Bollworm pressure is extremely high this year. Many cotton growers have become used to Vip3A cotton (Bollgard 3, TwinLink Plus, and WideStrike 3) providing excellent bollworm control. As a result, some growers may not be checking fields as closely for bollworm damage as they did in the past. This is the year to start looking again.
Our pheromone traps have been consistently catching high populations. This has been coupled with lots of bollworms early-season in full-season soybean and plenty that passed a generation in corn. Light traps also showed a sustained bollworm flight across much of North Carolina. Even in places where light trap catches trended down, they are popping back up again. With this many moths moving through the system, even a small percentage of surviving larvae could result in damaging levels in some fields.
Mississippi entomologists have also reported bollworms surviving and unexpected injury in a cotton variety that produced Vip3A. Most reports appear to involve one cotton variety, although it is too early to know exactly what might be causing that issue.
This does not mean that Vip3A cotton is failing across the Cotton Belt. Vip3A varieties still provide very good bollworm protection. However, no Bt trait should replace field scouting, especially during a year with unusually high pressure.
What Should Growers Look For?
Scout fields for bollworm larvae and damaged fruit. Pay close attention to blooms and small bolls, where surviving larvae may be easier to find.
Finding eggs or a few small larvae does not mean a field needs to be sprayed. Bollworm larvae must feed on cotton tissue before they receive a dose of the Bt toxin, so some small larvae may be found before they die. More concern is warranted when medium or large larvae are found alive and feeding on bolls. This shows that the larvae have survived long enough to cause damage.
Treat Based on Boll Damage
In North Carolina, treat when 4% of bolls are damaged and live larvae are present. Check bolls from several areas of the field so that the sample represents the entire field. Do not spray based only on moth catches, egg numbers, or reports from other fields. High regional pressure tells us where to look. Scouting tells us whether an individual field needs treatment.
Good insecticide choices include Vantacor, Besiege, and Elevest. Applications are most effective when made soon after a field reaches threshold. Control becomes more difficult after larvae grow larger and move into large bolls.
The message is not that Vip3A cotton has stopped working. The message is that bollworm pressure is unusually high, survival has been reported in Vip cotton elsewhere in the South, and growers should not assume their fields are protected without checking them. Scout now, treat only at threshold, and report unusual survival in Vip3A cotton to your county Extension agent.
