean plants with heavy aphid infestations. Under autumn conditions of shortening photoperiod and cooler temperatures, the aphids on soybeans produced migrants that had to choose one of the test plants on which the overwintering eggs would be deposited. Eggs were deposited on only two of the 11 potential winter hosts, Rhamnus cathartica and Rhamnus alnifolia. The latter is a rare native plant that is found in wetland habitats in northern Illinois. Rhamnus caroliniana, a shrub that is common in the southern states, was not readily accepted by the aphids.|
Figure1. Fall migrants and oviparous
The experiment will be repeated this fall. If the results are similar, it would suggest that the aphid might be limited in its southern distribution by a lack of satisfactory hosts on which to overwinter. In addition to the above experiment, small potted buckthorns (not caged) were observed during the fall of 2002 for natural infestation of migrants from soybeans in the Champaign–Urbana area. They were at least a kilometer distant from the nearest soybeans, and migrants began colonizing them during the last week of September. They were continuously colonized until near the end of October, and eggs were deposited on the buckthorns. The potted buckthorns were mulched in for the winter, and aphid egg hatch began in the last week of March this spring. Spring migrants that fly from buckthorn to soybean were produced from the middle of April to the early part of May.