The landscape for diamondback moths (DBM) is evolving, and the pest is taking a bigger bite out of growers’ profitability. Thanks in part to the increased planting of long-season cole crops, which require a four- or five- month growing season, and the emergence of overlapping generations of DBM, management of this pest has become increasingly difficult.
The 2019 season was particularly tough on Brussels sprout growers, who experienced elevated DBM populations and damage from the pest – broccoli and cauliflower crops were also significantly impacted. At the season’s population peak in August, Alejandro Del Pozo-Valdivia, entomologist and area IPM advisor for the University of California Cooperative Extension, was observing 40-60 adult moths a day in his traps across the Salinas Valley.
Del Pozo-Valdivia says that the cause of this population boom can be traced to two factors: overlapping generations and lack of DBM overwintering.
“In theory, we expect an overwintering generation in December and January as the temperatures drop really low. We don’t expect to see any moths flying, but that’s not the case in the Salinas Valley,” Del Pozo-Valdivia says. “Based on our trapping since February last year (2019), we know moths are always flying and that means we’re having overlapping generations. If you go in and scout, you’re going to have caterpillars, early instars, mid and late instars and adults. You’ll have this mixture of stages, and that is really problematic.”
"Even with the rains and changes in temperature, the populations are holding up in the landscape... we’re expecting to have 40- 60 adults per trap in August 2020."
He says growers can expect 2020 DBM populations to be a repeat of 2019. Looking at his trapping results collected from February through April 13 of 2019 and 2020, moth numbers were similar in terms of adults caught per trap per day. “That tells me, even with the rains and changes in temperature, the populations are holding up in the landscape,” Del Pozo-Valdivia states. “Based on those numbers, we’re expecting to have 40- 60 adults per trap in August 2020.”
Proactive, preventative, season-long control crucial in 2020
Every stage in a cole crop’s life is critical and is threatened by DBM. To ensure season-long control that protects from planting through harvest, Del Pozo-Valdivia advises growers to be as proactive as possible and points to these difference makers:
Know the history of your field. Avoid putting Brussels sprouts in areas that were hot spots for DBM last year, especially in August.
Weed and alternative host management
Keep an eye on weed control. Non-crop areas will be the source for DBM populations coming in and out. Manage these areas. “You underestimate the power of this creature (DBM) if you take for granted a non-crop area and leave it with weeds,” Del Pozo-Valdivia says. “In addition, don’t leave leftover plant material for the DBM to lay its eggs. As soon you harvest your cole crop, kill it, cultivate it, plow it under.”
Scouting is key to timing management practices, especially if you’re in a hot spot and you know the moths are flying. “I can’t stress it enough. You have to be walking the fields every three to five days. Seven to 10 days could be too late. You need to know where you are in these overlapping generations for timely treatment,” he adds.
In places where DBM are prevalent, multiple insecticide applications are usually needed to keep the pest from causing unacceptable crop damage. But, due to its rapid reproduction and exposure to insecticides, DBM is prone to developing insecticide resistance. “You don’t want to use the same mode of action back to back on DBM. Within the rotation, you have to use all the tools you have,” notes Del Pozo-Valdivia. “Diamides are great partners for the insecticide rotation. Cyantraniliprole, like Exirel® insect control, is a really good one.” When used accordingly in the rotation, Exirel insect control with its unique cross spectrum and rapid protection is a critical addition to management of this highly prolific pest.
With the Central Coast and Salinas Valley continuing to plant more long-season cole crops to meet consumer demand, the behavior of the DBM has changed, creating an environment that fosters high populations. Staying on top of management practices with a proactive and preventative approach for season-long control will pay dividends come harvest and for seasons down the road.
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