Low Densities of Cereal Aphids Present in Both ND and MN this Past Week (6/23/11)
Populations in most areas are still very low, but there may be an issue of concern other than yield loss from aphid feeding. This year, small grains were planted late in most areas making fields especially susceptible to Barley Yellow Dwarf Virus (BYDV). BYDV is vectored by aphids, especially the bird cherry oat aphid, one of the most common aphids that we find in our wheat. The bird cherry oat aphid is olive green with a brownish patch at the base of cornicles. Two other common aphids in our area include the English grain aphid (bright green with long black cornicles) and greenbug (pale green with a darker stripe down the back). The bird cherry oat aphid feeds primarily on leaves in the lower part of the small grain plant. When aphid populations are high, BYDV can spread quickly through small grain fields. At greatest risk are later-planted fields which attract migrating aphids that are moving from more mature fields.
BYDV has already been confirmed by the NDSU Diagnostic Lab in winter wheat samples in ND. In other areas, some symptoms that look like BYDV have been reported. Given that we've just started to see aphids showing up, we suspect this means that aphids are arriving already carrying virus.
Barley yellow dwarf virus is easily confused with a nutritional disorder or the effects of adverse weather. Visible symptoms depend on the plant's stage at infection. Leaf discoloration in shades of yellow, red (oats), and purple (sometimes) appear starting from the tip to the base and along the leaf margins to the midrib. If plants are infected with BYDV in the 4- to 5-leaf stage, growth will be slowed, maturity delayed and stunting is possible. Later infested plants do not suffer as much impact and are less likely to benefit from an insecticide application. Infected barley and wheat plants appear quite yellow against the rest of the field and infected oat plants seem to have reddish leaves. This disease is more pronounced during cool weather (60-70F) with ample sunlight. Infection sites can appear as stunted yellow single plants or as groups or clusters of yellow plants among healthy plants.
“Losses from BYDV depend on various factors including the cultivar, the virus strain, the time of infection, the number of vectors, and environmental factors. Losses can approach 100% if plants are infected early. Average losses have been calculated to be 55% for plants artificially infected in the seedling stage, 23% for those infected in the tillering stages, and 19% for those infected during stem elongation. These numbers should be considered worst-case estimates, because transmission efficiency is lower in natural infestations. Estimates of yield losses due to natural infection are commonly around 15%. Losses result from fewer heads per plant, decreased head length (and consequently fewer seeds per plant), partial sterility, and reduced kernel size and weight. Losses in grain yield are related to the decrease in plant height, the percentage of leaf area discolored, and the intensity of the discoloration. Infected plants tend to open prematurely and are often predisposed to fungal diseases that darken the grain and reduce seed germination.” (Source: Compendium of Barley Diseases, edited by D.E. Mathre).
Because we're dealing with late-planted small grains, and the long term forecast calls for cool weather through June, small grains in the area are at a greater than normal risk for BYDV if there is an aphid infestation. As a result, producers/crop consultants should be proactive with their cereal aphid control in late-planted wheat. The typical economic threshold for aphids in wheat is when 85% of the stems have more than one aphid present prior to complete heading. However, knowing that most of our late-planted wheat is at high risk for aphid infestation and BYDV transmission, we recommend that producers/crop consultants be more preventative against aphid infestations and treat as soon as aphids are detected or BYDV is observed in the area. Field scouting should begin earlier than normal (typically before stem elongation) and continue up to the heading stage of wheat to prevent economic yield losses. Due to the high risk status, best pest management of aphids may require more than one application of insecticides this year to prevent yield losses from aphids and BYDV. Even if early aphid populations require treatment because local BYDV symptoms have been reported, uninfected wheat may still require treatment for aphids later in the season if aphid populations rise to yield limiting numbers.
Pyrethroid or organophosphate insecticides registered in small grains should provide adequate control. A listing of insecticides registered for cereal aphid control in small grains is available from the ND Field Crop Insect Management Guide 2011, E-1143, NDSU Extension Service.
Janet J. Knodel - NDSU Extension Entomologist
Ian MacRae -MSU Extension Entomologist