As discussed in the article, Review of Barley Yellow Dwarf in 2012, the spread of barley yellow dwarf virus (BYDV) is completely dependent on aphids transmitting the virus which causes the infection. There are four aphid species occurring in Delaware that are capable of transmitting BYDV from infected grasses into wheat including the English grain aphid, bird cherry-oat aphid, corn leaf aphid, and the greenbug. For photos to help you identify aphids go to: http://kentagextension.blogspot.com/2007/10/know-your-small-grain-aphids.html
Aphids can infest small grain fields in the fall and again in the spring. The number of aphids arriving in fields in the fall is often dependent on the growing conditions the preceding summer. In general, cooler summer conditions with adequate rainfall followed by a warm, dry fall favors aphid development, especially in early planted fields. In drier summers, fewer aphids are produced due to reduced host plant quality. It should be noted that greenbug aphids (which cause direct damage to small grains as well as transmit BYDV) are favored by cool, late summer conditions. Aphid population densities in the fall are also affected by when the first hard frost occurs in relation to wheat seedling emergence. Crops that emerge long before a hard freeze have a greater potential for aphid infestation (and exposure to BYDV). Planting after the fly free date can help to help to manage aphids as long as the freeze occurs when expected. Aphids arriving in the fall will continue to feed and reproduce as long as temperatures remain above 48°F.
In areas where you have seen BYDV in the past, where you are planting early (before the Hessian fly-free date), or you have seen direct damage by greenbug aphids, a commercial applied seed treatment which includes an insecticide would be a good control option for fall infestations. Another option would be to scout fields and apply a foliar insecticide. Information from Kentucky indicates that planting date is the most important factor determining the intensity of an aphid infestation. The most important time for controlling aphids in the fall is the first 30 days following emergence. The second most important time is the second 30 days following emergence. So it will be important to scout wheat starting at plant emergence if you plan to use a foliar insecticide for fall aphid management. The following link to a fact sheet from Kentucky provides more information on aphids and BYDV in wheat (http://www.ca.uky.edu/entomology/entfacts/ef121.asp).
Since the incidence of BYDV has not been widespread in past years in Delaware and Maryland, we do not have current data from our area evaluating thresholds to time sprays for fall aphid management. The following thresholds from Kentucky (included in the above fact sheet) could be considered when making a decision to apply a fall foliar insecticide : (a) the first 30 days after planting treat if you find an average of three or more aphids per row-foot, (b) from 30-60 days after planting treat if you find six or more aphids per row-foot, and (c) more than 60 days after the plants emerge treat if you find ten or more aphids per row-foot. Depending on weather conditions, a second application could be needed, especially if temperatures remain warm. As we saw this past season, aphid populations remained high throughout the winter and early spring so you will need to continue scouting if conditions remain mild again. They also indicate that in some years these thresholds may be too high and in others too low but currently this is the best available information. In addition, the risk of BYD infection varies from year to year. At this point, we are still not certain if the 2012 season was just an unusual one for BYD or if we will continue to see an increase in problems. We hope to survey for aphids this fall and evaluate these thresholds under our conditions.