Sterile Insect

(originally published May 2018)


Aiming to suppress burgeoning navel orangeworm (NOW) populations in California pistachio orchards, the pistachio industry is following in the footsteps of the cotton industry and continues to pursue a sterile insect program adding another control tool for growers.

Delivered to a 1,250 acre test plot in western Kern County this spring will be five million sterile navel orangeworm adults. Taking a page from the cotton industry’s successful pink bollworm eradication program, the project is aimed a reducing numbers of NOW early in the season to head off much larger crop damaging NOW populations later.

Sterile Insect Releases

The release plan, said Bob Klein director of the California Pistachio Research Board, is for one million moths per day to be delivered via fixed wing aircraft, to the test plot. The releases will be made in the early morning hours from 500 feet up. Another 1,250 acre block of pistachios will be used as a control. Both blocks will continue to receive standard NOW control with sanitation, mating disruption and pesticide applications.

These releases, Klein said, are the first in a three year program focused on NOW suppression and lowering crop damage percentage and to determine success of the delivery method.

Sterile insect releases are being scheduled to match emergence of native adult NOW moths from overwintering sites. The aim is to end the reproduction cycle for as many NOW adults as possible by releasing sterile NOW when males are seeking females to mate. High numbers of sterile insects are needed to out-compete native NOW.

Klein said the sterile insect releases are believed to work best early in the season when NOW populations are lower. The sterile Insect program is a suppression program, Klein stressed, and if it proves successful, can be another tool for growers to protect their pistachio crops.

Crop Loss

Navel orangeworm (Amyelois transitella) has caused record high crop damage in pistachio nuts for the past two harvest seasons, reaching two percent in 2016 and over two percent last year. From those crops, there were 18 million pounds of pistachio nuts rejected by processors each year. In addition, there were uncounted millions of pounds of damaged nuts left in orchards. Navel orangeworm feeding on pistachios also introduces the toxic mold, aflatoxin, in the nuts, causing loss of export markets.

Loss of pistachio crop volume and processing costs to sort out the damaged nuts, plus market rejection due to aflatoxin are costing pistachio growers and processors $400 million per year, Klein reported at the annual Pistachio Day event. Stories about consumers finding NOW larvae in their bags of pistachio nuts made the rounds on the Internet last year.

NOW Management

Pistachios are not the only nut crop affected by NOW feeding, but their split shells offer an easy feeding opportunity and a depository for NOW eggs. NOW is also a pest in almonds, but after that crop is harvested, NOW readily moves into pistachio orchards to feed and lay eggs in split nuts.

Most pistachio growers have adopted better orchard sanitation practices, are using mating disruption and have chemical control programs in place to combat NOW. Each of those activities adds to total production costs and in the case of chemical control, growers have experienced lower efficacy due to resistance development to many chemicals.

Sterile Insect Research

The Administrative Committee for Pistachios and the California Pistachio Research Board committed funding in 2017 to continue sterile insect research. The first step was to determine if NOW could be raised in sufficient numbers to have an impact. The mass rearing project has been conducted at a United States Department of Agriculture (USDA) operated facility in Phoenix, AZ which was used by the cotton industry for many years to rear pink bollworm for their sterile insect eradication program. With funding, the Pistachio Research Board has been able to keep the facility open while determining if NOW can be mass raised.

Mass raising pink bollworm, sterilizing the adults and releasing them in cotton fields was hugely successful, Klein said, as the facility was able to produce 60 million per day when needed. Mass rearing of NOW has some challenges due to its biology.

Producing high numbers of adult NOW are critical to the feasibility of the sterile release project. Jeff Gibbons with Setton Pistachio said that unlike pink bollworm, NOW fly away after pupating and capturing the one-third inch size moths is difficult. Progress in mass rearing has been made in the last year, however, and the facility can now produce about one million NOW a day. Work at the facility has also included development of a NOW diet, conducting NOW irradiation testing for sterilizing the moth. A new, more fertile colony of NOW has also been established at the facility.

Gibbons, who serves on a technical subcommittee with Western Agricultural Processors Association, said another benefit of testing the efficacy of the sterile insect release would be gaining interest of the almond industry.


Irradiation or sterilization of the NOW moths to be released is the final process prior to release. Klein said the Arizona facility now has the capacity to irradiate only one million moths per day. Scaling up the process will eventually be possible, but would add to the cost. Gibbons also said that additional material for the radiation process would be required. The facility can handle the volume, he said, but costs will increase at higher production numbers.

If this season’s sterile release program proves successful, Gibbons said generating the numbers of sterile NOW to cover all of the state’s producing pistachio acres would be critical to success of the program.


Following release of the sterile NOW, Klein said there would be a trapping program to determine the ratio of released and native NOW. The sterilized moths, he said had a pink dye in their diet prior to release and when they stick to the trap and are squashed, they can be identified by the pink dye. Researchers will also monitor egg laying rate in the almond orchard. Klein noted that the desired result at the end of the season will be a lower percentage of NOW damaged nuts harvested.


A Sterile Insect Program is expected to cost the industry $45 million per year or about $30 per acre for growers. The incentive for grower support are processor bonus programs that pay 40 cents per pound for quality. With yields of 3,000 pounds per acre, the bonus payments can add another $1,200 per acre for a $30 cost.

Relying on chemical controls for NOW suppression will cost growers more in the long run. Navel orangeworm has a propensity to become resistant in fewer generations, making it difficult for chemical companies to develop new insecticides that will be effective.

Roger Isom, president of Western Agricultural Processors Association said support from all commodities affected by navel orangeworm damage would be critical to the success of a sterile insect program. There are still unanswered questions about the impact the sterile releases will have on the native NOW population, he said.

“There has to be total buy in from all involved in fighting this pest. There is a high level of interest from some and some are skeptical, but one thing is for sure: what we are doing now is not enough.”