Monday, March 4, 2024

Insect Management in ThryvOn Cotton

⋅ BY DOMINIC REISIG & ANDERS HUSETH ⋅

Bollgard® 3 ThryvOn™ cotton with XtendFlex® has now received full commercialization for 2023 planting. Our programs have worked with the toxin expressed in ThryvOn cotton since 2014, looking at both basic and applied aspects of thrips and tarnished plant bug (also called Lygus bug) control and the potential impact on beneficial insects. In summary, ThryvOn cotton provides excellent control of thrips without additional insecticide inputs and has very little impact on beneficial insect species. However, ThryvOn cotton will require supplemental management with foliar insecticides if tarnished plant bug numbers exceed the economic threshold.

What changes should growers expect with ThryvOn?

Thrips. ThryvOn cotton expresses a Bt toxin that deters adult feeding and egg-laying for our main pest thrips species, tobacco thrips. Because of this, grower should expect to see both adult and larval thrips on ThryvOn cotton, but in greatly reduced numbers compared to non-ThryvOn cotton.

Our current economic threshold for thrips is either an average of 2 immature thrips per plant OR an average of 1 immature thrips per plant for each true leaf from cotyledon to the 4 true leaf stage in non-ThryvOn cotton. Based on extensive field testing, however, we recommend that growers do not use in-furrow or foliar insecticides in combination with ThryvOn cotton for thrips. In field studies, adding insecticidal seed treatments, in-furrow, or foliar insecticides to ThryvOn cotton can reduce thrips numbers and visually improve the look of seedlings, but we have never shown an impact on yield, even under extreme thrips pressure and adverse environmental conditions.

ThryvOn cotton will be packaged with an insecticidal seed treatment (imidacloprid) for resistance management. Experimental results combined with modeling suggest that the rate of resistance to the Bt toxin expressed by ThryvOn cotton can be delayed when paired with an insecticidal seed treatment. Growers should not add additional insecticides to ThryvOn for resistance management beyond what is provided by the company.

Tarnished plant bug. ThryvOn cotton expresses a Bt toxin that kills some portion of the tarnished plant bug population, but is mainly lethal to small nymphs (first and second instar). Like the mechanism for thrips, it serves as a deterrent to plant bug feeding and (probably) egg-laying. Field and laboratory research has shown that this toxin is not highly lethal to adult tarnished plant bugs. As a result, growers should not expect to see a reduction in adult tarnished plant bugs, especially earlier in the season when plant bugs migrate into cotton from other hosts. Furthermore, tarnished plant bugs can also reproduce on ThryvOn cotton. For this reason, we recommend that growers follow the same economic threshold for tarnished plant bugs in both non-ThryvOn and ThryvOn cotton.

Despite no changes to the economic threshold, we have seen several benefits relative to tarnished plant bug in ThryvOn cotton. First, when ThryvOn was tested under heavy pressure (4+ sprays for tarnished plant bug/season in non-ThryvOn cotton), the economic threshold was not exceeded as frequently in non-ThryvOn cotton. On average, this technology should save growers one to two insecticide sprays per season in heavy pressure scenarios. A second knock-on effect from reducing the number of sprays is that beneficial insects can be preserved, resulting in bio-residual that will keep other pests down, including spider mites, bollworm, cotton aphids, etc.

Thirdly, North Carolina cotton producers are often limited by weather more often than other places in the Cotton Belt, experiencing a shorter season due to our northern location and tropical weather systems during the growing and harvest season. This can make it difficult to treat tarnished plant bugs directly from weather delays or they can “nickel and dime” growers throughout the season even under lower pressure scenarios. Tarnished plant bugs not only rob yield by causing square abscission and internal damage of small bolls, but they can delay maturity. Too often North Carolina growers that try to manage a top crop are disappointed because late-season conditions are not conducive to bolls development. We have seen ThryvOn cotton provide protection that should help (but not eliminate) some of these issues that North Carolina cotton producers face managing tarnished plant bugs due to weather.

In summary, ThryvOn cotton provides excellent protection from thrips and fair supplementary control of tarnished plant bug with little impacts to beneficial insect populations. Key Take Points No additional in-furrow or foliar insecticide needed for thrips on ThyrvOn cotton Use the standard economic threshold system for tarnished plant bugs Continue scouting the crop to ensure plant bugs and other pests (stink bugs) do not exceed economic thresholds As with every change to the cotton system, there will be unpredictable outcomes as more ThryvOn cotton is planted. We will continue to work to provide timely updates as the system changes.

Read more at: https://cotton.ces.ncsu.edu/2023/02/insect-management-in-thryvon-cotton/


Dominic Reisig is Professor and Extension Specialist at NC State University. He may be reached at dominic_reisig@ncsu.edu
Anders Huseth is Assistant Professor and Extension Specialist at NC State University. He may be reached at ashuseth@ncsu.edu.

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