NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665
June 8, 2000
Aphid populations in southern states such as Texas and Kansas were damaging this spring, and recently, low populations of these northward-migrating insects have been detected in parts of North Dakota. Meanwhile, crop surveyors working with the Integrated Pest Management (IPM) program administered by the North Dakota State University Extension Service have identified symptoms of barley yellow dwarf virus (BYDV) in winter wheat fields in southern North Dakota.
Aphid feeding can reduce the yield of small grain crops if populations are large enough, and these cereal aphids can also transmit BYDV, which likewise can threaten yield. To minimize this double-edged production threat, North Dakota growers need to begin monitoring their small grain fields for aphids, say two extension specialists at NDSU.
"Barley yellow dwarf in wheat has been very high in Kansas this year, with incidences of infected plants sometimes reaching 90 to 95 percent in a single field," says Marcia McMullen, an extension plant pathologist at NDSU who also coordinates the IPM program. "There's a good chance that the aphids migrating here will be coming in with the disease, because many of the areas they're coming from have high infection levels."
BYDV symptoms in wheat and barley include random patches of stunted, yellowing plants ranging from one or a few plants up to fairly large areas. The bright yellowing of leaves begins at the leaf tip and margins and works downward and inward on the leaf. Under cooler conditions, leaf discoloration in wheat may have a purplish hue as well. Similarly, red leaf symptoms are common in oats infected with BYDV, McMullen explains. The severity of an infection will depend on the number of aphids present, the percentage of virus-carrying aphids, crop growth stage and other factors. Late-seeded crops are at greatest risk.
"Infection while the crop is young and vegetative can lead to significant stunting and leaf discoloration. Infection at the flag leaf stage or after heading often is seen only as discoloration of the upper leaves," McMullen says. "However, adverse growing conditions can aggravate symptoms and ultimately increase yield loss."
The English grain aphid, corn leaf aphid, greenbug aphid and bird cherry-oat aphid are primarily responsible for transmitting BYDV, says Phil Glogoza, extension entomologist at NDSU. Limiting the incidence of BYDV means trying to manage a disease by managing an insect--a difficult undertaking. Complicating the effort to suppress BYDV is the fact that spraying to control aphids will also kill the beneficial insects that are aphid predators.
"If you take out those beneficial insects, the aphid population may recover rapidly and reach levels where yield losses from direct feeding of the aphids can occur," Glogoza explains.
An aggressive approach for trying to minimize BYDV would be spraying with an insecticide when 50 percent to 75 percent, or fewer, of the small grain stems have at least one aphid present, although this guideline has not been thoroughly researched, Glogoza says. To prevent yield loss due to aphid feeding alone, research has shown that spraying when 85 percent of the wheat stems have at least one aphid present is effective.
"The 85-percent threshold is for tillering to head emergence," Glogoza says. "After wheat is fully headed, treatment for aphids is rarely justified."
If fields are treated early, close monitoring of the aphid population is recommended to determine if aphid populations increase, Glogoza says. Since insecticides also kill predators and parasites, aphid populations can increase quickly without any natural controls present to suppress the population. It is possible for aphid populations controlled early to recover to threshold levels before the heading stage is reached.
"Other natural controls of aphids include fungal infections that occur during humid weather," Glogoza explains. "It is common to see these parasitic fungi affect aphid colonies very quickly, sometimes in a matter of days. If weather events favor such natural control, scout fields before treating with an insecticide to determine if treatment is still warranted."
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Sources: Marcia McMullen (701) 231-7627
Phillip Glogoza (701) 231-7581
Editor: Dean Hulse (701) 231-6136