Although it is more important to recognize the economic potential of an infestation than to be able to identify the different species, the following are brief descriptions of each species along with information on the damage they do.
A recent survey of eastern Virginia showed that of the four species, English grain and bird cherry-oat aphids were the most abundant during the fall, winter, and early spring. English grain aphid was most abundant in late spring.
Corn leaf aphid can be abundant in the fall, but is more commonly found on sorghum (milo). Greenbug can reach high levels, but ecnomic infestations are not common.
Greenbug:
Small grains, primarily wheat and barley, provide the major fall and winter hosts for greenbugs. Oats, sorghum, johnsongrass, and other grasses can serve as summer and interim hosts. Green bugs are pale green with a darker green stripe running down the middle of the back, but are more readily recognized by the damage they cause to small grain plants.
Greenbugs inject a toxic substance when feeding that causes feeding sites, and eventually whole plants, to turn yellow or reddish, then brown if infestations are severe. Seedling plants are very sensitive to injury, which may result in plant loss, stunting, delayed maturity, and reduced kernel size.
Field infestations can be detected by the presence of localized areas where leaves have reddish or orange-looking spots (feeding sites), or an overall yellowish appearance. Greenbug damage can be distinguished from moisture or temperature stress, or nitrogen deficiency, by inspecting plants for the presence of aphids.
Greenbug is also a known vector of barley yellow dwarf virus (BYDV). The disease is spread as aphids feed, and losses from the disease can be serious if infection occurs in the fall or early winter.
Corn Leaf Aphid:
Corn leaf aphids are distinctly blue to dark green in colour with a darker area at the base of the cornicles. They feed on small grains, wild grasses, and corn, but are most commonly found in sorghum whorls during vegetative growth stages.
Corn leaf aphid feeding is much less injurious to grain than greenbug feeding because toxins are not injected during feeding. Large infestations can
cause yellowish mottling of leaves, but injury is seldom economic.
Bird Cherry-Oat Aphid:
Formerly known as the oat-bird cherry aphid, this species infests small grains but is not found on sorghum. These aphids are olive green to blueblack in colour and have reddish patches at the base of the cornicles.
Like corn leaf aphids, bird cherry-oat aphids do not inject toxins while feeding, therefore, even though heavy infestations can develop, injury is not readily apparent and plants appear to be able to tolerate large infestations without economic yield losses.
However, bird cherryoat aphid is another known vector of BYDV, and if the virus is present in aphid populations, feeding can vector the disease and yield losses can occur.
English Grain Aphid:
English grain aphids are usually grass-green in colour, but can be pinkish, especially in the spring. Their cornicles and antennae are black and longer in relation to their bodies compared with other species.
No toxins are injected during feeding, so, like bird cherry-oat aphids, injury is not apparent and usually not economic unless associated with BYDV transmission. It is the most common species in late spring, when it develops and feeds inside the upper leaf sheath (boot) and on the flag leaf.
As grain begins to head, the aphids move to the grain head and cluster among bracts and kernals. Feeding by large numbers causes the growing kernels to discolor and shrivel. Heavy head infestations can reduce yields by 13%.
Scouting is especially valuable in predicting problem infestations because English grain aphids have only been found in heads of plants infested in the lower canopy earlier in the season. It appears that they move up plant stalks as grain heads develop, rather than fly in from areas outside the field. English grain aphid can also vector BYDV, and like bird cherry-oat aphids, can cause reduced yields if the virus is present in the populations at the time they are feeding.
When to Treat:
The following guidelines should be considered prior to using insecticidal controls for aphids in small grain which will be harvested for the grain: Insecticide-resistant aphids have been found in other wheat producing areas of the United States.
Using insecticides only when actually needed may reduce the likelihood of resistant strains developing. Aphid control is not advised if the crop is approaching the hard dough stage or if there is good predator/parasite activity.
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