Citrus thrips, a significant, rind-scarring pest of San Joaquin Valley Navel oranges, as well as desert citrus and coastal lemons, can be easily mistaken for flower thrips, which does not cause economic damage.
With that in mind, Beth Grafton-Cardwell, director of the University of California's Lindcove Research and Education Center at Exeter, recently scheduled a field day at the center to help growers and PCAs take a closer look to monitor and distinguish the two tiny insects they encounter in citrus groves.
Joseph Morse, professor of entomology at UC, Riverside, was also on hand with tips on managing citrus thrips, which can vary in populations from year to year.
Citrus thrips eggs, embedded in leaves, hatch in time for new spring growth. The first-instar larvae are very small, and the larger, second instar, whose feeding causes most significant damage, goes to tender foliage and fruit, especially under the sepals of young fruit. Their punctures create scabby, grayish or silvery scars on the rind.
Since flower thrips will leave citrus after petal fall for other plants and is not a concern, Morse said it's important to check and make certain what's there is indeed citrus thrips before treating.
Identification requires a hand lens. Citrus thrips larvae are fast-moving across foliage, particularly in warmer temperatures and direct sunlight. Flower thrips, larger at a given stage, tend to be sluggish and move in an S-shaped path.
While citrus thrips have a shorter body with a thick waist, flower thrips have a longer body, a slight waist, and stout hairs at the tip of the abdomen. Adult-stage citrus thrips are always pale yellow, and adult flower thrips are variable, brown to golden, in color.
“You will see foliar damage from citrus thrips,” said Morse, “but generally we don't consider that of economic importance.” He and Grafton-Cardwell did two studies that concluded that trees not treated for the pest were no smaller in trunk diameter than those that were treated multiple times.
Morse noted that the multiple treatments invite resistance to the materials used. “So, we suggest you withhold treatments on young, non-bearing trees, and wait until later to treat to prevent fruit scarring. The problem we have with citrus thrips is a lack of effective control materials, and the thrips have the ability to develop resistance quite quickly.”
Second-instar citrus thrips tend to move under sepals, and their feeding shows as circular damage as the fruit expands. Rind spots also can be caused by wind abrasion, and deeper scars may be caused by other pests.
Citrus thrips, very responsive to hot weather, have a developmental threshold of 58 F, and development accelerates with warming temperatures.
The first and second instars combined, take about four days at 80 degrees, but only a couple of days at 90 degrees. Given favorable weather conditions, there can be up to eight generations in a year.
“When sampling, look for citrus thrips on the sunnier, southeastern outer parts of your blocks, because that's where they appear first,” Morse said. A typical sample, starting at petal fall and twice a week afterward, would be 100 fruit, with only one or two fruit from any one tree. The first-instar hatch is a good signal to begin sampling. Midday, from 10 a.m. until 2 p.m., is the best time of day for sampling.
“Record the percent infested with immature citrus thrips, differentiating them from flower thrips, and ignore any adults because they aren't responsible for the damage. Once fruit gets to about 1.5 inches in diameter, the rind tends to be tough enough to resist citrus thrips feeding. The idea is to protect the really small fruit,” he added.
Treatment threshold is 5 percent of Navel fruit infested when natural enemies are not present. The threshold is doubled for Valencia, since the thrips don't damage that variety as easily.
While cultural control is helped by wet, cool spring weather that lowers pupal populations, Morse said biological control is difficult, since the predatory mite, Euseius tularensis, does not reduce heavy populations below the economic threshold. Other natural enemies of citrus thrips include spiders, lacewings, dustywings, and minute pirate bugs and field monitoring of them is also helpful.
Stressing that citrus thrips in the SJV has already developed resistance to pyrethroids Baythroid and Danitol, as well as Cygon and Carzol in some cases, Morse recommended generally avoiding broad-spectrum materials where possible.
Among the softer materials, Delegate, available for the first time this year, is similar in chemistry to Success, but in trials shows an edge on Success for efficacy.
“But keep in mind, if you've been using Success for a number of years there is a concern with resistance. Agri-Mek is also fairly effective. Veratran will give some knock-down of citrus thrips during hot weather, but it is really not persistent.” He recommended only one treatment per year with any one of the above four products.
Another tip for treatment is complete outer foliage coverage with lower sprayer wind velocity. “Timing is important, generally right before the hatch, starting from the outside of the block and getting the treatment on quickly, except with Veratran, which is a little more effective when the thrips are at higher numbers.”
Morse also recommended using oil, up to 1 percent, with Delegate, Success, or Agri-Mek to improve persistence. “The key is to rotate as much as possible between materials, and try not to put on treatments unless they are needed. That will both save you money and delay the onset of resistance.”
A new material, Movento, is expected to be registered later this year for citrus thrips in California, but Morse said it also will be intended for red scale control and must be used judiciously to prevent resistance. “You won't want to be spraying in the summer for red scale and then again for citrus thrips the next spring.”
Yet another new material is in the wings, but Morse said it is not likely to be registered until 2012.
He also urged growers to work closely with their packers to learn whether foreign destinations for their citrus have residue issues with any new material.
Growers and PCAs got hands-on experience locating citrus thrips on trees and viewing their various developmental stages under microscopes in the center's mobile laboratory during the field day.