Aphid Populations in Wheat

Published: February 29, 2012

Aphid populations are present in low numbers in some wheat fields, but have reached or exceeded threshold levels in others. Most economic infestations have been found during the past 2-3 weeks in Western and Southwest Missouri fields. Aphid numbers vary greatly between fields, so be sure to look at each field before applying insecticides. Although five species of aphids are often found in Missouri wheat fields (greenbug, bird-cherry oat, corn leaf, English grain, yellow sugarcane), at present the bird cherry-oat aphid is the one species causing problems. Because this aphid species tolerates cold conditions, it can overwinter in Missouri wheat fields. Damage from bird cherry-oat aphids may occur throughout the growing season, even during winter months. All species of aphids found on wheat damage plants by removing plant juices using their piercing-sucking mouthparts. The bird cherry-oat aphid and greenbug are the most damaging species because in addition to direct damage caused by feeding, they also transmit the viral plant pathogen yellow barley dwarf (BYD). All five aphid species reproduce parthenogenically which means they produce several generations of living young, mostly females, which are pregnant when born without mating. Occasionally males will be produced, but several generations of aphids may occur without mating taking place.

• Bird cherry-oat aphids are medium sized aphids with olive colored bodies and reddish-orange patches on back at base of cornicles (tailpipes). Antennae, eyes, and tips of legs and cornicles are black in color.
• Greenbug aphids are small pear-shaped aphid, 1/16-inch in length. Pale yellow to pale green in color with black legs, cornicles, eyes, and a predominant dark green line running down the length of the back.

The bird-cherry oat aphid and greenbug are often serious pests of small grains including wheat. Whereas the bird cherry-oat aphids may be present throughout the growing season, the greenbug usually does not overwinter in Missouri, but instead migrates into the state each spring during early season movement of storm fronts from more southern and western states. Similarly, bird cherry-oat aphids may occur over the entire plant and often be at ground level during periods of cold or windy weather. In contrast, the greenbug is generally found in colonies on leaf surfaces whenever present on the wheat plants.

Thresholds are based on the average number of aphids present per foot of row depending on plant height and stage of growth. There is much controversy as to appropriate thresholds for each of these aphid species. The use of high performance wheat varieties, the high price of wheat sold for grain, difficulties in finding certain aphids during scouting, recent research in Missouri, and many other factors suggest that our traditional thresholds are no longer suitable. With this in mind, our current recommendations for control of bird cherry-oat aphids and greenbug in wheat are as follows.

Bird cherry-oat aphid thresholds: Treatment is justified if 12-15 or more aphids are present per foot of row during the seedling stage in the fall through head formation the following year.

Greenbug thresholds: Scout several locations in the field to determine number of aphids present per linear foot of row. Treatment is justified if the average number of greenbug per linear foot of row are 50 or more greenbug on wheat with less than 3 tillers, 100–300 aphids or more on 3-6 inch height wheat with 3 or more tillers, or 300-500 aphids or more on 6-10 inch height wheat prior to harvest.

Regardless of aphid species present, producers should consider the number of beneficial insects present (examples: adults and larvae of pink ladybugs and other species of ladybird beetles, parasitic wasps), the stage and condition of the wheat plants, and whether the wheat is under additional stressors such as drought. The presence of high numbers of beneficial insects will increase the threshold and reduce the need for insecticides, whereas, increased stress on plants will call for lower thresholds as stressed plants are less able to withstand aphid infestations. If economic threshold levels of bird-cherry oat aphids have been reached or exceeded, use one of the following recommended insecticides. Insecticide efficacy will be greatest on days when temperatures reach 60 degrees Fahrenheit or warmer.