The information and recommendations in this newsletter are applicable to North Carolina and may not apply in other areas.
The information and recommendations in this newsletter are applicable to North Carolina and may not apply in other areas.
From: Randy Weisz, Small Grain Specialist, North Carolina State University
Severe aphid problems are being reported in small grains, especially in the Piedmont. I have had calls about wheat so severely infested that the tillers have died completely back. Such high levels of Fall aphids in small grains are rare in NC since a hard freeze will kill the aphid population, and we usually have had a good freeze by this time of year. But, this year, the weather has been unseasonably warm and these high aphid populations have developed. Fall aphid infections cause two potential problems: 1) tiller dieback, and 2) possible Barley Yellow Dwarf Virus infection.
Fields that look yellow, or are suspected of having nutrient, or herbicide carry over problems should be checked for aphids since the symptoms can appear somewhat similar.
I recommend that all small grain fields be scouted for aphids as soon as possible. The standard threshold for applying a foliar insecticide for fall infestations is 20 aphids per foot of row. This threshold is, however, probably too high. If the threshold is met or being approached, and a freeze is not expected, a foliar insecticide may be a good idea. If warm weather is expected to continue, a long residual insecticide like Warrior might be a good idea.
What about fields that are already damaged? I would not expect tillers that are already yellowing and/or dieing to recover. Additionally, if Barley Yellow Dwarf has historically been a problem in the general area, the aphid infested small grain stands a good chance of already being infected with this virus. Growers with severely infested and damaged fields, and those who are concerned about Barley Yellow Dwarf, may want to consider re-planting. At $2 per bushel this would most likely not be feasible, but at today’s prices, and with the continuing warm weather, it may be a good option for consideration. If fields are replanted, the infested wheat should first be burned down or tilled under to prevent the spread of Barley Yellow Dwarf. Additionally, the newly planted wheat should be protected from aphid feeding if a hard freeze has not already occurred. This could be done with the use of an insecticidal seed treatment like Cruiser or Gaucho, or by applying Warrior at the two to three leaf stage.
What about fields that have just recently been planted? These fields should also be scouted and a foliar insecticide considered if the threshold is being approached or exceeded. If recently planted fields have not yet reached threshold, but are adjacent to earlier planted and severely damaged small grains, I would suggest that an application of Warrior be considered as a “preventative”.
John Van Duyn, Extension Entomology Specialist, North Carolina State University, adds that there have been several aphid infested wheat fields identified in NE counties. In addition, aphids have been showing-up regularly in the Piedmont. This situation is not unusual during a fall that remains warm into November. After cold weather arrives (events below freezing) the problem usually quickly subsides.
Aphid identification:
Different kinds of aphids may be found within and between fields. Usually the English grain aphid is most common but corn leaf aphid, bird-cherry-oat aphid, and greenbug may also be found. All four species have cornicles (tail pipes). Here are a few distinguishing characteristics of each (wingless form only):
I. – Cornicles completely dark:
A. English grain aphid -- The cornicles are long and narrow. The body is relatively large, broadly spindle shaped and legs are skinny. Leg segments are black near the ends and the antennae and cornicles are completely black.
B. Corn leaf aphid – The cornicles are short and broad and the bases of the cornicles are often dusky looking. The body is green (often very dark green) with a bluish tinge (blue green). The head, legs, tail end, and antennae are darker looking. Antennae and cornicles are short.
II. -- Only tips of cornicles dark:
A. Bird-cherry-Oat aphid -- There is a reddish-orange spot on the aphid’s back around the bases of the cornicles. The body is olive-green and broadly oval. Tops of the leg segments and tips of the cornicles are dusky to black.
B. Greenbug -- Without reddish spots. The body is light green to yellow-green with a darker stripe down the middle of the back. The tips of the legs, tips of the cornicles, and most of the antennae are black.
All species except the greenbug are about equal in their direct damage potential and it takes many aphids to inflect yield loss. This does not account for barley yellow dwarf virus transmission. The greenbug is the most damaging since it can inject the feeding site with toxic saliva. Normally, around 20 aphids per foot (15 per foot for greenbug) are required to justify treatment in fall wheat. As plants grow the thresholds go up proportionately.
There may be difference between conservation tillage and conventional tillage planted wheat. Aphids may transfer from grassy weeds that may be burned down in conservation tillage and wheat often grows more slowly in this situation. It may be a good idea to concentrate scouting efforts on the conservation tillage wheat, but do not ignore conventional wheat.
Several insecticides are recommended in the 2002 Agricultural Chemicals Manual. You can also consider Mustang Max, as it was recently labeled. The pyrethroids have been most effective at controlling aphids and reducing yellow dwarf virus (Warrior and Mustang Max). However, if fields are now full of aphids the time for reducing virus has passed for this crop.
Note: While all the calls that have come in are about aphids, I am also concerned that some of these symptoms may be signs of Hessian fly damage. Where tiller yellowing and die-back is being seen, I would recommend that the tillers be dug out of the ground and checked for Hessian fly larvae and/or pupae. Hessian fly recommendations have just been updated and can be found on the internet at: http://www.ces.ncsu.edu/depts/ent/notes/Grain/Hessian/HflyAG-368.html
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Last modified on November 25, 2003 by Stephen J. Toth, Jr.