Aphid Populations Present in Missouri Wheat
By Wayne Bailey, University of Missouri
Over the past few weeks, greenbug and bird cherry-oat aphid numbers have increased in some wheat fields in the state. There are five aphid species that can be commonly found at various times in Missouri wheat. These include the greenbug (Schizaphis graminum Rondani), bird cherry-oat aphid (Rhopalosiphum padi Linnaeus), corn leaf aphid (Rhopalosiphum maidis (Fitch)), English grain aphid (Sitobion avanae (Fabricius)), and occasionally the yellow sugarcane aphid (Sipha flava (Forbes)). Of these five aphid species, the greenbug and the bird cherry-oat aphid are the most damaging to Missouri wheat. These two important aphid species can cause direct feeding damage by sucking plant juices, but may also transmit barley yellow dwarf virus (BYD) in wheat. 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. This type of reproduction allows for rapid increases in population levels over short periods of time. Typically greenbug aphids damage seedling wheat in the fall when they feed on plants and often transmit the barley yellow dwarf virus. In contrast, the bird cherry-oat aphid can damage plants in the fall by feeding and transmission of barley yellow dwarf and also during spring by feeding on plants that have reached head emergence. Although producers are encouraged to scout individual fields to determine greenbug and bird cherry oat aphid numbers at this time, most wheat fields in Missouri do not currently support aphid numbers requiring applications of insecticides.
Greenbug is a traditional pest of Missouri wheat and may cause severe damage. Although Greenbug migrate into the state each spring, they typically damage seedling wheat plants during fall and much less often during spring. They damage wheat in three ways which are by sucking plant juices, injecting a toxic saliva when feeding, and transmission of the barley yellow dwarf virus(BYD). Greenbug populations can change rapidly depending on weather conditions and the number of beneficial insects present. Thresholds for this aphid are based on the average number of aphids present per linear foot of row depending on plant height and stage of growth. Traditional economic thresholds for greenbug in Missouri wheat are as follows: treatment is justified if 50 or more aphids are present per linear foot of row in the seedling stage; 100 or more present during the 3- to 6-inch stage of growth, and 300 or more aphids when the plants reach 6- to 10-inches in height. Recent research in Missouri and other states suggest that these traditional thresholds may be too high and allow for more damage and subsequent yield loss than necessary. Additionally, the high market price of wheat justifies the use of lower economic thresholds than traditionally used. Other factors influencing economic thresholds include: variability in plant size, stand density when infestations occur, the variety of wheat under production, the number of aphids and amount of BYD virus present, spring growing conditions and the presence or absence of biological control agents such as ladybugs, parasitic wasps, and predators.
Based on the preceding information, the current recommendation for management of the greenbug in wheat is as follows: Scout several locations in the field to determine number of aphids present per linear foot of row. If the average number of greenbug per linear foot of row equal or exceed 50 to 100 on wheat plants 6-inches or less in height, then treatment may be justified. In wheat taller than 6-inches the economic threshold would be reached when 300 or more greenbug are present per linear foot of row. Producers should consider the number of beneficial insects present (examples: pink ladybugs and other species of ladybird beetles, parasitic wasps) and whether the wheat is under other stressors such as drought. The presence of high numbers of beneficial insects will increase the threshold and reduce the need for insecticides, whereas, the greater the stress on the plants, the lower the thresholds as stressed plants are less able to withstand aphid infestations. Wheat plants in the later stages of plant growth (taller) can withstand greater numbers of aphids than younger or shorter plants. The greenbug is generally found in colonies on leaf surfaces whenever present on the wheat plants.
- Greenbug migrate into Missouri during early spring.
- Small pear-shaped aphid, 1/16-inch in length.
- Range in color from pale yellow to pale green with black legs, cornicles, and eyes.
- Exhibit a predominant dark green line running down the length of the back.
- Dark green line is the insect's digestive tract which can be seen through the somewhat transparent aphid body.
- Damage wheat by (1) feeding on plant sap, (2) injecting toxic saliva when feeding, and (3) transmit barley yellow dwarf virus.
- Economic threshold of 50 to 100 or more aphids present on wheat 6-inches or less in height during fall. In wheat over 6-inches in height the economic threshold would be 300 or more greenbug present per linear foot of row.
- Rarely causes damage in spring.
The bird cherry-oat aphid can transmit BYD, but because this aphid often attacks the wheat at boot to heading stages of growth, thresholds are based on numbers of aphids present per tiller. In general, if an average of 12-25 or more bird cherry-oat aphids are present per tiller from seedling to wheat head emergence treatment is justified. The lower threshold is appropriate if plants are under additional stressors such as severe drought. Feeding by this aphid may result in damage to the flag leaf with the head taking on a hooked appearance as it emerges. This aphid is capable of overwintering in wheat fields and may be present throughout the growing seasons of wheat. Bird cherry-oat aphids may occur singularly or in small groups over the entire plant and often move to ground level during periods of cold or windy weather.
- Medium sized aphid dark olive to black in color.
- Rosy to reddish-orange color found on back of abdomen and surrounding the cornicles.
- Antennae, eyes, tips of legs and cornicles are black in color.
- Damage wheat by (1) feeding on plant sap and (2) transmission of barley yellow dwarf virus.
- Economic threshold of 12-25 or more aphids present per tiller plant emergence up to head emergence.
- Bird cherry-oat aphids occur singularly or in small groups anywhere on wheat plants.
- Aphids may move to ground level in very cold or windy conditions.
Corn leaf aphids are bluish-green to black in color with black eyes, antennae, cornicles, and legs. They damage plants by removing plant juices resulting in increased plant stress. This aphid attacks corn, sorghum, wheat and other grasses. This aphid rarely reaches population levels in wheat to cause economic damage.
English grain aphids are bluish-green in color with brown head and eyes. Antennae, cornicles, and tips of legs are black in color. Antennae and legs are long in comparison to body size. This aphid species attacks wheat, barley and oats and causes increased plant stress by removal of plant sap. It often takes 100 aphids or more per linear foot of row to cause economic damage in wheat. English grain aphids rarely reach economically damaging levels due to being heavily parasitized.
Yellow sugarcane aphids are yellow in color with numerous spines covering the body. They also have two rows of black spots running down the length of their backs. Although not considered an economic pest in Missouri wheat, this aphid is a severe pest of sorghum and sugarcane in the Southern United States. Main hosts include sugarcane, sorghum, johnsongrass, and dallisgrass. The occasional infestations of this aphid in Missouri wheat often occur when winged aphids move to wheat from sorghum and various grasses. This aphid is not considered an economic pest in Missouri.