Identification and general discussion of the cereal aphid species most commonly found in Nebraska small grains, corn, sorghum and millet.
G. L. Hein, Extension Entomologist
J. A. Kalisch, Extension Technologist
J. Thomas, Extension Assistant
Cereal aphids can be a serious threat to several Nebraska crops. Aphid feeding may cause direct damage to the plant or result in transmission of plant diseases. Aphids also may cause damage by injecting toxic salivary secretions during feeding.
In Nebraska the most serious cereal aphid problems result from Russian wheat aphid infestations on wheat and barley and greenbug infestations on sorghum and to a lesser extent on wheat. Growers must monitor their crops for these aphids. Several other cereal aphid species also may be present, but they seldom cause significant damage. Accurate aphid identification is necessary to make the best management decisions.
Figure 1 illustrates how to identify the major cereal aphids found in Nebraska.
Refer to http://ianrpubs.unl.edu/insects, UNL Extension Insect Publications Web site, the http://entomology.unl.edu, Department of Entomology Web site, or the http://highplainsipm.org High Plains Integrated Pest Management for more information on managing cereal aphids.
Plant viruses also can be injected into the plants during aphid feeding. Barley yellow dwarf virus can be transmitted to wheat and barley by the bird-cherry/oat aphid, corn leaf aphid, greenbug, and English grain aphid. The bird-cherry/oat aphid, corn leaf aphid and greenbug also transmit maize dwarf mosaic virus to corn and sorghum in Nebraska. These aphid-vectored diseases usually are not serious in Nebraska; however, in some years they can be significant. Disease can be transmitted whenever a few aphids move in or out of the field rapidly, making their presence difficult to detect.
The primary means of reproduction for the aphid species discussed here is asexual (parthenogenesis), with eggs hatching inside the female aphid and the female giving birth to living young. A female may produce two to three young per day under warm conditions, and females may mature in 7-10 days. This tremendous reproduction potential can result in rapid aphid population buildup. Males of some species are seldom if ever seen.
Both winged and wingless aphids may be present in the field. Winged forms are produced when the quality of the host plant declines, such as at maturity. Other factors, including temperature, photoperiod or seasonality, and population density also may be involved. The ability of aphids to use flight for dispersal is an important factor that contributes to the status of these insects as pests.
| Cornicles much reduced, cone-shaped, not pigmented. | Cornicles quite apparent, cylindrical, pigmented. | ||||
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| Only tips of cornicles dark | Cornicles completely dark | ||||
| Tip of abdomen with single tail" (only cauda present) | Tip of abdomen with double "tail" (extra tail above cauda) | Reddish-orange spot on back around bases of cornicles | Without reddish-orange spot, but with dark green stripe on pale green back | Cornicles short and broad; bases of cornicles often dusky | Cornicles long and narrow |
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| Western Wheat Aphid (Diuraphis tritici) |
Russian Wheat Aphid (Diuraphis noxia) |
Bird Cherry-oat Aphid (Rhopalosiphum padi) |
Greenbug (Schizaphis graminum) |
Corn Leaf Aphid (Rhopalosiphum maidis) |
English Grain Aphid (Sitobion avenae) |
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| Wingless Aphids: Body light green, spindle shaped and often heavily powdered; legs, cauda and tips of antennae dusky; cornicles light green reduced and cone-shaped; antennae very short. | Wingless Aphids: Body light green, spindle shaped; all of legs dusky; cornicles light green, reduced and cone shaped; antennae very short and dark at tips; projection above cauda. | Wingless Aphids: Body olive-green and broadly oval in form with reddish-orange patches around bases of cornicles; tops of leg segments, tips of cornicles and antennae dusky to black. | Wingless Aphids: Body light green to pale yellow-green with darker green stripe down middle of back; tips of legs, tips of cornicles and most of antennae black. | Wingless Aphids: Body green with bluish tinge; head region, legs, cauda, and antennae dusky; cornicles black and dusky at bases; antennae and cornicles short. | Wingless Aphids: Body bright green, large and broadly spindle-shaped, often skinny; legs' antennae and cornicles long; leg segments black towards ends; antennae and cornicles completely black. |
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| Winged Aphids: Body darker green than wingless form; all of legs and tips of antennae dark; median vein in front wings two-forked. | Winged Aphids: Body dark green and black; legs dusky with tips of segments black, antennae black; median vein in front wings two-forked. | Winged Aphids: Body green and olive-tan with darker green stripe down middle of back; tips of legs and cornicles black; antennae mostly black; median vein in wings one-forked. | Winged Aphids: Body dark green and black; legs, cornicles, cauda and antennae black; antennae short; median vein in front wings two-forked. | Winged Aphids: Body green with head thorax, antennae, cauda, cornicles and antennae long; median vein in front wings two-forked. | |
Greenbugs are the most important insect pest of sorghum in Nebraska. They seldom overwinter north of southern Kansas, where they can be a major pest of winter wheat, especially when mild winters permit reproduction and survival. Infestations in Nebraska originate from winged greenbugs blown northward by spring winds. Greenbugs moving into Nebraska early in the spring infest winter wheat and perhaps other small grains. Later flights from the south, along with offspring from the earlier flights, begin to infest sorghum from late May into June. Damaging levels of greenbugs usually occur in late June and July in sorghum. By August populations usually begin to decline, often as a result of control by natural enemies. Greenbugs only rarely damage Nebraska wheat in the spring, but may occasionally develop damaging populations in the fall.
