This article is running a little late, but there are questions about whether or not to use an insecticide seed treatment in wheat for control of aphids, and if you do or don’t, how does this affect management down the road.

What aphids occur on wheat in Tennessee?  Aphid identification is tricky because we are dealing with small insects that may have winged and non-winged adults.  The identification guides linked below do a far better job than I can in helping to distinguish different species.
University of Nebraska Identification Guide for Aphids in Cereals
University of California Key to Aphids in Small Grains

In Tennessee, the most common species of aphids found in wheat are:

• Bird cherry – oat aphid:  The most common aphid found, its body is olive-green and broadly oval with reddish-orange patches around the bases of cornicles.  The tops of the leg segments, tips of cornicles and antennae are dusky to black colored.  The near black colored winged adults can be found in the field at this time.
• English grain aphid:  This species is most easily identified by its relatively long legs, antennae and cornicles.
• Greenbug:  I typically see more greenbugs in the spring.  It has a light green body with a darker green stripe down the middle of its back. The tips of legs, tips of cornicles and most of antennae are black.  Greenbugs inject a toxin when they feed, and thus are more likely to cause economic damage if they occur in sufficient numbers.
• The corn leaf aphid and Russian wheat aphid may also be found, but in my experience, usually in lower numbers than the other species.

What harm do aphids cause?  Aphids feed on the sap (phloem) of plants.  Fortunately, with the exception of greenbug, it usually takes very high populations of aphids to cause serious, direct injury to wheat.  We are talking about several hundred aphids per foot of row before plants are seriously affected, and these population levels typically do not occur until spring after temperatures have warmed and the wheat is larger (and able to tolerate higher aphid populations).  The bad news is that much lower populations may transmit barley yellow dwarf virus (BYDV).  Fall or early spring transmission of this disease can substantially reduce yield during some years.

How does planting date affect aphids and other pests?  Planting wheat in September is discouraged.  October 15th is the “Hessian fly-free date”, and this date corresponds to the approximate date of first freeze in West Tennessee.  Adult Hessian flies begin emergence from summer dormancy in late August and September. After a freeze, adult Hessian flies are no longer active.  Thus, you can avoid potential infestations by the first (fall) generation of hessian fly if you abide by the fly-free date.  Early planted wheat is also more likely to be colonized by aphids during the fall.  Wheat planted in October will probably not have high populations present before aphids go dormant during the winter months.  This helps to reduce the potential for fall transmission of BYDV.

Should I use a seed treatment for aphid control and the prevention of BYDV?  There are a several options to consider for managing aphids in wheat. The first is to do nothing. The occurrence of BYDV (and aphids) varies considerably from field to field, year to year, and across planting dates.  It is not a sure bet that treating for aphids will increase yield. But keep in mind that you can’t put the horse back in the barn if aphids do show up in numbers. By the time aphids are common in the field it is too late to treat with the hopes of preventing transmission of BYDV.

There are several insecticide seed treatments options that do a good job of reducing aphid infestations including Cruiser (thiamethoxam), Gaucho (imidacloprid) and NipsIt (clothianidin), although I’ve not done much testing of clothianidin on wheat.  These treatments will typically keep fields nearly aphid free in the fall, and this also translates into lower springtime populations.  A 10-year study by Dr. Albert Chambers showed an average yield response of about 4 bushels per acre in Tennessee by using an insecticide seed treatment (he used Gaucho).  In my tests over the past four years, I’ve seen a 4.7 bushel response (this was either Gaucho or Cruiser).

So a no brainer, right?  Not so fast!  I should point out that over the same four years, I’ve observed an average yield response of 6.9 bushel per acre by making a early spring (mid to late February) foliar application of insecticide for aphid control.  This was on untreated seed for wheat planted between October 7 – 22 where aphid populations were very low during the fall and at the time of application (typically less than 2 aphids per foot of row).  My conclusion is that this application is suppressing the springtime spread of BYDV before resident aphid populations begin to build.  So my recommendation is to consider using an insecticide seed treatment on early planted wheat or make a foliar insecticide application about 3-4 weeks after emergence.  On wheat planted in October, especially after October 15th, a single application of insecticide during mid to late February application may accomplish the same.  It may be beneficial to make both a fall and early spring application on early planted wheat if an insecticide seed treatment was not used, but I have little data to support this.   After several years of testing, I have been impressed with how long treatment effects last from a springtime insecticide application.  Aphids are often almost undetectable at the time of application, but treatment effects will become obvious has temperatures warm and the wheat grows.  Dimethoate or the pyrethroid insecticides such as Karate/Warrior, Baythroid XL, Declare, or Mustang Max are common choices for aphid control. 

Insecticide seed treatments do offer some control of Hessian flies during the fall, and this is a potential benefit for early planted wheat.  Below ground insects sometimes reduce stands and vigor of wheat.  These include critters like wireworms.  Foliar insecticide applications will have little effect on below ground pests.  On the plus side, foliar applications are relatively inexpensive particularly if they can be piggybacked with herbicides or fertilzer applications.