There are three major species of aphids that can be found on cereals in Manitoba. These are the English grain aphid, the birdcherry-oat aphid, and the greenbug. Other less numerous species of aphids are also found on cereals in Manitoba. The term cereal aphid is sometimes used as a generic term to describe the complex of aphids commonly found on cereal crops.
The English grain aphid is bright green with black antennae, long black cornicles (a pair of tubes at the back of the aphids abdomen), and has legs that are banded with areas of green and black. The greenbug is pale to bright green with a darker stripe down the middle of the back; antennae are black; and legs and cornicles are pale green with black tips. The birdcherry-oat aphid is dull olive-green with black antennae; legs and cornicles are pale green with black tips, and there is a reddish-orange patch between and at the base of the cornicles. On wheat, the birdcherry-oat aphid prefers the stems and lower leaves, whereas the English grain aphid is found mostly on the head and upper leaves of the plant.
The birdcherry-oat aphid is the main vector of barley yellow drawf virus on the prairies. See the section on diseases in cereals for more information on barley yellow dwarf virus. The greenbug has a saliva which is toxic to the plant and causes discoloration and brown spotting on the leaves it is feeding on. English grain aphids can often be found on the heads, feeding on the ripening kernels of cereal crops. When sufficiently large populations develop, their feeding shrivels the growing kernels. Aphids also produce large amounts of honeydew that can promote the growth of saprophytic and pathogenic fungi on the plant.
Monitor July through early August. Check 20 stems in each of five areas. Counts should be at least 50 paces apart, and observations should be made well into the center of the field. Too frequently farmers become alarmed after checking a few plants along the margins, especially near shelterbelts, where populations are high. Record the total number of aphids and calculate the average per plant.
Look for symptoms of damage, including leaf discolouration in host plants. Severe infestation may appear as large, bronzy or brown patches in the field, often visible from the road. Later in the season, these patches may appear as thin stands.
Agronomists routinely take stem samples for aphid counts. Cut the stem close to the ground and then hit it sharply against a stiff piece of white paper or into a white pan. This dislodges the aphids so they can be counted.
When counting aphids, note the presence or absence of predators, parasitized aphid mummies, or aphids killed by fungus.
The economic threshold for aphids on cereals is an average of 12 to 15 aphids per stem prior to the soft dough stage.
Natural predators (such as ladybird beetles, hover fly larvae and lacewings) and parasites usually keep populations of aphids under control. See the section in this guide on predators of insects for more information on predators of aphids.
Early seeded crops grow and become vigorous and pass the susceptible stage before aphid populations reach damaging levels.
For further information, contact your GO representative.