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Dr. Lisa Neven is studying the survival of codling moth larvae in tropical conditions.

Dr. Lisa Neven is studying the survival of codling moth larvae in tropical conditions.

There is little risk of codling moth larvae shipped in apples to Taiwan resulting in the pest becoming established in that country, research by Dr. Lisa Neven, insect physiologist at the U.S. Department of Agriculture’s research laboratory in Wapato, Washington, suggests.

Taiwan has a “three strikes” policy designed to protect its own small apple industry from the risk of codling moth coming into the country with imported fruit.  If a codling moth larva is detected when fruit is inspected on arrival in Taiwan, the company that shipped the fruit is generally prohibited from shipping for the rest of the season. After three detections, the U.S. export program to Taiwan is suspended for the season, causing a major trade disruption. Taiwan is Washington’s third largest apple export market after Mexico and Canada.

Neven’s study focused on how a codling moth larva in diapause (the dormant phase before the insect emerges as an adult) could survive in Taiwan’s tropical environment and on the amount of chilling that would be needed for a larva to break diapause and continue its development.

The average temperature in Taiwan is between 68 and 70°F, and the temperature never drops below 55°F, Neven reported to the Washington Tree Fruit Research Commission, which has supported her research. The average day length is 12 hours, and it only varies by 1.5 hours. These conditions are not conducive to codling moth breaking diapause, she found.

No emergence

In two years of experiments, diapausing larvae that were held at a day length of 12 hours and not subjected to either cold storage or a chilling period of at least two weeks at 50°F did not emerge as moths.

Even when larvae were subject to cold storage (for between 2 weeks and 4 months at 33°F), only 0.5 percent emerged as moths, and only 25 percent of those emerged during the six-week emergence period that the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) has used in its risk assessment.

Neven calculates that the risk of any moth emerging from unchilled fruit within that six-week period is only 0.125 percent. She found that female moths tend to emerge first, and males emerge much later. With moths only living for perhaps seven to ten days (based on observations in the Pacific Northwest), the chance of a moth emerging during the six-week period, encountering another moth, and mating to produce a second ­generation is almost zero.

In its current risk assessment, APHIS’s Plant Epidemiology and Risk Analysis Laboratory (PERAL) has assumed that 100 percent of the codling moth going into Taiwan would successfully exit the fruit, go into diapause, and emerge within a six-week period. “We have solid information that this is not going to happen,” said Neven.

She is conducting a third year of experiments with the intention of publishing the results, and will provide the information to APHIS. She believes the risk assessment should be based on biologically and ecologically relevant information.

“I understand that they are trying to be conservative, but I’m a biologist and part of my job is to be able to develop good data for APHIS to use,” she said.

Dr. Mike Willett, vice president for scientific affairs at the Northwest Horticultural Council, said no one has done this kind of research before. It provides good data to ­support the hypothesis that codling moth is unlikely to become established in Taiwan.