By Doug Johnson, Extension Entomologist, University of Kentucky
Over the past two growing seasons a difficult new insect pest of grain sorghum has been found across the mid-south. This new pest is an aphid called the sugarcane aphid, Melanaphis sacchari. Be sure not to
confuse this with the yellow sugarcane aphid, Sipha flava, which has been around the lower Mississippi river valley for several years. These are two completely different insects. Before we go any further, it is important that you understand thatat present, NEITHER of these aphids have been found in Kentucky. My concern is one of preparation, as both of these aphids are found in western Tennessee and southeastern Missouri. The information that follows is specific to the sugarcane aphid, which has shown itself to be a very damaging pest on grain sorghum. What I can tell you about this pest is a compilation of information obtained from my extension entomology colleagues in states to the south of Kentucky. We are fortunate to have information developed by my colleagues in Tennessee, Missouri, Arkansas, Mississippi, Texas and Louisiana to to draw upon before this pest shows up in Kentucky; IF it does show up in Kentucky. So here is the story.
Figure 1. Sugarcane aphid on sweet sorghum
In July of 2013 the state of Louisiana discovered a new species of aphid in grain sorghum not previously noted in the mid-southern region to be a pest of grain sorghum. This turned out to be sugarcane aphid (SCA) which produced economically important problems in Louisiana and Texas. In 2014 SCA rapidly spread throughout much of the South including Mississippi, Arkansas Tennessee and Missouri causing severe yield losses in many fields of grain sorghum.
No one really knows how this happened. It could be that SCA adapted to feeding on grain sorghum or it could be the introduction of a variant of SCA that was already adapted to grain sorghum. Some taxonomist suggests the aphid may be a new species. Additionally, SCA may feed on sorghum grown for forage, sweet sorghum and Johnson grass. The SCA was first verified in Tennessee in early August of 2014, although it was subsequently apparent that it was present prior to this in some counties of West Tennessee. This species was confirmed in nearly all counties of West Tennessee by the end of the 2014 growing season, and it was likely present in other areas of the state at low numbers. Fortunately, for our Tennessee neighbors, it was late-planted fields that were primarily affected in 2014 owing to the relatively late arrival of SCA in Tennessee. Other southern states along the Mississippi River had similar experiences.
Figure 2 Known Distribution of sugarcane aphid in the 2013 & 2014 growing seasion. Texas AgriLife Extension Service.
You will notice in figure 2. That sugarcane aphid (SCA) data are not listed for Kentucky and Missouri. The insect has not been collected in Kentucky, but it has been identified in MO. In 2014 SCA was found in Pemiscot and Dunklin Counties, MO. These counties are south and west of the KY line and thus not directly on the KY border. So, thus far the aphid has not been collected as far north as Kentucky within the Mississippi River valley. This is good news, but may not be much protection as the aphid has been reported in Oklahoma and Kansas further north than the Kentucky – Tennessee state line. Not good news for Kentucky, but those areas are much less humid than the Purchase area of Kentucky so they may not be a direct comparison. SCA is now confirmed in 11 states and may spread further.
Taken from the Texas A&M publication (see reference below): the sugarcane aphid is gray to tan or light yellow. Unlike other common aphids on feeding on sorghum, sugarcane aphids have dark, paired, tailpipe-like structures called cornicles, at the rear, and their tarsi (feet) are dark at high magnification. The dark cornicles and tarsi contrast distinctively with the lighter body color of the sugarcane aphid.
Figure 3. Common aphids on grain sorghum. Texas A&M AgriLife Extension.
Sugarcane aphids differ from other aphids that attack grain sorghum:
• Greenbugs have a distinctive darker green strip down the back that SCA does not have.
• Yellow sugarcane aphids have hairs on the body (seen with magnification).
• The legs and head of corn leaf aphids are dark.
Sugarcane aphid infestation begin by clustering on the under surface of lower leaves, then move to the upper leaves. If not controlled they may even infest the grain sorghum head. Colonies may grow quickly in favorable conditions and produce large quantities of sticky honeydew. It is believed that this sticky substance may help protect the aphids from some predators.
Feeding damage results in yellow to red or brown discoloration on both sides of the leaf. The honeydew will support growth of sooty mold fungus. Infestations on young plants may result in plant death, while very late infestations may prevent grain from forming.
Heavily damaged plants can result in the clogging of harvest equipment, and difficulty in moving the grain through machines. The sticky leaves may prevent grain from separating from the stalks and leaves. Combines may require service to wash off the honey dew and remove lodged stalks form the heads.
Insecticidal control options for SCA are limited in grain sorghum. Most currently labeled insecticides are marginally effective. Most of the neighboring states that have had problems with this insect have sought and received a Section 18 Emergency Exemptions for the use of Transform WG™, a Dow product, against sugarcane aphid on sorghum and ONLY sugarcane aphid on grain sorghum. I have provided to the Kentucky Department of Environmental services (within the KY Dept. of Ag.) biological information concerning this pest, again, obtained largely from our sister states, as they consider submitting a Section 18 Emergency Exemption for Kentucky. It is important for all concerned to realize that if this exemption is granted to Kentucky, there will be follow up information collection required such as but not limited to; number of treatments, number of acres treated, timing of treatments, and application methods etc. If this information is not supplied to EPA the Emergency Exemption will be revoked.
In addition, an insecticide, Sivanto 200 SL, a Bayer CropSciences product, has just recently been labeled for use on grain sorghum. Though sugarcane aphid is not listed on the label, Bayer CropSciences has issued a 2ee label for use against sugarcane aphid. Originally Kentucky was not included in the list of states which may use the 2ee label, but after communicating with individuals with the company, Bayer CropSciences has agreed to add Kentucky to their 2ee label, and this label will be available in 2015.
Beyond rescue insecticide applications what can you do to prevent / mediate this pest? Our colleagues in Mississippi have published an outline for SCA control on grain sorghum that seems to be the best approach (See Catchot, Gore and Cook, Reference Below). Briefly:
1.) Plant Early.
2.) Use high plant populations and / narrow rows.
3.) Use insecticide Seed treatments.
4.) Avoid all Disruptive Sprays.
5.) Use tips and GPA to maximize coverage to the bottom of the canopy.
6.) Scout 2X per week when the first aphids are fond.
7.) Watch for late season head colonization.
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