With the advent of heat-tolerant blueberries, growers have established a California blueberry industry. But, along with the new opportunities offered by this crop, comes a new struggle — citrus thrips.
Citrus thrips have expanded their host range to become a major pest of blueberries. Help is on the way, though, from a team of researchers funded by the University of California Statewide Integrated Pest Management Program (UC IPM).
Adult citrus thrips are small, orange-yellow insects with fringed wings. During spring and summer, females lay about 25 eggs in new leaf tissue. Both adults and immature thrips feed on and damage newly developing leaves and stems. In fall, overwintering eggs are laid mostly in the last growth flush of the season. Citrus thrips also are prolific and can produce up to eight generations during the year if the weather is favorable.
UC IPM Farm Advisor David Haviland is looking at nonchemical ways to keep the thrips from spreading. Haviland, UC Riverside entomologist and citrus thrips expert Joseph Morse, and blueberry expert and Farm Advisor Manual Jimenez have found what time of year citrus thrips are present in blueberries, where to look for thrips on the crop, how to monitor for thrips, and some of the damage they cause.
During the first year of this three-year project, Haviland and his team held field trials in three commercial blueberry fields in Tulare and Kern counties. They identified May through early October as the period when citrus thrips are present in blueberry canopies, and learned that thrips densities peak in July and August.
The team's data also indicates that an increase in thrips density causes a decrease in blueberry growth. This occurs when adult and immature thrips feed on the newly developing flush, causing the leaves and stem to become discolored, scarred, or even die. “Since the new growth at the end of one season is where berries form the following spring, any damage to the quality of the current year's new wood, and the buds it bears, can have negative impacts on harvest the following year,” says Haviland.
The team is also testing nonchemical controls, such as the use of high-pressure water. Field trials suggest that applications of water at high pressure with an air blast sprayer may be a way to reduce the number of pesticide applications per season. “The theory behind these treatments is that the combination of air and water would blow or knock thrips off of the plants,” says Haviland. “We speculated that adult thrips would likely fly back into the plant canopy, but that a portion of dislodged immature thrips would die. Initial trials suggest that this theory may be true, but we need to do more testing.”
Unlike citrus, which has many pests and dozens of natural enemies to biologically control them, blueberries are relatively pest-free, with insect pests in the plant canopy limited to citrus thrips and, periodically, some worms. Other pests such as grubs and wood-boring beetles are either below ground or within the canes and, therefore, do not serve as alternate hosts for biocontrol organisms. Since there is no food source for natural enemies, they do not hang around. Haviland says, “The relative lack of natural enemies is one of the reasons that insecticides are being used at seven- to 10-day intervals, sometimes more than 10 times per season on some fields to combat citrus thrips.
“Citrus thrips have a history in citrus of rapidly developing resistance to chemicals that are used repeatedly and frequently for control,” he says. “With the limited number of pesticides available to blueberry growers, it's wise to monitor citrus thrips levels carefully, to limit treatments only to populations that are causing, or are expected to cause, significant levels of damage, and to time and apply treatments optimally so that reapplications aren't required. Our main objective is to develop information that will make such an IPM program a reality.”
Growers also report large differences in thrips densities among different blueberry varieties. “In the initial stages of evaluating blueberry varieties, our field testing showed that the variety, Star, is extremely susceptible to citrus thrips, often having three or more times that of surrounding varieties.”
In the next two years the researchers plan to refine sampling techniques to determine economic thresholds of the pest, as well as investigate insecticides that are acceptable for organic production.