Sterile Insect Technique Shows Promise Against Asian Citrus Psyllid
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Sterile Insect Technique Shows Promise Against Asian Citrus Psyllid

Close-up image of a small insect perched on a thin silvery surface. The insect has a mottled green and brown exoskeleton with delicate wings that show intricate patterns. The background is completely black.
With no known treatment for citrus greening disease, researchers are evaluating myriad control techniques for the insect that spreads it, the Asian citrus psyllid (Diaphorina citri), shown here. A new study shows early evidence that the classic sterile insect technique could be an effective strategy to manage the invasive citrus pest. (Photo by USGS Bee Inventory and Monitoring Lab via Flickr, public domain)

By Andrew Porterfield

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Andrew Porterfield

The Asian citrus psyllid (Diaphorina citri) has been identified on every continent except Antarctica and Australia. It is known for spreading the bacterium Candidatus Liberibacter asiaticus, which causes citrus greening disease (also known as Huanglongbing).

Citrus greening is one of the most serious citrus diseases in the world. Infected trees produce immature (and unmarketable) fruits and die within five to 10 years. In the U.S., it was discovered in Florida in 1998 and is now found in virtually every citrus-producing state (Georgia, Florida, Puerto Rico, Virgin Islands, Alabama, California, South Carolina, and Texas).

No known treatments exist for citrus greening. In places like California, where the psyllid is not yet established in orchards, area-wide applications of insecticides are used. But cost, insecticide resistance, and integrated pest management (IPM) programs are discouraging the use of these agricultural chemicals. So, researchers at the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) and University of California, Riverside, turned to sterile insect technique as a possible alternative way to control D. citri and, in turn, reduce the spread of citrus greening. Their results, published in May in the Journal of Economic Entomology, show that sterilizing X-rays had a significant impact on fertility, survival, and fecundity of the Asian citrus psyllid.

The researchers, led by supervisory agriculturalist Greg Simmons, Ph.D., at USDA APHIS, applied sterile insect technique by rearing psyllids, sterilizing them with X-ray radiation, and releasing them to compete with wild psyllids for mating. Irradiation damages the insects’ chromosomes, leading to the formation of lethal mutations in the DNA of their germ cells, thus rendering them sterile.

In their study at UC Riverside, Simmons and his team used curry tree (Bergera koenigii) plants to host and rear the psyllids. The nymph psyllids were collected and either prepared for radiation or incorporated in their bioassays as non-irradiated partners, or controls. A major part of the study included determining the most effective X-ray dose to achieve acquired sterility in the treated psyllids or inherited sterility in the treated psyllids’ first generation of offspring (referred to as F1).

A close-up view of a circular industrial machine component featuring six labeled compartments arranged in a ring around a central hub. The compartments are numbered 1 through 6 and secured with tape. The metal surface has a shiny, polished finish.
With no known treatment for citrus greening disease, researchers are evaluating myriad control techniques for the insect that spreads it, the Asian citrus psyllid (Diaphorina citri). A new study shows early evidence that the classic sterile insect technique could be an effective strategy to manage the invasive citrus pest. Here, numbered cylinders hold insects for the irradiation process inside the radiation chamber of X-ray equipment used in the study at the Salinas, California, facility of the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, Plant Protection and Quarantine Program. (Photo courtesy of USDA)

To prepare for irradiation, 100 adult psyllids were shipped from Riverside to a USDA-APHIS irradiation facility in Salinas, California. After irradiation at several doses—40, 60, 80 and 100 Grays (Gy), or joules per kilogram— individual irradiated virgin females were paired with a non-irradiated virgin males (10 pairs per radiation treatment level). In addition, virgin irradiated males (at 80, 160, 240, 320, 400 and 480 Gy) were paired with non-irradiated virgin females (again, 10 pairs per radiation dose). Each pair was confined in a test tube, with a curry leaf seedling. Each pair was transferred to a new tube (with a new seedling) every three days and allowed to mate and lay eggs for 15 days. Nymphs that emerged were collected and reared to adulthood, to be used in F1 sterility tests.

Sterile insect irradiation showed a significant negative effect of dose on fecundity. Among pairs with irradiated females, 92 pairs laid eggs, versus 683 eggs from control (unirradiated) pairs. The doses of irradiation also had a negative effect on egg hatching. All control females produced at least one egg, while no females irradiated at 80 or 100 Gy laid viable eggs. Among pairs with non-irradiated females and irradiated males there was a marginal effect on fecundity. However, viable egg hatching was significantly hampered. Among males, 400 Gy was necessary to achieve some sterility (about 95 percent).

Progeny of males exposed to 320 Gy or more were 100 percent sterile.

“We were pleased to confirm that F1, or inherited, sterility exist in this species,” says Simmons. “While it is known to exist in Hemiptera, relatively few studies have been studied to confirm the presence of this phenomena.”

The researchers were also pleased that the psyllids could be collected, irradiated, and shipped without much impact on their survival or performance. “This is a very important finding and suggests that there may be potential for further development of sterile insect technique as a control strategy for Asian citrus psyllid,” Simmons says.

But, could enough irradiated psyllids be raised? Future studies will be needed to answer this question. “The number of sterile insects needed depends on the control strategy employed,” Simmons says. “Having a supply of sterile psyllids ready for release in a newly detected infestation outside of a generally infested area could be effective in reducing the pest’s spread or for local eradication efforts.”

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.


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