Be Prepared to Spray Bollworm in Bt Cotton During 2017
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Bollworms are abundant again during 2017. Many survived the winter an emerged early. This could mean trouble for cotton, since we have the potential to crank out an additional generation, and since most of our cotton is late this year. Furthermore, we are seeing as many, or more breakthroughs in Bt corn this year compared to last. Bollworms that are produced in Bt corn are adapted to survive on Bt cotton. One bright spot is that plant bug pressure has been relatively light compared to previous years. If we can avoid spraying insecticides prior to a bollworm flight, we can preserve some of the natural enemies that help us ward off bollworm infestations. Another bright spot is that we have more cotton acres relative to corn this year. But with close to one million corn acres in the state, there are going to be a lot of moths moving from corn into cotton during July and possibly even persisting into September.
Cotton growers should enter bollworm season with a plan and be prepared to spray Bt cotton if they want to preserve yield. Bollgard II, TwinLink, and WideStrike are most at risk. The few acres planted to Bollgard 3, TwinLink Plus, and WideStrike 3 will only need sprayed under the most extreme pressure situations. There are two strategies that growers can take. While both have benefits and costs, both rely heavily on good scouting for proper implementation.
1) Preventative strategy:
Begin scouting cotton leaves and squares (focusing on bracts) for eggs starting mid-July. You can keep an eye on when flights are happening by using our light trap site. Eggs are small and take a sharp eye to spot. Unfortunately, bollworm and tobacco budworm eggs are difficult to tell apart. This is important because tobacco budworm is 100% controlled by Bt cotton. Although tobacco budworm is more abundant this year than during 2016, to play it safe, assume it’s a bollworm egg. You can also pay attention to moths that you flush in the field. Look for the double chevron on the wings of budworm in contrast to the single dot on bollworm wings.
Eggs can be laid on any plant part and sampling the entire plant can be challenging. Recent research conducted in NC during 2016 found that eggs are laid throughout the plant, but are most common on leaves and squares, especially near blooms. If there are 25 eggs on 100 terminals, stems, or fruit (squares/blooms/bloom tags/bolls), and if stink bugs or plant bugs are not an issue, apply Prevathon. If stink bugs or plant bugs are an issue, apply Besiege (note that this will kill more beneficials compared to Prevathon). Do not use this strategy if eggs have hatched and 2nd instar larvae are present. Also, this strategy will provide the most return for Bollgard II, TwinLink, and WideStrike; expect less return for Bollgard 3, TwinLink Plus, and WideStrike 3 (which are pretty effective for bollworm thanks to the Vip protein).
2) Reactive strategy:
Our tried and true threshold for bollworms in Bt cotton is based on a scouting procedure focused on finding 2nd instar larvae (see the scouting guide). The huge advantage to this strategy is that it allows Bt cotton to do its job by killing all the tobacco budworm larvae that are newly hatched from eggs. The problem with this strategy is that it has the potential to let bollworm get a foothold. Once larvae obtain some size (3rd instar or larger) and move into squares and bolls, they are extremely difficult to control. So, like the preventative strategy, the reactive strategy has benefits and costs as well.
To use this strategy, only apply an insecticide when thresholds are reached (three 2nd stage bollworm (or larger) in 100 fruiting tissues on one scouting trip, two 2nd stage bollworm (or larger) in 100 fruiting tissues on two consecutive scouting trips, or one 2nd stage bollworm (or larger) in 100 fruiting tissues on three consecutive scouting trips). You could use any of the insecticides noted for bollworm control in the 2017 NC Ag Chem Manual (page 82).