Hopes that sterile insect technology (SIT) can offer a viable alternative for controlling navel orangeworm (NOW) in tree nut crops appear to have undergone a reversal of fortune as researchers were able to dramatically improve trial results between 2018 and 2019.

“After the initial field trials in 2018, we realized we were facing a lot of challenges with these sterile NOW,” said Houston Wilson, a Cooperative Extension Specialist with the Department of Entomology at University of California Riverside, who is leading the research along with Chuck Burks, a research entomologist at USDA-ARS in Parlier, California. “As we go into 2020, it now feels like we have a lot clearer idea of what specific issues need to be addressed.”

In 2019, the second year of releasing irradiated moths into pistachio orchards, Wilson said researchers documented significantly improved sterile moth recovery rates – a key indicator of moth performance in the field – and eliminated several concerns they initially thought contributed to the low recovery rates of 2018.

“I’d say we are about half-way there,” Wilson said. “At present, our best estimate of the expected recovery rate (for healthy, nonirradiated NOW) is somewhere around 5 percent (of the total number of moths released into an orchard). We weren’t seeing that all the time with the sterile NOW, but we went from essentially zero recovery in 2018 to something in the range of 0.5 to 5 percent in 2019.”

Wilson added that researchers working on sterile moth programs in other parts of the world have said they are happy with recovery rates as low as three percent. “They said, ‘We’d be over the moon with five percent,’” he said. “Granted, that is for a different species of moth than NOW, and we expect each moth’s recovery rate to be different. So, is three-percent or five-percent recovery good enough for NOW? We’ll need more research to have better confidence in the numbers we’re currently seeing.”

Sterile insect technology, which has been around for decades, was first used as a pest control technique in the 1950s to control the New World screwworm, a pest of livestock in the southeastern United States. At its most basic level, the technology sterilizes moths that are mass-produced and then releases them into commercial orchards, where they hopefully mate with wild moths. Wild moths that mate with a sterile moth become infertile, which can lead to an overall reduction, or even eradication, of the target pest population. Mating with sterile moths can reduce populations by either directly blocking reproduction or through inherited sterility. The sterile NOW program in California leads to direct infertility of wild moths.

Discussions about the use of SIT for NOW began about five years ago, when the California pistachio industry partnered with the USDA’s Animal and Plant Health Inspection Service (USDA-APHIS) to explore the idea of utilizing a moth mass-rearing and irradiation facility operated by the USDA near Phoenix, Arizona, which was originally designed for the production of sterile pink bollworms. With the eradication of pink bollworm in 2018, the USDA-APHIS facility had become available for production of another insect, and the California pistachio industry jumped at the opportunity to see if SIT could work for NOW. The almond industry is now also getting involved and investing in the research project.

Since 2018, Wilson and Burks have conducted a series of experiments to investigate several suspected causes of poor performance of the sterile NOW, including the effects of irradiation, the mass-production process, the shipping and handling and release methods.

They found that by tweaking the release mechanism, or how the moths were released into orchards, recovery rates jumped noticeably. Researchers used wing traps baited with pheromone to recover male moths and baited with pistachio/almond meal to recover females.

Further, other experiments showed that the methods of collecting and storing the irradiated moths appeared to have a significant effect on their competitiveness and flight ability.
“Going into 2020, we are hyper-focused on how they are collecting and transporting the moths from the USDA-APHIS mass-rearing and irradiation facility, specifically how they are cooling them, how the moths recover from that chill period, as well as further aspects of the release mechanism,” Wilson said. “Of course, we will also continue to explore improved release mechanisms and generate data on the expected recovery rate using healthy, locally produced, non-irradiated moths.”

He added that the temperatures they used to cool and transport the sterile NOW were originally developed for the pink bollworm program. “And there are a lot of issues when you simply swap out one insect and put another in its place,” he said. “We now realize there is something about the current collection and/or cold storage process that is having a significant negative impact on the sterile NOW.”

Wilson cautioned growers from getting overly excited about using the technology in the immediate future. Typically, he said, developing successful SIT programs for individual pests takes many years, if not decades. The pink bollworm SIT program, part of a larger areawide IPM program to eradicate the cotton pest, took four decades to fine-tune, he said. Canadian researchers worked several decades to develop and implement a sterile codling moth program in use in British Columbia’s Okanagan Valley, which is home to about 6,000 acres of apples.

Wilson said that even in an optimistic scenario, pistachio growers probably won’t be able to commercially utilize sterile NOW for five to ten years, at the earliest.

“In a best-case scenario, where, say this coming year we got everything to work: we got a totally competitive moth; we know how to rear it; we know how to ship it; we know how to release it; it is right there with the wild moth; it is mating; it is flying as far. Even if we were able to get there this year, we still would have to do additional field work to understand the timing and the densities of sterile moths that need to be released into orchards to actually drive down crop damage, and that would take at minimum of two to three years – and again, that is a time estimate where everything goes perfectly well, which it rarely does,” Wilson said.

There are also questions about what size of orchard block is needed for sterile insect technology to be effective, he said.

In British Columbia’s Okanagan Valley, where growers operate under more government control, participation in the SIT program is mandatory. “That obviously won’t happen in California,” Wilson said, “since this is not a quarantine pest.” Instead, growers may need to come together to develop regionwide programs that improve the effectiveness of a SIT program, or like mating disruption, it could be led by private industry.

And there are questions about how the technology will work with existing integrated pest management programs, such as mating disruption.

“If mating disruption is trying to confuse moths from finding each other to mate and SIT is hoping the sterile moths will find and mate with wild moths, how do those two approaches coexist? This is an important question, but there are examples where SIT and mating disruption work together,” Wilson said.

When perfected, researchers envision that the sterile moth technology will be part of a suite of methods for controlling navel orangeworm in pistachios that includes sanitation, mating disruption, monitoring, timely sprays of pesticides and early/timely harvest, Wilson said.

“Optimistically, we’ve made some progress towards getting a more competitive sterile moth,” Wilson said. “Is this going to be the thing that just wipes out navel orangeworm entirely? No. For us, this is one more IPM tool that we are trying to develop and understand how to use in conjunction with the other tools.

Another issue that awaits the team of researchers working on the technology is where to release the moths. “We have 1.5 million acres of tree nuts, so how do you decide where these moths go?” Wilson asked. He added, “The USDA-APHIS facility is currently able to produce about 1 million sterile moths per day, but it may be possible to increase moth production in the future.”

And commercializing the technology to handle large acreages is another hurdle California research has yet to confront.

Wilson pointed out that in Washington state, which has begun utilizing the SIT to help control codling moth in apples, growers are paying a commercial provider to release the sterile moths with drones. “The drones fly over the top of an orchard and drop the moths from a small container,” Wilson said.

Wilson and Burks had mixed results in experimenting with drones last year, and plan to do more experiments with drones this year.

“We are making progress, and we are optimistic about our ability to make some additional gains,” Wilson said. “But whether or not SIT actually plays into a viable strategy is yet to be determined. At present, we are simply trying to develop a process to mass-produce, irradiate, ship and release NOW in a way that allows it to effectively behave similarly to and compete with wild NOW.

“It is still a long road ahead of us,” he said. “These sterile moth programs typically take multiple years to even get to the point where you have a competitive moth, much less be able to use it in a way that actually reduces crop damage, and much less take it to a phase after that in terms of implementation on a statewide basis.

“But it is nice to see that after making some tweaks (in 2019), we were able to get improved performance of the moths,” he said, “and we’re optimistic about our ability to make additional gains in 2020.”