Citrus thrips may seem like an unusual pest of blueberries, but the pest has brought new meaning to the word damage to growers of the small, round bluish fruit in California's Central Valley.

Beginning in 2006, a team led by University of California Cooperative Extension farm advisor David Haviland worked to develop monitoring and treatment guides to help blueberry growers fight the pest.

"Citrus thrips are best known for the scarring damage they cause to navel oranges in the San Joaquin Valley, but with the recent plantings of blueberries, this pest has taken damage to a whole new level," Haviland says. "With the high value of blueberries and potential damage in the thousands of dollars per acre, management of this pest is critical."

Growers typically sprayed for the pest beginning after summer harvest and running through the fall. Not only was the practice costly, but it also could speed resistance to spinosyn-based insecticides, the only products registered for blueberries.

Unlike flower thrips that prefer to feed within blossoms, citrus thrips prefer to feed on new growth. This makes blueberries an excellent host because they produce new, tender growth at the end of their shoots from June through October.

Citrus thrips feeding on blueberry foliage results in a wide range of symptoms including crinkled or misshapen leaves, stem scarring, stem discoloration, shortened internodes and even death of the shoot tip. In some cases, death of the tip causes the buds at the bases of leaves to

begin to grow, giving the shoot the appearance of an upside-down witch's broom.

Haviland and his team showed that an average of 35 thrips on a shoot tip for a one-month period in August caused a 52 percent reduction in the length of new shoots.

But damage didn’t stop there. Blueberry fruit during the spring develops at the tips of the shoots from the previous year. This means that citrus thrips feeding causes reduced growth that results in less fruiting wood, and, therefore, less fruit.

This was confirmed when harvest data revealed that for every 10 citrus thrips per shoot tip during a one-month period in August 2006, there was a 5.3 percent reduction in yield at harvest in 2007. In the field where the research took place, this was the equivalent of an 18.4 percent yield loss in the untreated fruit.

The team is developing an integrated pest management program for citrus thrips. So far, they have developed information on the seasonal biology of citrus thrips in blueberries, a monitoring program and treatment guidelines.

They also are investigating alternatives to insecticides, such as the use of repeated applications of high pressure water to knock immature thrips off of the plants, and the use of Beauverria bassiana, a fungus that attacts the pupal stage of the thrips that reside in the soil.

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