Aphids In Winter Wheat: What To Do In Case Of Fall Infestations

By Anitha Chirumamilla

Fig. 1. Bird cherry-oat aphids (note the dark green color and rusty brown patch at the tip of the abdomen)

Aphids infesting winter wheat made news last week in South Dakota. It appears that the populations are higher in the central and southern counties and aphids are starting to build up in the western counties as well.

Two possible reasons that could explain aphid infestations this fall are 1) warm fall weather conditions and 2) early planting of wheat. Mixed populations of the two aphid species were identified in infested wheat fields. Most of them are bird cherry-oat aphids (BCOA) and less frequently greenbug aphids. The BCOA is dark green with a characteristic rusty brown patch near the black cornicles at the tip of the abdomen (Fig. 1), whereas greenbugs are yellow to green in color with a dark green stripe running down their back (Fig. 2). Both species feed by sucking plant juices causing yellowing of leaves and compromising plant growth.



Fig. 2. Greenbug aphid (note the light color and dark stripe)

Damage to Crops

Apart from feeding damage, both species are known to transmit the barley yellow dwarf virus (BYDV) and the cereal yellow dwarf virus (CYDV), two of the serious diseases in wheat known to cause yield losses. The symptoms of BYDV and CYDV are stunted growth and yellowing or reddening of the leaves. These symptoms tend to be more apparent in the spring even if transmission of the virus occurs in the fall. The virus transmission occurs at the early stages of aphid infestation i.e., when winged aphids fly into wheat fields and it takes two weeks for the plants to show symptoms. Once aphids get settled and begin to colonize, the movement between plants is limited. However, as the aphids get crowded on small plants, they produce winged aphids that fly to nearby plants and transmit the virus.

With the weather forecast predicting more warm days this fall, early planted winter wheat remains vulnerable to virus spread and aphid feeding damage, and increases plant susceptibility to winter kill as well. As the night temperatures fall below 20oF, aphids switch to producing winged male and female aphids which fly to their overwintering host (cherry trees in case of BCOA), mate and lay their eggs that hatch in following spring. It is important to note that not all BCOA or greenbugs are vectors of BYDV or CYDV – it is impossible to estimate the risk of infection and it is not always a given that mere presence of BCOA or greenbugs indicates high probability of the BYDV and CYDV infections.



Fig. 3. Barley yellow dwarf virus symptoms in wheat

Managing Infestations

Spraying winter wheat for aphids in late fall is usually not recommended, as cool fall temperatures do not favor aphid population build up. Moreover, aphids like BCOA tend to be at or below the soil level (Fig. 4), which are not covered by foliar sprays. However, a combination of warm fall conditions, high populations of aphids (at or above thresholds and at least a portion of aphids on foliage and not below soil level) and the threat of spreading BYDV and CYDV may require insecticide treatments (Table 1) to protect wheat seedlings. There are not many studies documenting effectiveness of fall applications of insecticides to manage aphids, but one report noted that pyrethroid insecticides provided better control of these pests in the fall. The economic thresholds for aphid infestations in fall have been defined as an average of 15-25 BCOA per linear foot of row or 20 aphids per plant during the seedling stage in fall through head formation in the following spring. For greenbugs, the threshold is 50 or more greenbugs per linear foot of row (Source: University of Missouri Extension).



Fig. 4. Bird cherry-oat aphid colony near soil line


Table 1. Insecticides recommended for foliar application (aphids) on wheat in South Dakota
 

Preventative Measures

• Taking weather forecasts into consideration to adjust planting dates will help winter wheat to escape aphid infestations.

• There are no treatments available for viral diseases and prevention is the only available option. Seed treatments with neonicotinoid insecticides are known to be very effective in controlling fall infestation of wheat aphids for about four weeks after planting. Producers should consider this option whenever warm fall weather is forecast and especially if they elect to plant winter wheat early.

Source:SDSU