June 7, 2008
Concerned that citrus thrips are becoming resistant to the few control options available to San Joaquin Valley growers, Joe Morse, University of California, Riverside entomologist, offered a resistance management strategy for this troublesome pest during a spring citrus seminar in Tulare.
Citrus growers should avoiding treating for citrus thrips on young trees, monitor carefully for pest populations, and time applications when they do treat to target thrips early in the pest's life cycle to get the most out of citrus thrips control sprays.
An economic pest in San Joaquin Valley citrus for nearly 100 years, citrus thrips have a long history of establishing resistance to insecticides.
“We have a very real history of resistance in insecticides when managing citrus thrips and there are very few classes of chemistry available,” Morse said. “We just don't have as many chemistries as we'd like to have.
Research at the UC Lindcove Field Station in the 1980s provided support for the use of Baythroid to replace Cygon and Carzol, which had both sequentially developed resistance in citrus thrips. When Baythroid resistance appeared in the late 1990s, Lindcove research again supported registration for abamectin (Agri-Mek) and spinosad (Success).
In recent years, Lindcove has been evaluating reports of resistance to Success and cross-resistance to Agri-Mek. While the two are technically listed in different classes of chemistry, Morse said there appears to be some cross-resistance between the two. The latest product registered to control thrips, Delegate, is also in the same chemistry class as Success.
Resistance is also showing up in some areas of the Valley to more broad-spectrum materials such as pyrethroids and neonicotinoids, and they tend to disrupt natural predation in the orchard. Morse noted that applications of broad-spectrum chemistry often flare citrus thrips populations within weeks after spraying.
“If you don't time OPs right you seem to get the thrips angry, and they come back with a vengeance,” he said.
Unfortunately, there are very few biological control options that are effective against citrus thrips. Parasites do not attack thrips on citrus. And while several natural enemies, including spiders and lacewings, can help reduce citrus thrips numbers, pest populations commonly exceed predation levels.
As a result, growers must practice resistance management in treating citrus thrips to preserve what few control options are available. Researchers continue to study new materials at Lindcove in an effort to make additional classes of selective chemistry available to growers and help them integrate new products into a resistance management program.
In the meantime, monitoring remains extremely important to be sure treatment thresholds have been reached. Insecticide applications should be made judiciously and timed to the most vulnerable life stages of the pest.
Morse suggested growers rely on selective materials and rotate between different classes of chemistry where possible. Delegate, Success and abamectin (Agri-Mek or one of many generic formulations on the market) should be grouped together as a single class and rotated with another selective product such as Veratran D or the newer insecticide Movento.
Insecticide applications should be timed early to manage the first and second in-star stages. Second in-stars cause the most damage to fruit because they concentrate their feeding under the sepal and lead to ring scarring. Young fruit is also the most vulnerable, and once fruit reaches about 1 inch in diameter rinds are able to fairly resist economic damage from fruit scarring.
Growers might also consider variety susceptibility before making the decision to treat. Morse's recent long-term study on fruit damage caused by citrus thrips on 16 different varieties showed that Navel oranges have the highest level of citrus thrips scarring, followed by tangerines and some grapefruit. Atwood, Tulegold, and Bonanza navels are the most susceptible of the common navel varieties.
Morse said growers can also improve the efficacy of their sprays by adding oil to Success, Delegate or abamectin and suggested targeting spray patterns to the outside of the tree canopy.
“If you do decide to treat for citrus thrips be sure to use outside coverage. Citrus thrips like the outside of the canopy near the sun and that's where materials will be effective. So cut back on the wind velocity of the sprayer,” he said.
Morse said growers should refrain from treating citrus thrips in young non-bearing trees because damage to young foliage, while unsightly, will not impact tree growth and productivity or lead to economic damage.
“In our trials we could not see economic differences between treating and not treating, so I suggest you not treat because if you do, you will have real resistance potential develop in the future,” Morse said.
Growers should also take care to determine for certain that scarring is in fact attributed to citrus thrips. Citrus thrips can be distinguished from other scarring causes such as rubbing by the distinct ring scar. Lepidoptera can also cause similar ring scarring, but worms tend to burrow deeper into the sepal area.
Citrus thrips populations vary widely from year to year and require that growers and pest control advisers (PCAs) monitor for their presence using a hand lens to distinguish them from other related insects, such as the Western flower thrips. Flower thrips are typically found feeding on nectar where flowers dropped. Citrus thrips don't feed near flowers and typically do not begin feeding until after petal fall.
Citrus thrips are more active than flower thrips and move in a stiff pattern, while flower thrips move more slowly in an S-shaped pattern. Growers and PCAs can also distinguish in-star citrus thrips by its yellowish color and by an abdomen that is rounder and shorter than the cigar-shaped thorax of the Western flower thrips.
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