Aphids on the upswing but make they are present before you spray

There has been a recent uptick in the amount of calls and reports we have been getting on the increasing presence of sugarcane aphids in the western grain sorghum production regions of Oklahoma, Kansas, and Texas.  This is particularly in the panhandle region where sorghum is just now reaching the reproductive stages of growth.  This has not only resulted in concern for these pests presence early in reproductive growth but has also increased the number of fields that have been sprayed or intend to be sprayed in the coming week.  This has been meet some indication that applications are being made premature or when sugarcane aphids are not even present in the fields.

Proper identification:

Probably the most challenging thing for consultants and producers working to manage grain sorghum in sorghum production is proper identification of sugarcane aphids in the field.  This is due to the fact that in small populations or with immature nymphs, many of the distinguishing features are difficult to see with the naked eye.  A hand-lens or magnifying glass should be used to allow for proper identification.  These can typically be purchased at a local store or purchased online at a reasonable price.

Many in-field indicators can aid growers in the identification of sugarcane aphids in the field.  Honeydew on the top side of the leaves is one of the most telling signs of a sugarcane aphid infestation.  However, two issues typically exist with using honeydew for identification or make recommendations on chemical control.  Firstly, when noticeable honeydew is present in the field, especially enough that can be seen during quick field checks, often the population has grown large enough to create serious losses to the sorghum crop.  Furthermore, several other insects in sorghum can create honeydew.  One of these in-particular is corn-leaf aphids.  These aphids are typically found in the whorl during vegetative growth or in the collar region of sorghum during reproductive growth stages.  While these can create a large amount of honeydew, they typically cause little to no yield reduction of the sorghum plant and application is rarely ever recommended.  These corn-leaf aphids also aid in the sustenance of predators that can be helpful in controlling sugarcane aphids if communities establish.  Therefore, making an application to control these pests are frequently cost adverse and can negatively impact natural predator populations.

Corn_leaf_aphids
Corn leaf aphid at flowering in sorghum (Photo courtesy of Jason Warren).

There has also been increased reports of sugarcane aphids in the sorghum head during early flowering.  While this is always within the realm of possibilities, these aphid pests do not like sunlight and will typically not be in the head at these early stages due to the high amount of sunlight that is present.  We can see sugarcane aphids in the heads but often much later in the season during grain fill or ripening when there is much less exposure to sun.  Sugarcane aphids, for the bulk majority of the season, will be found on the underside of the sorghum leaf.  Aphids can be present on the upper side of the leaf, but these will typically be yellow sugarcane aphids not the white sugarcane aphids we are concerned with.

While honeydew and location are good indicators that sugarcane aphids could be present, physical identification needs to be done prior to making an application.  This is the only true way to positively identify these pests as well as know the magnitude of populations growers will have to manage around.

When to make an application:

When to pull the trigger on a chemical application is very challenging and several factors (including population, stage of the crop, and environment) come into play in this decision.  Furthermore, based on the potential growth patterns of these pests, the line between under threshold and exceeding threshold is very narrow.  Growers do not want to become overwhelmed with aphids prior to making an application.  Not only could this drastically effect yields but incomplete coverage applications could allow for lower efficacy of applications resulting in not achieving a complete kill.  However, growers do not want to apply before the economic threshold.  These applications can be costly, especially when applying high gallons.  Early application do not use the potential predators in the field to help control these pests and may result in an application that was unwarranted.

Growers should make an application when a minimum of 50% of plants have between 50-100 aphids per leaf.  To visualize, 50-100 aphids will result of being able to be completely covered by a quarter.  Higher populations would be considered over threshold.

Should I treat aphids in forage sorghum/BRM sorghum/Sorghum-Sudan:

This is a difficult question to answer.  While there has been some indication that aphids do impact these forage-type sorghums in-season, whether or not to spend money on an application is more difficult.  The challenge with application is two-fold, both return on investment and application efficacy.  Determining the return on investment is difficult as we do not fully understand or know the amount of impact sugarcane aphids can have on tonnage or quality.  Initial indication from Texas A&M has indicated that quality did decrease in forage-type sorghums that were infested by sugarcane aphids but not to the level that was expected.  The second challenge come with application.  As the current chemistries used to manage sugarcane aphids do not have true systemic nature, coverage is critical.  On forage-type sorghums, this can be very challenging, especially later as the crop nears cuttings.  Therefore, control of the upper canopy may be achieved but mid- to lower canopy would only be marginal.

So what should I do?

One of the best management for sugarcane aphids in more maturity forage-sorghums is to harvest.  Cutting and baling will remove or minimize the impact these pests can have on the crop.  However, there has been reports that aphids on the cut and windrowed hay have caused issues in the baling process.  There current is not solid answer for what to do if aphids persist on the cut hay.  Both chemicals have harvest and grazing restrictions that growers will have to adhere.  However, the return on investment and efficacy still come into question and unfortunately there is no solid answer to this currently.

Managing aphids late-season:

Outside of the far west regions, sorghum producers have begun to finish up the growing season, with several fields being harvested prior to recent rains.  However, with increased aphid pressure, questions arise on how to manage a crop that is infested with aphids at or near harvest.  Discussions were had last year that growers could use desiccants to management sugarcane aphids at harvest.  The results on the success of this practice have been highly variable.  A rapid dry-down of the plant may dissuade the pests from those plants and active populations could deplete food stores and die out, resulting in success of this practice.  However, it has been shown that more gradual dry-down of sorghum can result in the aphids moving into the head of the sorghum plant and causing greater harvest issues there.  It should be noted that sugarcane aphids present after black-layer will no longer effect grain yield but may decrease the harvest efficiency and lead to lower yields realized on a per acre basis through increased honeydew.  This should be evaluated on a case-by-case basis.  Low populations down within the canopy of the plant should result in little to not issue at planting and a desiccant may be sufficient to control these pest at harvest.  However, if populations are high, particularly higher in the plant canopy or in the head of the plant, the application of a desiccant and insecticide (Transform® or Sivanto®) may be warranted.  Be cautious as there are harvest restrictions on both of these chemicals.

Overall, this is a good crop in front of much of Oklahoma.  Grower should make sure to take steps to ensure that yields and profits are optimized on each field individually because with insect management, fields can change across the road.  Sugarcane aphids have been a minimal pest up until now throughout the region.  However, now is the time to get out into sorghum fields to be scouting for sugarcane aphids.  For further questions please visit your local Oklahoma County Extension office for additional information and management details.

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