As we have seen in previous years from monitoring corn, Bt resistance is on the rise. They also are more numerous than 2018 in corn and I expect our major July-August flight to arrive a bit earlier than usual. Bollworms that are produced in Bt corn are adapted to survive on Bt cotton. What we have done in cotton prior to this point in the season may determine how many bollworms we have to contend with. If we can avoid spraying insecticides prior to a bollworm flight (except selective ones), we can preserve some of the natural enemies that help us ward off bollworm infestations.
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. Bollgard 3, TwinLink Plus, and WideStrike 3 varieties will only need to be sprayed under the most extreme pressure situations. There are two strategies that growers should take. While both have benefits and costs, both rely heavily on good scouting for proper implementation.
1) Preventative strategy for Bollgard II, TwinLink, Widestrike (note this is the NC State Extension recommendation for these varieties):
Begin scouting cotton terminals and leaves starting mid-July. Note that recent North Carolina research by Dr. Lewis Braswell suggests that most eggs are laid in these locations. If you scout there, rest assured you aren’t missing anything. Don’t neglect scouting leaves in the canopy where blooms are present. Bollworm moths lay eggs in these areas, following blooms up the plant throughout the season.
Eggs laid on Bt cotton during 2016. Solid black line represents the top of the plant.
Eggs laid on Bt cotton during 2017. Solid black line represents the top of the plant. Photo of the cotton bloom represents the location of blooms within the canopy during the two major ovipositional (egg-lay) events.
Corn earworm moth left. Tobacco budworm moth right. Bollworm egg on leaf. Photo credit Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
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. 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.
If there are 25 eggs on 100 terminals or leaves (search multiple plants), 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. This strategy will provide the most return for Bollgard II, TwinLink, and WideStrike; expect no return or to lose money if you use this strategy for Bollgard 3, TwinLink Plus, and WideStrike 3 (which are pretty effective for bollworm thanks to the Vip protein).
2) Reactive strategy for Bollgard 3, TwinLink Plus and Widestrike 3 (note this is the NC State Extension recommendation for these varieties):
Our tried and true threshold for bollworms in Bt cotton is based on a scouting procedure focused on finding 2nd instar larvae (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. However, replicated research in North Carolina under heavy bollworm pressure has not shown a yield or economic benefit to treating Bollgard 3, TwinLink Plus and Widestrike 3 at the egg threshold mentioned in the preventative strategy above (#1).
In other words, you don’t need to spray Bollgard 3, TwinLink Plus and Widestrike 3 at egg threshold with Prevathon.
To use the reactive strategy, only apply an insecticide when thresholds are reached (three 2nd stage bollworm (or larger) in 100 squares, blooms or bolls on one scouting trip, two 2nd stage bollworm (or larger) in 100 squares, blooms or bolls on two consecutive scouting trips, or one 2nd stage bollworm (or larger) in 100 squares, blooms or bolls on three consecutive scouting trips). You can use any of the insecticides noted for bollworm control in the 2019 NC Ag Chem Manual
(pages 80-84). Below result from a spray trial for larvae in WideStrike cotton.
Percent damaged bolls 7-9 days after treatment- WideStrike- in Plymouth, NC, 2018. Red line represents 6% damaged boll threshold used in some states (not North Carolina).