Sorghum Tips

Grain Sorghum Hybrid Thresholds to Sugarcane Aphid—No Hybrid Specificity

This tip was provided by:

Calvin Trostle, Extension Agronomy, Lubbock, 806-746-6101, ctrostle@ag.tamu.edu

Statewide

Grain Sorghum Hybrid Thresholds to Sugarcane Aphid—No Hybrid Specificity

In the Feb. 23 issue of TGSA’s “Sorghum Insider” Texas A&M AgriLife Extension entomologists discussed potential options for varying the aphid count on specific hybrids to determine control thresholds. This included raising the threshold to as high as 200 aphids per leaf for grain sorghum hybrids deemed resistant to SCA. This is in contrast to a base level of 50 or 100 aphids per leaf for susceptible lines.

Subsequent grain Sorghum Tips have included additional information pertaining to this option. That guidance was not yet viewed, however, as part of AgriLife Extension entomology’s recommendations.

Recently, Extension entomology decided not to advance any suggestion about differing thresholds to trigger SCA sprays (further research may be conducted). Dr. Allen Knutson, Extension entomologist, Dallas, (972.952.9222, Allen.Knutson@ag.tamu.edu) commented.  “We have decided not to promote different thresholds for resistant and susceptible hybrids. Populations of SCA can grow quickly from 50 aphids per leaf on one day to 100 per leaf the next. So fine-tuning a threshold for resistant hybrids is probably not very practical. Also, Extension entomologists on the Texas High Plains do not rely solely on aphid numbers, so this suggested revised threshold using differing SCA numbers is not applicable. Finally, there is concern that some resistant hybrids do not perform as well in the High Plains as they do in south Texas.”

For these reasons, Dr. Knutson notes, AgriLife Extension is revising their forthcoming report and SCA control guidelines.

Also, AgriLife Extension entomologist Dr. Pat Porter, Lubbock (806.746.6101, pporter@ag.tamu.edu) notes that “guidance” (as discussed by AgriLife entomology staff particularly in South Texas) does not mean it has been accepted for our official publications. That is why our new guide will not address thresholds for resistant hybrids. AgriLife Extension now has two SCA treatment thresholds for grain sorghum hybrids. These treat susceptible and resistant hybrids identically. One guideline emphasizes the proportion of plants infested and plus the number of aphids in colonies on leaves. A second guideline will emphasize counting aphids. The former threshold is widely applicable. The latter is likely more applicable in a smaller zone in South Texas. The forthcoming guide will present both approaches, with note of geographic applicability. Readers will be invited to choose which one they prefer.

Another change Dr. Porter notes is the former High Plains SCA threshold has been tweaked for 2018.  The action level for pre-boot stage is slightly less conservative (lower number of SCA per leaf) than in the past

We will share the updated entomology information in Sorghum Tips as it becomes available.