Aphids commonly present in wheat

It is still cool and damp in the east but southwestern parts of South Dakota are faring much better with regards to moisture and temperatures. We’re nearing the time when we may be seeing some aphid activity in wheat.

Aphids are small, soft-bodied insects that range in color from pale yellow through all shades of green and reddish brown to black, and some species of aphids can have many color morphs. Their bodies are usually spindle-shaped or somewhat oval with rounded abdomens, a pair of antennae, and a pair of cornicles or tail pipes protruding from the abdomen. Some aphids may have long cornicles, while others can have greatly reduced cornicles. Aphids have sucking mouthparts, a small tube-like structure that they use to puncture through the plant tissues. They excrete droplets of sugary substance called honeydew. This sugar-rich liquid is often harvested by ants that feed on honeydew and in return tend the aphids protecting them from their predators. Presence of ants often indicates the potential for aphid infestations. Honeydew can also lead to mold growth when in drips onto leaves and other plant structures. Many aphids that attack wheat do not overwinter in South Dakota, and are carried from the south by wind in the spring and early summer.

Bird-Cherry/Oat Aphid

Bird-cherry/oat aphids are among the aphids that attack wheat and vector Barley Yellow Dwarf Virus (BYD). The bird cherry-oat aphid, Rhopalosiphum padi, is a large (1/8 inch), dark olive green aphid that has long antennae and a pair of long tubular cornicles at the end of its abdomen. Wingless forms of the bird-cherry/oat aphid have a characteristic orange to brown spot that is visible at the base of the cornicles. This species of aphid is typically active early in the season, but activity is dependent on environmental conditions. The bird cherry-oat aphid causes both direct and indirect damage. Direct feeding on early-seeded wheat by the bird cherry-oat aphid can result in golden-yellow streaking on the foliage of wheat and other small grains while indirect damage is the result of BYD transmission. It is important to actively scout wheat fields for populations that are at or above the economic threshold.

Greenbug

Greenbugs, Schizaphis graminum, are small (1/16 inch), pale green aphids that are pear-shaped with a dark green stripe running down their backs. They can also be identified by their black-tipped legs and cornicles. Greenbugs feeding damage on wheat appears as discoloration or striping, wilting, and if present at high number, these aphids can kill the plant. Typical greenbug damage appears as yellowing foliage with red spotting. Greenbugs do not overwinter in South Dakota and are carried in by southern winds during the early spring. They prefer to feed on the leaves and can often be found on the underside of leaves. Early and consistent scouting is critical to establishing a successful management of these pests.

English Grain Aphid

The English grain aphid, Sitobion avenae, can be found in South Dakota, though it is seldom a concern to small grain producers in the state. This aphid is about 1/10 inch long, light green to brown with black antennae, cornicles, and joints. It is a known vector of BYD, but the major damage to wheat is caused by the aphid’s direct feeding injury to the heads of small grains. Foliage damage occurs until grain begins to head. Once heading initiates, these aphids move and aggregate at the heads and aggressively feed upon the ripening kernels.

Management Guidelines

The most important aspect of managing aphids in general is scouting. Scouting should begin roughly 100 ft or 20 paces from the field edge. Individual stems and leaves should be inspected for aphids or signs of damage. Field ants can be an indicator of an aphid threshold as they harvest the sugary secretion called honeydew that the aphids produce. The general economic thresholds for grain aphids is reached if 85% of the stems have one or more aphids on them or aphid densities reach 12-15 aphids per stem prior to complete heading, and the specific thresholds for each species and wheat stage are described in the table shown above. Applying insecticides before aphids reach this threshold has severe negative effects on populations of beneficial insects that can successfully suppress low levels of aphids. Predators of aphids are diverse and include ladybeetles, lacewings, parasitoid wasps, and many species of the predatory bugs such as minute pirate bugs and damsel bugs. Elimination of beneficial insects through unnecessary pesticide applications will likely result in subsequent outbreaks of aphids as well as other pests.

References

· University of Nebraska, Lincoln (Extension Publication G1284)

· North Dakota State University (Extension Publication E-493)