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Citrus peel miner continues northward trek

Citrus peel miner continues its northward movement from the desert to California’s central valley. The pest caused major problems first in 1995 in Coachella Valley grapefruit. It’s now as far north as Fresno County, Calif. As its name implies, citrus peel miner mines the peel of citrus, rendering it cosmetically unacceptable for market. As many as 10 to 12 peel miners can inhabit one piece of fruit and one individual can mine up to 20 inches, said David Headrick, assistant professor of crop science at Cal Poly, San Luis Obispo. "This pest has got people really worried. Unlike the glassy-winged sharpshooter, which doesn’t cause economic damage to the (citrus) fruit, this little guy is right there on the fruit and even just a little bit of damage is very noticeable. It gets people really upset," Headrick said. Peel miner damage becomes evident in June or July when the larvae become active on the fruit. This year Bakersfield growers are already seeing signs of peel miners on stems and twigs in the trees. The peel miner population, which generally peaks in August, usually start feeding on fruit inside the tree canopies and work their way out, Headrick said. "As the season progresses, damage becomes more evident on the outside of the trees." Growers wanting to check for damage early in the season should inspect fruit inside the trees’ skirts and turn the fruit all the way around, Headrick said. Citrus peel miner was first identified in 1863 and has been a sporadic problem in Arizona and Southern California citrus for many years. However, in 1995 is when it became a very serious problem, resulting in grapefruit losses of 80 to 90 percent in some Coachella Valley groves. The pest showed up in Bakersfield three years ago and is now in Fresno County, Headrick said. It’s northward trek has already caught the attention of overseas buyers. "Chile has listed peel miner as one of its major concerns. It got some last year, and this year they could put a quarantine on the fruit. We would have to be able to certify certain areas as peel miner free to avoid growers getting quarantined." Dr. Beth Grafton-Cardwell, associate IPM Extension specialist and research entomologist with the university of California at the Kearney Agricultural Center, Parlier, said that the citrus peel miner has always been present in Kern County in small amounts, mostly in grapefruit groves near oleander plantings. But in the last year, the pest has spanned out throughout most of Tulare County, hitting all varieties of citrus, including navels and Valencias. Oleander, willow, tree tobacco, and grapefruit are some of the citrus peel miners’ primary hosts. Industry experts speculate that the peel miner could have been transported to the Central Valley on an infested grapefruit shipment from the Coachella Valley. Most Central Valley orchards only had miner peel miner damage this last season, however, a few Lindsay orchards suffered 70 percent to 80 percent damage, Grafton-Cardwell said. David Haas, a PCA, who works for Helena Chemical Co. in Lindsay, Calif., said that industry-wide, the peel miner has only damaged a relatively small portion of fruit. "What has me concerned is that blocks that had 80 percent damage in 2000, in 1999 only had 1 percent to 2 percent damage." Rather than preferring certain geographical areas, the peel miner seems to gravitate to certain varieties of citrus, including the Melogold grapefruit and Fukimoto and Atwood navels, Haas said. "The pest also prefers younger trees-trees under eight-years old," Haas said. Grafton-Cardwell said the pest may prefer certain varieties, such as the Fukimoto because of their softer, smoother skins. One of the problems in trying to combat the peel miner is that it probably reproduces four to six times a year, Grafton-Cardwell said. This could force growers to spray several times to kill multiple generations of pests, she said. "That’s what we’re trying to avoid. We want to spray as little as possible." In Arizona researcher obtained peel miner control with Alert, Success and Lorsban. Provado and Admire also have been used there. It’s difficult to predict the exact life cycle of the peel miner and that makes spraying a tricky proposition, Headrick said. While most pests pupate after a fixed number of in-stars, the peel miner has up to seven instars, and can pupate any time between the fourth and seventh molt, he explained. "This is a bizarre staggering of the generations, and one of the things that makes it difficult to control. You can’t do one treatment and get all the larvae. You’d have to do at least two or more," Headrick said. If growers spray too many times for the peel miner, it could disrupt IPM techniques in the field, which are used to treat red scale and other pests, Headrick said. Another development that Headrick discovered in the past year, is that peel miners have the capability to overwinter as larvae. After pest populations peak in August, the moths continue laying eggs and larvae keeps developing on the fruit. "We presumed, based on other pests in the central valley, that the frost would knock back the population. That assumption did not hold for peel miner. They continue to develop on the fruit as well as the ones that exit the fruit rind and drop to the soil and pupate in the soil," Headrick said. Headrick and other scientists are working on using biological controls as a strategy to reduce citrus peel populations in the Central Valley. This summer, Headrick and other scientists, plan to introduce a small parasitic wasp from the Coachella Valley — Cirrospilus coachellae — into Central Valley orchards. The parasite has successfully parasitized peel miner in Coachella and there is hope that the wasp will do the same thing in the Central Valley, Headrick said. If the parasite doesn’t work, Headrick may look into using pheromones as another option to combat the peel miner. Growers who are in areas with severe peel miner damage can help themselves to avoid the pest by getting rid of cull fruit in the field at the end of the season so the peel miners don’t have fruit to overwinter in, Haas said. "When we removed fruit, we didn’t just windrow them and shred them, we took them out of the fields and disposed of them and skirted up the trees as much as possible," Hass said. "This is a new deal for us here in the valley, and it’s real frustrating. It’s not like red scale where we know what the pest is and what control measures will work," Haas said.

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