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Startling results launch new direction in navel orangeworm study

Chemical ecologists at the University of California, Davis, are changing their navel orangeworm research direction after an elementary school student's science project found that the major agricultural pest prefers pistachios over almonds and walnuts.

Gabriel Leal, an 11-year-old sixth grader at Willet Elementary School, Davis, prefers pistachios over all other nuts so he figured that the navel orangeworm would, too. “Pistachios taste better,” reasoned Gabriel, whose family says he can eat an entire bag of pistachios at one sitting. Pistachios have long been his favorite nut, so why wouldn't the navel orangeworm prefer pistachios over almonds and walnuts, too?

Why not?

So Gabriel hypothesized that the insect would lay more eggs in pistachios than in almonds and walnuts, contrary to widely published research that indicates an almond preference.

“Everybody knows that navel orangeworms prefer almonds,” said his father, Walter Leal, a chemical ecologist and professor of entomology at the University of California, Davis. Research published recently in the California Agriculture journal also indicates the preference.

“But in science,” Leal said, “we should believe what we see, not what others tell us. I know that Gabriel prefers pistachios, but I assumed the navel orangeworm's taste receptors were different.”

Wrong. Gabriel's research showed that the insects preferred pistachios, just like him.

The findings led to a report at the Almond Board of California's 32nd Almond Industry Conference, held Dec. 1-2 in Modesto, and launched a new direction of navel orangeworm chemical ecology research at UC Davis.

Gabriel, a student in Leslie Whiteford's class, performed his research in his father's UC Davis lab, under the volunteer supervision and mentoring of Chemical Ecologist Zain Syed.

“It was a ‘choice’ experiment where Gabriel placed mated and gravid (egg-filled) females in a cage,” Syed said. “He used four commercially available navel orangeworm traps (Ovitraps). One trap was filled with 50 grams of shelled pistachios, another with 50 grams of almonds, and the third with 50 grams of walnuts. The empty trap served as the control to check if the trap itself had any effects on attracting egg-laying moths. The eggs laid in the ovitraps were counted for two consecutive nights.”

“Gabriel got enough replicates to demonstrate that female orange navelworms do prefer pistachios over walnuts and almonds. We are very excited with our little scientist's discovery. I reported ‘our’ findings at the state almond industry conference in Modesto. And these findings changed our research direction, because we are now interested in determining what chemistry in pistachios attracts female navel orangeworms,” said Leal.

“Oviposition (egg-laying) attractants derived from almond oil are used to monitor female populations in the field,” Leal explained, “but during hull split, the chemical from the natural source (crop) competes with the synthetic material in traps. If we use pistachio-derived attractants in the almond field there will be no competition throughout the flight season.”

Research Entomologist Brad Higbee of Paramount Farming Co., Bakersfield, called the boy's research “interesting, provocative and intriguing.”

“It's provocative in the sense that we know little about the natural preference of the navel orangeworm (NOW),” Higbee said. “NOW is a pest that attacks tree crops planted on over one million acres in California and it is the primary and most destructive pest on almonds and pistachios, which represent about 800,000 of those acres. About 152,000 are in pistachios. The economic impact of NOW damage varies from year to year, but it can easily reach $10-15 million for our company and much higher statewide.”

“I find Gabriel's discovery intriguing,” Higbee said, “as it relates to trapping studies we have conducted that suggest that NOW females lay more eggs on traps baited with pistachio kernels compared to almond kernels at certain times of the year in both almond and pistachio orchards.”

Researchers and growers often use egg traps baited with a mixture of almond meal and almond oil to attract the pests.

Higbee said that developmental studies conducted in the Paramount laboratory and in the Joel Siegel lab, USDA-Agricultural Research Service, Parlier, confirm that NOW develops much faster when fed pistachios relative to almonds.

“That suggests,” Higbee said, “that not only do NOW have preferences, but there may be biological advantages to the preferences they exhibit.”

Gabriel initiated the project in September after text-messaging the idea to his father, who was 7,000 miles away. Leal was in his native Brazil to deliver the keynote address at an entomology conference on the mode of action of the mosquito repellent, DEET. Contrary to 50-year-old assumptions, DEET does not mask the smell or jam the senses, the Leal lab discovered in groundbreaking research published in August. Mosquitoes can smell DEET and avoid it because it smells bad.

So while the father was thinking DEET, the son was thinking how neat it would be to do research on an insect that's a fellow fan of pistachios.

“When I received the text message, I thought ‘No way,’” Leal said. “No way would the navel orangeworm prefer pistachios over almonds.”

Syed also thought “No way.”

But they figured that Gabriel, despite a wrong hypothesis, could learn more about the scientific method and about an agricultural pest that wreaks havoc on California nut orchards. “Gabriel was excited about the project, especially when he was counting the number of eggs laid in the pistachios,” said Syed.

“The results,” Syed said, “shocked us.”

The navel orangeworm is a major agricultural pest in California, said Frank Zalom, UC Davis integrated pest management specialist and entomology professor, who has studied the insect for more than 30 years.

“Navel orangeworm (Amyelois transitella) is considered the key insect pest of almonds and pistachios in California because the larvae feed directly on the nut meats making them unsuitable for the marketplace,” Zalom said. “Damage in excess of 2 percent of nuts is considered unacceptable. Navel orangeworm larvae also feed on walnuts, pomegranates and a number of other crops.”

Zalom said almond growers manage the navel orangeworm “through a combination of winter mummy nut removal, rapid nuts removal and pick up after hullsplit, post harvest fumigation. An insecticide spray targeting navel orangeworm is applied at hullsplit to the majority of almond and pistachio acreage.”

The adult moths, about 1 inch long, are gray with black markings, with a snoutlike projection on the head. At two days old, the adult females begin laying eggs.

The take-home message? “Well, in science we should never underestimate anyone's idea,” Leal said. “That's why the academic environment is so enriching: students come with new ideas, but I never imagined we would benefit so much from a science project for elementary school.”

TAGS: Management
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