Aphid Numbers Increasing in Mississippi Wheat Fields

Over the last week I have received several calls of very high aphid numbers in wheat.  The two species being found are Bird Cherry-Oat aphids (BCO) and Greenbugs.  I have had reports of treatable levels of Greenbugs being found but most are BCO’s.  The very warm temperatures over the last couple weeks has allowed for populations to increase rapidly.  Greenbugs are most injurious this time of year because of the toxin they secrete when they feed that causes localized chlorotic/necrotic lesions.  Although Mississippi has no threshold for BCO aphids and direct injury from this pest is generally not yield limiting, there are exceptions to every rule.  Here are some criteria I would use when making a decision to treat BCO aphids in wheat in the absence of an established threshold in Mississippi. 

 (1) Are BCO aphid numbers at extremely high levels? (50-100 per individual stem).  (2) Is honeydew accumulating?  (3) Is the wheat visibly being damaged by the aphids?  Often you will see aphid “hot spots” showing up in fields first.  If these hot spots are lagging behind adjacent wheat and showing chlorotic symptoms from aphid feeding, it may be necessary to treat before spots increase in the field.  (4) Is the wheat already stressed from some other factor?  When wheat is stressed from some other factor like drought, waterlogged, herbicide injury etc. aphids can cause more damage than usual.  We generally want to avoid if possible compounding stress factors, or in other words, stress on stress.  (5) Beneficial insects are not adequately maintaining aphid populations.

The data in Table 1 is from a yield study that Dr. Jeff Gore conducted in 2007 under extremely high bird cherry-oat populations at Stoneville.  This was an unbalanced test with 9 replications of plots with low numbers of bird cherry-oat aphids and 7 replications of plots with extremely heavy numbers of bird cherry-oat aphids.  You can see from the data that although rare, bird cherry-oat aphids can sometimes reach numbers that would require treatment.  Also keep in mind that it was extremely dry during this period adding an additional stress factor to the plants.  The low treatment averaged less than 50 aphids per plant and the heavy treatment was greater than 200 aphids per individual stem.  Populations are rarely uniform across large acres.  Looking at the whole field, approximately 40% would have been classified as “heavy”.

 Table 1.  Wheat yield comparisons in plots with low and heavy bird cherry-oat aphids

Keep in mind that not all wheat fields are experiencing this problem, and higher aphid numbers will likely be found in the earliest planted wheat or the more lush areas of the field.

For more information refer to “Monitoring Aphids in Mississippi Wheat” you can simply type the word “aphid” in the search bar at the top right of the blog.