From the Carolinas to Texas, the cotton bollworm is leading an assault on Bt cotton again this year.
Growing resistance to the Bt traits found in the most common cotton varieties is proving problematic as southern states face another large population of the pest, said North Carolina State University Extension Entomologist Dominic Reisig.
“We’re seeing earlier and higher bollworm populations than normal,” he said. “As the season progresses, that means we’ll see more generations of the insect.”
For now, entomologists are trying to get growers reacquainted with how to scout for bollworm eggs and caterpillars in order to make timely insecticide applications.
MULTIPLE BT COTTON TRAITS FAILING
Last year, Texas A&M Extension entomologist David Kerns confirmed that some bollworm populations are resistant to Cry1Ac, the Bt protein found in both Monsanto’s Bollgard II and Dow AgroSciences’ WideStrike cotton varieties.
Now Kerns has also confirmed that some populations tested from Louisiana and Tennessee last year have resistance to Cry2Ab — the second protein found in Bollgard II. Nor can growers lean on the second protein in WideStrike cotton, Cry1F, which has only sublethal effects on the bollworm, Reisig said.
Growers may also see problems with Bayer’s TwinLink cotton, which expresses the proteins Cry1Ab and Cry2Ae, which act similarly to the proteins found in Bollgard II, Reisig said.
Many bollworms might be moving into cotton from corn this year (where they are called the corn earworm). There, they have been exposed to similar Bt proteins and may already be predisposed to survive on Bt cotton, Reisig added.
“Growers can’t just lean back on Bt cotton and expect it to work this year,” he said. “Especially not with the bollworm populations we’re seeing.”
The good news is that Monsanto, Dow and Bayer have all added the Bt protein Cry2Ae, to new cotton varieties called Bollgard 3, WideStrike 3 and TwinLink Plus, respectively.
These products should supply good control of the bollworm, but their availability isn’t as widespread as the older varieties yet, Reisig noted. Bollgard 3, in particular, won’t have a full commercial launch until 2018, although some varieties are being planted on a limited scale this year.
HOW TO SCOUT
Growers have two options for scouting the bollworm — doing a pro-active search for eggs or a secondary search for emerged caterpillars.
The egg method is complicated by the fact that there is no good way to distinguish the eggs of the bollworm from the eggs of the tobacco budworm, which is well-controlled by Bt cotton, Reisig noted. The eggs will be white and tiny — smaller than a pencil point — and laid singly, not in bunches.
Most guides recommend looking on leaves in the terminal of the plant for eggs, but in North Carolina they are also commonly found in the middle of the plants, near blooms, Reisig said.
Reisig recommends spraying if eggs are found on 25% of the terminals scouted. In a university news article, Mississippi State entomologist Angus Catchot recommended Midsouth growers lower that threshold to just 10 to 15% egg infestation on “bloom tags” for WideStrike varieties. Click here to read Catchot’s article.
The second strategy involves scouting for emerged bollworm caterpillars and spraying when they are still quite small and haven’t yet burrowed into a bloom, square or boll.
This approach weeds out any tobacco budworm caterpillars, which will be controlled by the Bt proteins, but it requires being familiar with the size of different larval stages of bollworm. In a university article, Reisig categorizes the thresholds as follows: “Three second-stage bollworm (or larger) in 100 fruiting tissues on one scouting trip, two second-stage bollworm (or larger) in 100 fruiting tissues on two consecutive scouting trips, or one second-stage bollworm (or larger) in 100 fruiting tissues on three consecutive scouting trips).” Click here to read Reisig’s article.
Catchot recommends only diamides insecticides (or high rates of Intrepid Edge) for control of the bollworm in the Mid-South, as pyrethroids are not showing good control of the bollworm there anymore. Even with diamides, don’t expect 100% control if you’re spraying emerged caterpillars, he warned.
In North Carolina, pyrethroids are still among the insecticides recommended for use against bollworm, Reisig said.
Be sure to check for more local thresholds and insecticide recommendations with your state Extension office, as they can vary by state and region.
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