Small grains have looked rough this year for several reasons. Oats have suffered as they are more susceptible to cold and we’ve had another cold winter. Also, we’ve had leaching rains that removed nutrients. Most recently we are seeing some Barley Yellow dwarf virus (BYDV) transmitted by aphids. The darker aphid, Bird cherry- oat, is worse about causing this in small grains. We are seeing a good many of them now.
Here’s some info on aphids from Dr. David Buntin (Entomologist at UGA Griffin) handy for your review: (more details available here: http://extension.uga.edu/publications/detail.cfm?number=B1190).
Aphids are small, soft-bodied insects that can be found in wheat anytime during the growing season. The most common aphids found on wheat are the bird cherry-oat aphid, rice root aphid, greenbug, corn leaf aphid, and English grain aphid. The first four occur mostly in the fall and winter. Only the greenbug causes direct feeding damage that appears speckled brown and discolored with some leaf curling. The other aphids usually do not cause obvious feeding damage. The English grain aphid is mainly present in the spring and can reach large numbers on flag leaves and developing grain heads. Damage from this pest can reduce kernel size and lower grain test weight. For the most part, beneficial insects such as lady beetles are not active during the winter and only exert some control over aphids during the spring in wheat.
Aphids also vector a viral disease named barley yellow dwarf (BYD) and a related disease called cereal yellow dwarf. Wheat and barley can be severely damaged, but oats are most susceptible to this disease. BYD is present in most fields in most years throughout Georgia. Yield losses of 5-15% are common but losses can exceed 30% during severe epidemics. Infection can occur from seedling emergence through heading, but yield loss is greatest when plants are infected as seedlings in the fall. Although all aphids can potentially transmit certain strains of the virus, infections in the Southeast are mostly associated with infestations of bird cherry-oat aphid and rice root aphid. Planting date is the single most important management practice, with early plantings generally have greater aphid numbers and greater BYD incidence than late plantings.
Systemic seed treatments, imidacloprid (Gaucho, Attendant, Axcess), thiamethoxam (Cruiser), and clothianidin (NipsIt Inside), are available for controlling aphids in the fall and winter and may reduce infection rates of BYD. These seed treatments are more effective in the northern half of the seed treatments have been inconsistent in control and are not recommended for routine use.A single, well-timed insecticide application of the insecticide lambda cyhalothrin (Karate Zeon, Silencer, and similar products) or gamma cyhalothrin (Declare) also can control aphids, reduce the incidence of BYD and increase yields. The best time for treatment in northern Georgia usually is about 25 – 35 days after planting although an application at full tiller also may be beneficial. In southern Georgia, the best treatment time usually is at full-tiller stage in early to mid-February. But, scout fields for aphids at 25 – 35 days after planting and during warm periods in January to determine if an insecticide application is needed. A lambda cyhalothrin or gamma cyhalothrin treatment at full tiller can be applied with top-dress nitrogen. OP insecticides, such as dimethoate and methyl parathion, also will control aphids but are not effective in preventing barley yellow dwarf infection.
To sample aphids, inspect plants in 12 inches of row in fall and 6 inches of row in winter. In spring, inspect 10 grain heads (+ flag leaf) per sample. Count all aphids on both the flag leaf and head for making control decisions. Sample plants at 8 to 16 locations per field.
Treat when populations reach or exceed the following thresholds at various stages of development:
Seedlings: 2-3/row ft,
30-60 days after planting: 6/row ft;
6-10 inch plants: 1 to 2/tiller
boot to heading: 5/stem
heading to dough stage: 10/stem
hard dough to maturity: damage not economic.