Unlike some cereal aphids, salivary secretions of the greenbug are toxic to the plant. This destructive toxin kills plant cells and results in yellow discoloration of the leaves with reddish spotting. The reddish discoloration is especially distinct on susceptible sorghum varieties. Effects of damage are loss of stand, reduced vigor, poorly filled heads, light test weight and ultimately reduced yields. The first sign of a greenbug infestation is a circular, yellowish spot in the field. The center plants in these spots have the highest infestations and are the most severely damaged.
Greenbug management in sorghum relies on the use of resistant varieties, conservation of natural enemies and timely insecticide applications. Biotypes that have overcome varietal resistance in sorghum are common. In addition, some greenbug populations also have been found to be resistant to certain organophosphate insecticides. These factors have complicated greenbug management. Scouting to detect the presence of these insects and adherence to thresholds is essential to management.
Rolled leaves at the time of head emergence result in curved or “fishhook”; heads due to the awns being trapped in the curled leaf. The most serious yield losses result from damage to the flag leaf and incomplete seed set on the curved heads. Heavy infestations during heading can result in severe test weight reductions. The Russian wheat aphid has not been found to transmit any viral diseases.
Russian wheat aphid has become the most serious insect pest of wheat and barley in the arid areas west of the 100th meridian. Infestations begin in the fall when Russian wheat aphids move out of alternate grass hosts (several grasses and volunteer wheat) and into the newly emerging wheat. Severe fall infestations leave plants weakened and more vulnerable to winter kill. Infestations that result from overwintering Russian wheat aphids are the most severe in Nebraska because populations are able to build up early and cause serious damage in the spring. Late spring movement into Nebraska does not result in severe damage to winter wheat, unless there is little rainfall during the heading stages. During heading, the aphid is very susceptible to being dislodged from the plant, and populations do not build up in the heads if substantial rainfall occurs. Spring barley, however, is at severe risk for damage from aphid movement in the spring because it is infested at a much younger stage.
Since the mid-1990s, management of the Russian wheat aphid has relied on resistant varieties. Several winter wheat varieties resistant to the Russian wheat aphid were released from Colorado State University; all carried the Dn4 gene (e.g. Halt, Prowers 99, Prairie Red, Ankor, and Yumar). Beginning in 2002, a new biotype of the Russian wheat aphid was identified with the ability to overcome the resistance in the Dn4 varieties. This new biotype has been named “biotype 2”; and has been shown to have a widespread presence in western Nebraska along with the original “biotype 1”; aphid. Additional biotypes are thought to occur in other areas.
The current resistant varieties still provide some value in reducing risk, but management of the Russian wheat aphid also must rely on cultural practices. Controlling volunteer wheat, avoiding early planting of winter wheat, planting spring barley as early as possible and maintaining a healthy crop will help minimize the risks from the Russian wheat aphid. Monitoring aphid populations in the fall and spring and following treatment guidelines are necessary to minimize the risk of serious losses.
The western wheat aphid is not of economic concern in Nebraska; however, it is important because of its similarity to the Russian wheat aphid. It is distinguished from the Russian wheat aphid by its lack of the projection above the cauda giving it a single-tailed rather than a double-tailed appearance when viewed from the side. In general, it is found only in areas of highest elevation in western Nebraska.
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