Last week it was not hard to find sugarcane aphid in sorghum fields in central and south Alabama. About half the fields that were scouted turned out to have sugarcane aphids. In most areas, there were not yet enough aphids to require an insecticide spray.
The exception was in Baldwin County, where Kim Wilkins, Regional Extension Agent III, Agronomic Crops, estimates 40% of the sorghum fields needed to be sprayed. Aphids were reported from sorghum in Elmore, Autauga, and Escambia Counties, in addition to those counties reported previously (Baldwin, Dale, Marengo, and Tallapoosa). The aphids have not been reported yet this year from Chilton County or other counties in north Alabama.
It is important to scout for sugarcane aphids and spray when they have reached threshold levels. It is equally important not to spray too soon. Spraying too early will likely result in an extra insecticide application later on.
Aphid populations build rapidly, so once aphids are found in a field that field should be scouted twice a week. It is easy to overestimate how many aphids are in a field because they are often worse on the field edge. Since the aphids are not scattered evenly across a field, it is important to check different areas of the field to get a feel for how many aphids are actually out there.
When scouting, it is a good idea to walk into the field about 25 feet, then start pulling some leaves to look for aphids. Don’t make a decision based on plants right on the edge. Pick a lower leaf and an upper leaf from a plant and check for aphids on the underside of the leaves. Walk a little way then pick two more leaves, until you have looked at five plants (10 leaves).
Keep track of how many aphids you found on those 10 leaves, as well as how many of the plants had aphids. Then move to another area of the field and look at 5 more plants. Keep scouting until you have looked in at least 4-6 areas of the field. If the field is large, you will need to check more areas than if the field is small.
You may see the shiny aphid honeydew (sugary excrement) on the upper side of some leaves. This is an indication that aphids are feeding on leaves further up the plant (see photo, where you can see aphids on the underside of a leaf, next to the shiny upper surface of the leaf just below).
A reminder of some of the thresholds that have been proposed for sugarcane aphid:
- Walk 25 feet into the field and examine a lower leaf and the uppermost leaf from 5 plants. Estimate the number of sugarcane aphids found on each leaf. Scout plants in other locations of the field until you have examined at least 40 leaves per field. Apply an insecticide within 4 days if the field average is 50 – 125 aphids per leaf (Texas A&M AgriLife Extension).
- Treat when 50 aphids per leaf are colonizing 20 percent of the plants (before or during boot milk stage) (Louisiana).
- Treat when 25% or the plants are infested with 50 or more sugarcane aphids per leaf (Arkansas).
- Depending on growth stage, treat when 20 to 30 sorghum plants out of 100 have aphids and you can find areas of the field with numerous aphids and heavy deposits of aphid honeydew (Mississippi).
More information on sugarcane aphid in Alabama can be found in this publication: IPM Strategies for Managing Sugarcane Aphid in Alabama Sorghum
Some sorghum in Alabama is heading out, and some has not been planted yet. The later sorghum is planted, the greater the risk from sugarcane aphid, sorghum midge, whorl-worms, and headworms. More information on sorghum insect pests can be found in Dr. David Buntin’s publication: Sorghum Insect Pests and Their Management.