Every spring, California farmers brace themselves for signs of wriggling organisms destined to launch multigenerational attacks on their crops. 

Many insect species survive the winter as eggs or larvae and then emerge in early spring as the first generation to feed and breed on millions of acres of California vineyards, orchards and row crops. Climate change will complicate farmers’ efforts to control these pests in complex and unpredictable ways. 

The most alarming consequence is apt to be ramping up pesticide applications, with broad implications for the safety of California’s waterways—just as the state gears up for a future filled with drought.

Temperature strongly influences insect growth, development and reproduction, while carbon dioxide can affect insect feeding behavior. Higher temperatures will allow some insects to mature faster, helping them fit in extra generations and spend more time flying around fields, reproducing and feeding on crops. They include the moth that can destroy nearly a third of an almond or pistachio crop as larvae. Higher carbon dioxide levels can boost the growth of crops, only to give their primary pests more to eat.  

Such climate-driven changes may force farmers, faced with a boom of insects capable of doing more damage throughout the season, to spray more pesticides. And that could trigger a series of events that send more pesticides into sensitive aquatic ecosystems.

California has always cycled through droughts and storms, with both expected to become more frequent and severe under climate change. Heavy rains will saturate soils and aid the flow of sediments and chemicals from fields. 

“Eighty percent of the chemical load that gets into our stream network occurs during the 20 percent highest intensity rain events of the year,” said Linda Lee, an agronomy professor at Purdue University. Yet management practices can’t cope with the heavy downpours that account for most of the runoff, she said.

Moreover, when the ground becomes saturated, carbon dioxide can build up in soil and displace the microbial communities that break down pesticides. “Once you get drainage, everybody’s happy again,” said Lee. “But if climate change is causing more intense and more frequent intense rain events, then we could have a problem.”

Lee, though, is less concerned about carbon dioxide buildup in soil than she is about the effects of heavy storms on agricultural runoff. Most worrying are the storms that happen soon after a pesticide application. “That’s going to shove that pesticide into our aquatic system,” Lee said.

A spike in insect populations can lead to a jump in crop diseases, which could also lead to more chemical applications, said Jason Rohr, chairman of the Department of Biological Sciences at the University of Notre Dame.

Many infectious plant diseases are spread by insects, he said, raising concerns that as insect populations expand with the changing climate, so will the diseases they spread among crops.

That means climate change could deliver a twofold blow to agricultural ecosystems: first by increasing populations of harmful insects, triggering more pesticide use, and then by enhancing the movement of pesticides through the watershed, exposing threatened native fish and other aquatic species to higher concentrations of toxic agricultural chemicals.

Regrettable Substitutes

California pesticide applications reached nearly 210 million pounds in 2018, a few million more pounds than the previous year, according to the most recent state report. Insecticide use, however, jumped 8 percent during that time and more than 47 percent between 2010 and 2018, from about 31 million pounds to more than 46 million pounds. 

The San Joaquin Valley, one of the nation’s leading agricultural producers, led the state in pesticide use in 2018, with just four counties applying nearly 98 million pounds of pesticides. 

Pesticides applied to these fields can contaminate surface waters that feed into the Sacramento-San Joaquin Delta. More than a decade ago, scientists flagged the presence of toxic organophosphate insecticides as potential contributors to precipitous declines of native fish and their prey in the delta.

Pesticides were among the chemical contaminants scientists blamed for global declines of fish and other aquatic organisms in a report called Aquatic Pollutants in Oceans and Fisheries, released last month by the International Pollutants Elimination Network, a global nonprofit, and the National Toxics Network, based in Australia. 

Pesticides kill the invertebrates that sustain fish and poison the waterways where fish breed and spawn, warned the authors of the report, a synthesis of research documenting the impacts of chemical pollutants on the aquatic food chain, which was not peer reviewed.

As evidence grew that organophosphates—once the most widely used insecticides in California—had reached harmful levels in Central Valley rivers and estuaries by the 1990s, growers began to rely more on a class of pesticides called pyrethroids that were at the time assumed to be less toxic. 

Starting in 2011, applications of two pyrethroids, permethrin and bifenthrin, increased by about 50 percent in Monterey County, the nation’s “salad bowl,” scientists reported in the January issue of the Journal of Environmental Management. At the same time, the researchers found a marked increase in the number and frequency of surface water samples with levels of the chemicals high enough to be toxic to fish and aquatic insects.

Pyrethroids have relatively low toxicity to mammals, including humans, compared to organophosphates, so they became the “go to” chemical, said John Steggall, a coauthor on the paper and research associate at the University of California’s  Division of Land, Air and Water Resources in Davis. 

For a long time, no one knew their effects or even how to measure them, Steggall said. “Once they figured out how to measure this stuff, they started finding it all over the place.”

As often happens with pesticides, the longer they’re used, the more negative effects emerge. “And that’s certainly the case with pyrethroids,” he said. “It turns out that they’ve actually caused a lot of problems, and are especially toxic to invertebrates.”

Pyrethroids tend to stick to soil and stay put unless there’s enough rain or irrigation water to carry both soil particles and pyrethroids from fields. “But they also have a fairly long half-life,” said Steggall. “So if you spray in the fall and a few months later it rains, there will still be enough in the soil to wash into surface waters.”

More frequent and severe rains could send big pulses of all sorts of pesticides into rivers and streams, he said: insecticides, herbicides, fungicides, whatever farmers applied to crops.

One insecticide that appears to have little effect on aquatic species that are not targets is malathion—the chemical former Gov. Jerry Brown enlisted in widespread aerial spraying to combat a severe medfly infestation in the early 1980s.

In a series of lab experiments, as well as field work in Africa, Notre Dame’s Rohr has shown that malathion poses very low risk to crayfish, water bugs and prawns, important members of aquatic food webs. 

The results suggest that there are some chemicals within the classes of agricultural compounds that can help farmers control pests without poisoning local watersheds, Rohr said.

It’s also possible to curtail toxic runoff by creating buffer zones that restrict pesticide applications near sensitive waterways. But in Monterey, at least, significant improvements to water quality would require buffer zones of about one to two miles, according to the recent study of pyrethroid contamination. And that would require an end to most current pyrethroid applications.

“Growers always hate to take any land out of production because it’s a direct hit to their bottom line,” said Steggall. 

More palatable approaches might include installing drip lines or micro-sprinklers, which reduce irrigation runoff, planting cover crops to reduce sediment runoff or installing catchment ponds or vegetative “filter” strips between fields and surface waters.

“Or,” Steggall said, “they could potentially go with less toxic pesticides.”