Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
As you make plans to plant wheat, you should consider the following factors when making a treatment decision for aphids. In general, cooler summer temperatures with adequate rainfall followed by a warm, dry fall favors aphid development in small grains, especially in early planted fields. Early fall infestations of the greenbug aphid are favored by cool, late summer conditions.
In the fall, the aphid that can cause direct damage to small grains is the greenbug aphid species. It can inject a toxin into the leaves and cause death of plants. In outbreak years, we have seen entire sections of fields killed by this aphid. When it comes to Barley Yellow Dwarf Virus (BYDV) and its spread, all aphid species occurring in Delaware small grain fields (English grain aphid, bird cherry-oat aphid, corn leaf aphid, and the greenbug) are capable of transmitting BYDV from infected grasses into small grain fields. In the fall, the 2 most common species encountered in Delaware small grain fields are the bird cherry-oat aphid and the greenbug aphid.
Aphid population densities in small grains 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).
Although we do not have current data from our area evaluating thresholds to time sprays for fall aphid management, we will be conducting a project for the next few years to evaluate thresholds developed in states in the southern region. 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. If weather conditions remain warm through the fall and early winter favoring increases in aphid populations, you will need to continue scouting for aphids.
The following links from southern states provide information on additional management considerations: http://www2.ca.uky.edu/agcollege/plantpathology/extension/KPN%20Site%20Files/pdf/KPN1300.pdf
http://news.utcrops.com/2013/09/wheat-insect-control-begins-with-planting/