Greenbug
 Schizaphis graminum (Rondani), Aphididae, HEMIPTERA

DESCRIPTION

Adult - There are four types of greenbug adults: winged and wingless females which bear live progeny, winged egg-laying females, and winged males. Wingless females, the most commonly seen forms, are about 2 mm long and pale yellow to bluish green with a dark green stripe down the back. Winged females are about the same color but are slightly smaller. Males have wings and are very rarely seen.

Egg - The kidney-shaped eggs, about 0.8 mm long, are pale yellow when first deposited but turn black before hatching.

Nymph - The pale green nymphs are wingless and smaller than adults.

BIOLOGY

Distribution - Greenbugs are found throughout the United States and in most areas of the world. In this country, they are particularly damaging in the central states from Texas into North Dakota and Minnesota. Although greenbugs occur in North Carolina, damaging infestations are rare.

Host Plants - The greenbug is a particularly serious pest of wheat, barley, oats, and rye. Corn, sorghum, rice, forage grasses, Johnson grass, and western wheat grass may also be infested.

Damage - Greenbugs pierce plants with their needle-like mouthparts, secrete toxic substances which kill living tissue, and then extract the released juices. In the winter and early spring, this type of feeding produces "greenbug spots" which are usually circular areas of yellow plants in the field with brown, dead plants in the center. These spots grow as greenbugs feed and continue to move to new plants on the outer edges of the circle. Later in spring, heads of grain may be stunted as greenbugs feed inside the upper leaf sheath.

Greenbugs are likely to transmit virus diseases to crops at any time of the year. Occasionally, in the Plains states, fields may be invaded by swarms of migrating greenbugs. In this case, "greenbug spots" are not formed; all plants are killed at the same time. Light to moderate infestations may reduce yields significantly without destroying the whole field.

Life History - The greenbug is actually an aphid which causes some damage each year to small grains in the central and southeastern states. In the past century, 32 serious outbreaks have occurred. These outbreaks seem to have been favored by a cool, rainy summer followed by a mild winter and a cool, dry spring. There are now three different strains, or biotypes, of the greenbug in this country. Barley is the only small grain for which resistant commercial varieties have been developed.

The greenbug cannot survive winters in the northern states and Canada. Some adults overwinter in the central states; however, overwintering populations are highest in the southern states where wingless females survive and continue to reproduce throughout the winter. Of the four types of greenbug adults, wingless females are most common. When conditions are favorable they each produce three or four live progeny every day for about 25 days. Adult males, winged females, and eggs are rare, but occur sporadically in the North Central states. Most nymphs become wingless females in 6 to 30 days.

When adverse weather or poor nutritional conditions exist, winged adults develop and migrate to more favorable areas. As early as March or April, some winged female adults migrate northward in air currents to the central states. When small grain in the central states matures in May, greenbugs will migrate further northward. In late summer and fall, greenbugs return south. Therefore states such as Texas experience greenbug infestations in early spring and again in late fall and winter. In such areas there may be 20 or more generations per year.

CONTROL

In areas where the greenbug is a chronic problem, several cultural practices help reduce infestations. First of all, it is especially important in the southern states to destroy volunteer grain to prevent greenbug populations from growing rapidly and moving northward. Secondly, all practices which stimulate the growth of the plant such as fertilization, seedbed preparation, and use of certified, recommended varieties, help the crop resist greenbug injury. Last of all, avoid continuous planting of small grains. Rotate with legumes or corn, but avoid rotations with grain sorghum which is now susceptible to the greenbug.

Early in spring, when crop plants are small and few natural enemies are present, greenbugs may require chemical control. In North Carolina, 25 aphids per head of grain or 100 foliar or 100 foliar-feeding aphids per 30 cm (1 row foot) are recognized as economic thresholds. Later in the season, natural enemies are usually abundant enough to control infestations without the use of insecticides.

For specific control information, consult the current North Carolina Agicultural Chemicals Manual.