In California, a Warming Climate Will Help a Voracious Pest—and Hurt the State’s Almonds, Walnuts and Pistachios

California almond farmers enjoyed record-breaking harvests over the last five years, after production dipped in the wake of 2014’s historic drought. That year a chorus of headlines vilified almonds for sucking up a gallon of water per nut, though irrigation efficiency has been improving. 

Now, as global temperatures rise, a caterpillar barely the size of a paper clip may threaten California’s position as the world’s leading producer of almonds, walnuts and pistachios.

In a study published in Science of the Total Environment’s February issue (and online in October), researchers from three University of California campuses reported that warmer growing seasons will give the navel orangeworm an extra generation to eat into growers’ profits.

Moths of the navel orangeworm, named for the oranges it was infesting in Arizona a century ago, produce three to four generations of offspring a year. In nut trees, they lay eggs along the furrow of a nut shell, which splits as the fruit ripens. Their larvae grow through five stages, retaining a ghastly translucence while turning reddish-orange to offwhite and causing more and more damage. The moths often deposit spores of aflatoxin-producing fungi when they lay eggs, and caterpillars help the fungi spread as they eat through nuts. Feeding damage can ruin more than 30 percent of a crop, while cancer-causing aflatoxins render uneaten nuts unfit for consumption.

Temperatures in California—including the Central Valley, which produces all the state’s almonds, walnuts and pistachios—have already increased over the last 80 years, a trend that will likely continue. As cold-blooded organisms, insects are sensitive to temperature, which regulates their physiology, behavior, development and reproductive rates. 

Navel orangeworms cost growers millions of dollars in losses each year. Warming temperatures may help the dreaded pest wreak even more havoc in at least two ways, including expanding their range into previously inhospitable areas and accelerating their reproductive rates, boosting their numbers.

But just how global warming will affect insect pests, is likely to vary by geography and biology, an international team of scientists reported last year in Frontiers in Ecology and the Environment. Species need to be considered individually, the scientists warned, to ensure accurate predictions.

Toward that end, the researchers who conducted the recent study relied on models to predict how rising temperatures might affect navel orangeworm reproduction in Central Valley nut orchards. They used several climate models to estimate historic and future changes in the timing and number of the insect’s generations under different warming regimes.

Higher temperatures, they found, allow the nut trees’ arch nemesis to complete its first generation up to six weeks earlier than normal. 

“Climate change will likely make navel orangeworms grow at a much faster rate,” said Tapan Pathak, a UC Cooperative Extension climate change specialist in Merced, who led the study. That means a fifth generation of these voracious pests, which is rare today, could become routine within just two decades as the Central Valley heats up.

Double Trouble

The modeling results predicted that the effects of earlier springs and hotter growing seasons would be most severe in the warmer, southern portions of the valley, where most pistachios are grown. What’s more, the navel orangeworm would be likely to complete its life cycle even faster in pistachios.

A fifth navel orangeworm generation will emerge in almonds and walnuts in three southern counties by 2040 and eight more by 2100, according to the predictions. But in pistachios the fifth generation appeared in seven southern counties by 2040 and 17 counties by 2100. The findings flag the state’s pistachio crop—valued at nearly $2 billion—as the most vulnerable to damage and economic losses. 

It’s not unexpected that these insects are going to develop faster as it gets hotter, said Houston Wilson, an agricultural entomologist and director of the UC Organic Agriculture Institute, who was not involved in the study. “But we need to demonstrate it, show the timing, what it would look like and what would start to happen,” he said. 

“And it’s something that we can show to growers and industry and say, look, if the model’s right, we’re going to have more of these moths than we have now, and they’re already ruining everyone’s life,” Wilson added. “We need to be prepared.”

More generations of orangeworms spell double trouble for growers because nut quality increases over the season, which makes the insects bigger and more destructive. It will be critical for growers to follow recommended integrated pest management guidelines to reduce risks, Pathak said. 

One of the foundational recommendations involves removing remnant “mummy nuts” that harbor overwintering larvae and serve as prime egg-laying real estate. In addition, harvesting crops early will deprive later generations of food and closely monitoring crops can ensure timely use of insecticides, Pathak said.

Another strategy, called “mating disruption,” hijacks the chemical signals males use to find females with synthetic pheromones that throw males off a potential mate’s scent. “It’s a great technology that has been shown to reduce damage by 50 percent,” said Wilson. 

But the technology is expensive and needs to be deployed over a large area to thwart migrants from other orchards. Policy and economic incentives could encourage growers to take the type of collective action needed for the technology to work, Wilson said.

Whenever Wilson talks to growers, he urges them to use every pest control tool at their disposal: sanitation, mating disruption, timely harvest and targeted spraying. “None of those should be considered standalone,” he tells them. “Hit it with everything you’ve got.”