Releasing hordes of sterilized male insects for unfruitful mating with females—a process known as the sterile insect technique (SIT)—is a proven process for combating many species of pests. Its success depends on sterile males dispersing widely enough to outcompete their wild counterparts and mate with enough females to reduce reproduction of a population.
SIT has potential against the navel orangeworm (Amyelois transitella), a moth whose larvae decimates nut crops and, as its name implies, citrus, but scientists still are trying to figure out whether the idea will fly—literally. Researchers in California, where the moth’s larvae wreak havoc on almonds, walnuts, and pistachios, have conducted laboratory experiments that have revealed a potential glitch in dispersal ability that needs to be addressed before SIT can be effectively deployed against it. Their analysis is described in a report published this month in the Journal of Economic Entomology.
Researchers led by University of California-Riverside Ph.D. student Joshua Reger examined the impact of mass-rearing, sterilization, and transportation methods that are part of the sterile insect technique on navel orangeworm (Amyelois transitella), a moth whose larvae decimates nut and citrus crops.
The researchers, led by University of California-Riverside Ph.D. student Joshua Reger, examined the impact on the orangeworm of the same mass-rearing, sterilization, and transportation methods successfully used to combat the pink bollworm (Pectinophora gossypiella), a cotton pest. According to the new study, what works on the bollworm is iffy for the orangeworm, because the modus operandi seems to negatively impact flight performance, the key to dispersal. The researchers concluded that “the data from the current study demonstrate a substantial reduction in flight capacity in navel orangeworm, particularly males reared under the current conditions intended for use in a SIT program.”
The complexity and many facets of the research described in the paper testifies to the fact that using SIT successfully is more than a matter of collecting insects, rearing them, and releasing a big bunch into the target area. SIT demands techniques for mass production of males for release, with output numbering as high as hundreds of millions of individuals weekly. It is a complex process, involving elements that can stress the insects, including irritation, collection with vacuum tubes, and chilling before shipping to the release point.
Considerable tinkering with the method is needed before it can be applied to the orangeworm, says Reger. “The mass-rearing, irradiation, transportation, and release processes present various points of stress on navel orangeworm which can negatively affect flight performance,” he says. “Going forward, we are excited to examine the different factors such as insect cold tolerance, transportation, and field release method as this work continues.”
The study described in the paper, he says, “highlights the need for additional research focused on the key features of the SIT process, but this is in tune with other successful SIT programs. Notably, programs aimed at codling moth and pink bollworm were researched and improved over decades, ultimately meeting their goals of pest suppression and eradication, respectively.”
SIT is an ideal solution for dealing with the orangeworm, which has the unique ability to detoxify certain chemical compounds in the environment, including some pesticides. Even if that were not the case, growers are under pressure to use less toxic pest management tools. Major overseas markets for California almonds, pistachios, and walnuts, such as South Korea and the European Union, have strict regulations for pesticide residues. The Almond Board of California is committed to have growers adopt 25 percent more environmentally friendly pest management tools like mating disruption by 2025.
The tasks required to perform the flight-testing experiments used in the study were not for the fumble fingered. Individual moths—at most 11 millimeters long—had to be chilled then glued to a tiny insect pin at the end of an arm of hypodermic tubing on a device called a “flight mill.” It resembles a miniature carnival ride, with a thin arm of lightweight tubing that spins while mounted atop a post. The moth is counterbalanced by a dab of painter’s putty of the same weight, so each moth in the study had to be individually weighed; most weigh about 7 milligrams. Suspended by magnets and equipped with Teflon bearings, the arm spins so effortlessly it rotates when the moth flies. A sensor records each revolution and thus flight distance.
Reger and his colleagues assessed flight distance in both moths that were mass reared in the millions and those collected by hand and reared locally in much smaller numbers. Four groups of moths were compared. From a United States Department of Agriculture facility in Phoenix, Arizona, came moths that were irradiated and another batch that was not. Another two were non-irradiated moths that were locally reared at a facility in California, from the Phoenix colony, and a colony that originated in Fresno County, California, in 2011. It turned out, says Reger, that “female moths from both strains are capable of flying similar total distances and durations under all conditions.
The farthest females flew was up to almost 12 kilometers, while males went for three to more than seven. Mass reared months had more of what the researchers called “non fliers,’ which displayed less than two minutes of continuous flight. Overall, according to the paper, “In this study, only males appeared to be negatively influenced by the mass-rearing process.” Chilling, which occurs before irradiation and to immobilize moths for attachment to the flight mill, may impact the fitness of males, especially, the study suggests.
As for irradiation, Reger says, “If the irradiation regimen is found to have a large negative impact on insect fitness, there are multiple avenues to explore which may improve it. These include but are not limited to lowering radiation dose, manipulation of atmospheric conditions, changing radiation source, and incorporating radioprotectants into the artificial diet.”
Based on Reger’s assessment, the study did not produce a eureka moment but does bode well for the future of SIT as a weapon against the orangeworm. “With the results of this study,” he says, “our journey to improving and achieving a high-quality and competitive sterile navel orangeworm is just beginning.”
Header photo caption: Releasing millions of sterile insects to interfere with a wild pest population won’t work if irradiating the insects also hinders their flight capacity. A new study shows this may be the case with navel orangeworm moths (Amyelois transitella), and further fine-tuning will be necessary to build a successful sterile insect technique operation to manage the pest. (Photo by Mark Dreiling, Bugwood.org)