Our High Plains economic thresholds are valid
The treatment thresholds in place at the start of 2016 proved to be effective in preventing economic losses when insecticides were applied in a timely manner with good coverage at the appropriate rate. These thresholds were adopted from work originally done at LSU and reflect a higher damage potential the earlier in plant development that the aphids exceed threshold.
2017 Treatment thresholds:
- Pre-boot: 20% of plants with aphids.
- Boot: 20% of plants infested with 50 aphids per leaf.
- Flowering to Milk*: 30% of plants infested with 50 aphids per leaf.
- Soft dough through dough*: 30% of plants infested, localized areas with heavy honeydew, and established aphid colonies.
- Black layer*: Heavy honeydew and established aphid colonies with treatment only for preventing harvest problems.
(* Thresholds that are used successfully in the field but have not yet been validated by research.)
Control is effective if coverage is optimal, solid rates are used, and beneficials are protected
On-farm control of SCA was often achieved in 2016 thanks primarily to prompt application at threshold and a willingness to use solid rates of insecticide with plenty of carrier volume to achieve at least 5 GPA by aerial application or 15 GPA by ground application. Many of the control failures could be traced back to lack of coverage, rate, or letting the aphids become too numerous before applying insecticide.
Another major source of SCA control failure involved elimination of beneficial insects. In some cases this was done by tank mixing a hard insecticide like a pyrethroid, Lorsban or Dimethoate with either Transform or Sivanto, the two dedicated and effective aphid insecticides which were usually present in the tank at lower rates than if they had been used alone.
These hard insecticides killed the beneficial insects and allowed a quick resurgence of aphids in the field. The use of hard insecticides for other non-SCA pests like headworms or midge also resulted in a SCA resurgence.
Dr. Ed Bynum conducted a 2016 insecticide efficacy trial at Bushland with Transform, Sivanto and two non-labeled insecticides. In this trial he had excellent spray coverage from a backpack sprayer, a situation not likely to be encountered in a commercial application but which allowed direct comparison of insecticides when coverage was not an issue.
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The two registered SCA insecticides, Transform and Sivanto, provided equivalent initial control of the aphid. Residual control with Transform began to wane after eight days. The extreme rebound of aphids in the Transform plots is likely due in part to this being a small plot experiment; there were many aphids on nearby untreated plants that could infest the Transform plots after residual activity ended.
There is some discussion of whether Transform and Sivanto can be applied at less than 1.5 and 5.0 oz., respectively. Of course they can, and if the application is made at threshold and not too late, coverage is good and it is not too cold, then good control could be expected.
However, using a lower rate reduces the margin for error, the buffer against things being less than perfect. For that reason we are still going to suggest that 1.5 oz. of Transform and 5.0 oz. of Sivanto are the best rates for aerial application.
Yield losses if control is not achieved
In several of our 2016 research trials we had SCA build to high numbers at and after flowering and we lost almost 100% of the yield (not including subsequent lodging). Experience has taught us that High Plains SCA are capable of more damage at later growth stages than elsewhere in the country.
The reason(s) for higher potential damage on the High Plains are unclear, but our data from Lubbock, Halfway and Bushland suggest that sorghum grown under moderate drought stress may be more prone to yield loss than a more fully irrigated crop.
We conducted three experiments in 2016 that resulted in different amounts of leaf damage in plots. These experiments either had aphid numbers intentionally manipulated by various timings of different insecticides (Lubbock), or they were the result of different insecticide efficacy in our control trials (Halfway and Bushland).
At various times after flowering we rated leaf damage on a 0 – 10 scale. Zero indicates no aphid damage. A damage rating of 1 indicates the lower 10% of the canopy has damaged leaves. A rating of 5 indicates the lower 50% of the canopy has leaf damage, and a rating of 10 indicates that all of the leaves are damaged.
We harvested the plots, threshed the panicles and determined yield adjusted to 15.5% moisture. Yields were then regressed against the leaf damage rating taken at harvest.
Both Lubbock and Bushland experiments were conducted on sugarcane aphid susceptible sorghum grown under furrow irrigation. The Lubbock trial was pre-watered and received two supplemental irrigations during the season. The Bushland trial was pre-watered and then rain-fed for the rest of the season, and there was not much rain before early September.
Yield vs. Damage Rating at Lubbock, 2016, Kesheimer and Porter.
The Lubbock experiment lost 317 pounds of yield for each additional level of leaf damage incurred. For example, if the plants had been re-treated to stop aphid activity at a leaf damage rating of 2, the yield would have been approximately 4,700 lbs. per acre.
However, if the aphids were not controlled until they caused a leaf damage rating of 7, the final yield would have been only 3,115 pounds. (5 more levels of leaf damage x 317 pounds/damage rating = -1,585 lbs. of additional yield loss, or a yield of 4,700 – 1,585 = 3,115 pounds.)
Yield reduction at Ed Bynum’s experiment in Bushland was 409 lbs. per additional level of leaf damage rating. The Bushland field was less well irrigated than the Lubbock field.
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Our third experiment, conducted by Blayne Reed at Halfway, used a “resistant” sorghum hybrid grown under abundant drip irrigation. In this case yield loss was only 155 lbs. per additional level of leaf damage rating, but whether this lower yield loss is due to abundant irrigation, a “resistant” hybrid, or some interaction of the two factors is unclear.
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Second application treatment threshold
It is not economically feasible to make follow-on insecticide applications at the relatively low first application threshold, but our 2015 and 2016 data strongly suggest that a second application should be timed in order to protect the top 50 – 60% of the canopy. This second application should be made as quickly as possible; Ed Bynum’s 2016 Bushland experiment went from an average leaf damage rating of 3 to an average of 6 in just 5 days.
