As the 2021 walnut season begins, growers will again assess the best way to monitor for navel orangeworm (NOW) populations in their walnut orchards using attractant traps and lures throughout the season.
Historically, egg traps had been used to monitor NOW in orchards, with pheromone lures becoming commercially available in the mid-2010’s. Egg traps are still a useful tool for setting biofix in the spring, which is critical for accurately running degree day models for NOW, said Houston Wilson, a cooperative extension specialist with UC Riverside. However, since egg traps only work well in the early season, they do not work to monitor late-season populations or prevent crop damage, he said.
“These traps become less attractive to females as new crop nuts develop, which effectively dilute the attractive cue given off by the egg trap bait,” Wilson said. “As such, the ability of egg traps to provide accurate information declines as the season progresses.”
Unlike egg traps, pheromone lures remain attractive all season, attracting male NOW instead of female NOW. Like egg traps, though, lures do not correlate well with crop damage.
Pheromone lures also likely have a larger trapping radius, which Wilson noted is good and bad.
“A large trapping radius means you are likely pulling in distant (immigrant) moths, which doesn't necessarily reflect populations specifically in your orchard,” he said.
Additionally, according to Wilson, mating disruption interferes with pheromone lures and other traps by preventing male NOW from using pheromone cues, which in turn “shuts down” traps and prevents males from finding females or traps.
Current California Walnut Board-funded research is focused on using newer traps − bait bags (Peterson traps) and Phenyl Propionate (PPO) traps – to find the ideal population monitoring tool that provides good detection and damage detection of NOW in walnut orchards.
According to an article on the Walnut Board website, studies have shown that bait bags are more useful than egg traps in capturing mated females and preventing them from hatching and disappearing between monitoring days. PPO traps attract both males and females and capture more moths overall than bait bags, regardless of mating disruption being present.
Wilson, UCCE area IPM advisor Jhalendra Rijal and USDA-ARS research entomologist Chuck Burks are still determining whether pheromone, PPO and bait bag traps differ in their ability to distinguish between local and immigrant NOW.
“Previous studies have demonstrated that some NOW move between orchards,” said Wilson. “Currently, we are investigating the timing and extent of this NOW migration into walnut orchards.”
Wilson said that they have been monitoring moths in walnut orchards that are either mostly surrounded by other walnut orchards or mostly surrounded by almonds.
“We are doing this because we think almond harvest may catalyze movement of NOW between orchards, leading to an influx of moths into walnut orchards that originated in almond,” he said. “Pistachio harvest could trigger a similar influx as well.”
Wilson noted that this dynamic is important to understand as it has implications for how growers and researchers interpret trap catch, time sprays, predict damage risk and use mating disruption. NOW trap data does not correlate well with crop damage, he said, and this may be due in part to the rapid influx of migrant moths between orchards at certain times of the year.