Corn root aphid
Aphis maidiradicis Forbes (Homoptera: Aphididae)
Distribution. Corn root aphid occurs throughout the United States east of the Rocky Mountains. It is most abundant and troublesome in the mid-western states. Corn root aphid likely occurs in southern Canada also, but is not reported to be a pest there. Though likely a complex of closely related species, for the purpose of this discussion it is treated as a single species.
Host Plants. Corn and cotton are the only crops injured by this species. However, corn root aphid is reported to have a wide host range, existing principally on weeds. Although some of the host records may be erroneous because it is difficult to distinguish this species from some other Aphis species, it is reported to feed on aster, Aster spp.; crabgrass, Digi-taria sanguinalis; dock, Rumex crispus; foxtail, Setaria spp.; pigweed, Amaranthus spp.; plantain, Plantago spp.; portulaca, Portulaca oleracea; prairie sagewort, Artemisia frigida; ragweed, Ambrosia sp.; smartweed, Polygonum sp.; sunflower, Helianthus spp.; tansymus-tard, Descurainia sp.; and many others.
Natural Enemies. There seems to be little known about natural enemies of this insect, though fungi are known to attack colonies.
Life History and Description. In Illinois, hatching occurs during April and May. The aphids reproduce parthenogenetically during the spring and summer months. There are perhaps 16-22 generations per year, with a generation completed in just 6-8 days. Sexual reproduction occurs only in the autumn, followed by overwintering in the egg stage.
Egg. The eggs are collected by ants, commonly the cornfield ant, Lasius alienus (Forster) (Hymenoptera: Formicidae), and tended by the ants in their burrows throughout the winter. The ants pile them in their chambers and move them in accordance with the moisture and temperature conditions of the soil. The eggs are black or greenish black, elliptical in shape, and measure about 0.8 mm long and 0.4 mm wide.
Nymph. Upon hatching the young aphid nymphs, which are all wingless viviparous (parthenogenetic) females, are transferred by ants to the roots of weeds, where they normally complete 2-3 generations. The nymphs initially are pale green, but as they mature they become grayish, eventually attaining a length of about 2 mm. The nymphs usually complete their growth in about 7-14 days, and almost immediately they commence reproduction. Davis (1909), working during the cooler weather of October, reported development times of 2-4, 5-6, 9-14, and 13-21 days, respectively, for instars 1-4.
Adult. The various forms of the adult aphids differ in appearance. The wingless viviparous females are predominantly bluish-green, but bear a white floccu-lence on their body and have a black head and transverse bars on the thorax. In winged viviparous females the head and thorax are dark-brown or black, and the abdomen is light-green with three distinct black spots on each side. The wings are dusky. The oviparous females are wingless, and measure about 2.2 mm long. The head, and at least part of the thorax are dark, but the abdomen is bluish-gray and tinged with pink. Males also may be either winged or wingless. Wingless males are about 1.75 mm long, whereas the winged male aphid measures about 1.5 mm long. In both forms of the male, the head, thorax, and appendages are black and the abdomen is greenish but marked with 2-3 dark transverse bars on the apical (and sometimes basal) segments.
Reproduction during the summer is parthenogenetic. Viviparous, or parthenogenetically reproducing females give birth to 40-50 nymphs, averaging about five nymphs per day. Later, as corn plants develop, the ants transfer the aphids to the roots of corn seedlings. The aphids are redistributed and protected from predators and disease throughout the summer by the attending ants. The incentive for the ants to tend the aphids is, of course, the production of honeydew by the aphids. During the summer months a mixture of winged and wingless females develop and feed on the corn roots. The winged forms crawl through the ant tunnels to the surface, and disperse to new locations. If they alight in the vicinity of ants they are seized and carried below-ground by the ants and placed on a root, giving rise to a new infestation. Sometimes the aphids are transported up to 50 m to infest new locations. Males and egg-producing (oviparous) females are produced in the autumn, when sexual reproduction occurs and overwintering eggs are produced and deposited on the roots of corn or weeds.
Description and biology of corn root aphid were given by Davis (1909, 1917). Ecology of the cornfield ant, including its interrelationship with corn root aphid, was described by Forbes (1908).
Aphids cluster on the roots of corn and the underside of the crown at the base of the roots, where their feeding inhibits growth of young corn plants. Heavily infested plants are yellow or brown and stunted. Plants infested later in the season are more tolerant of aphid feeding and are not damaged unless the plant is under additional stress, such as lack of adequate water.
Sampling. The distribution of corn root aphids is governed by their attending ants. Ant colonies vary in size and frequency in corn fields, depending largely on whether insecticide application was made to earlier-grown crops of corn. In the absence of insecticide and presence of earlier crops of corn, individual ant colonies may be large, each encompassing areas of 50 sq m, and supporting high densities of aphids.
Insecticides. Insecticides applied to the soil for other root-feeding insects such as corn rootworms species typically kills ants and therefore aphids are not a problem. In decreased insecticide production systems, especially if accompanied by reduced tillage, the aphids become more of a problem.
Cultural Practices. Tillage disrupts ant nests, thereby decreasing the abundance of aphids. Deep cultivation is necessary because ant nests may occur at a depth of 18 cm or greater in the soil. Plowing and disking to a depth of 15 cm destroys most ant nests. Crop rotation can be beneficial because few other crops are suitable hosts. However, it is advisable not to plant corn adjacent to fields previously infested because the ants can relocate aphids to nearby fields.