Management of swarms of tiny, virus-packing thrips can be a daunting task, but Eric Natwick, Imperial County director of University of California Cooperative Extension, says it can be done on leafy vegetables with an array of practices, each significant although not a solution on its own.
Natwick, whose specialty is entomology, talked about thrips at a plant disease seminar in Salinas. Citing his experience with them in the desert conditions of his county, he said most of the guiding principles for management can have some application to vegetables on the Central Coast.
“Insecticides are often our only viable alternatives, so we need to keep the ones we have with good pesticide resistance management,” he said. “However, for thrips, and insects in general, trying to spray your way out of a problem rarely works.”
So small they can only be identified with a hand lens, the major pest species on leafy greens is western flower thrips, although onion thrips can also be a problem. Damage ranges from deformation of foliage to spotting or streaking, making produce unmarketable. More than 5,000 species of thrips exist, but only about 1 percent of those are considered pests.
“What we are primarily concerned about in the low desert is their direct injury to crops, including cabbage, beans, lettuce, onions and a number of others. Even when they are not vectoring a disease, they can be very damaging.”
Natwick's first step is to identify which species is present, particularly for biological control, where natural enemies are host-specific. For example, some insect-feeding thrips are hostile to plant-eating thrips.
Unfortunately, bio-control is often too late, the crop damage having occurred before the predators and parasites can build up to adequate numbers.
The key identifying trait for western flower thrips is the hair, visible only under high magnification, on the body segment behind the head. Natwick recommends using only mounted specimens under a powerful microscope for positive identification. The best results are from consulting an expert.
Western flower thrips, which has become common around the world, has developed resistance to a large number of classes of insecticides.
“So you should remember to check the label for the class of the material and rotate the different classes you use. It doesn't mean resistance will not occur, but hopefully it will not before a new material is available.”
Sprinkler irrigation can assist in thrips management by washing them off the crop. Another tactic is avoiding small grains or nearby grasslands where thrips build up populations that later migrate into the crop. “They will chase one planting to the next.”
“If you use transplants, make sure they are clean, not just free of disease, but thrips and other insects,” he added.
Another strategy in the desert, Natwick said, is to spray insecticides during cooler times in the day, since thrips avoid heat by moving inside the crop. Surfactants can be helpful in uptake of systemics by a crop and help reach recesses where the cryptic thrips tend to move.
In the desert, he said, thrips are present season-long, but they build up in the spring and when temperatures are in the 70s they can complete a life cycle in three weeks. That goes for the crop as well as weedy areas around fields.
In situations where viruses are a problem, insecticides should be timed to reach larval stages which are the vectors of pathogens. Adult thrips do not acquire pathogens, but the juvenile forms feed on infected plants, mature into adults, and move on to infect new plants.
“We don't have any thresholds for leafy vegetables when it comes to thrips control, so you need to closely examine plants. And if you find three to five of them on a plant with the naked eye, you can figure there are three times as many you don't see.”
Sticky traps are not used to determine numbers, but they can signal a movement of thrips into a field. Natwick said his preference is to beat plants over a sticky card or sheet for a better indication than examining plants. It is wise to sample the crop two to three days after an insecticide treatment to check its effect.
“Plant size is important because it contributes to insecticide efficacy. The larger the plant, the more difficult it is to get adequate coverage.”
And, he concluded, ground application with properly adjusted spray nozzles is more likely to succeed than an aerial application.
In another presentation at the seminar, Krishna Subbarao, University of California, Davis plant pathologist stationed at Salinas, traced recent research developments involving Verticillium wilt in spinach seed used in the Salinas Valley.
Infected spinach grown for the fresh market or processing does not reveal the soilborne pathogen, since the symptoms occur after the bolting stage. In spinach for seed, however, the disease reduces yields.
Another worry in Washington, where spinach seed is grown, is that infected soil will carry the wilt to potatoes there. Similar concerns have emerged in the lettuce industry on the Central Coast of California.
Vert wilt, first detected in Salinas Valley lettuce in 1995, has been spreading in recent years, newly infected fields discovered in 2008 bringing the total to some 1,500 acres, mostly around Salinas.
“Verticillium dahliae has been known in spinach for a long time, but only in 2000 was it found to be seedborne,” Subbarao said. With that in mind, he examined commercial spinach seed lots and learned that about half were infected with Verticillium.
The task of researchers now is to try to develop methods to reduce the seedborne inoculum. Lindsey du Toit, vegetable seed pathologist at Washington State University, is working on seed treatments to curb the pathogen.
Meanwhile, Subbarao is collaborating with Beiquan Mou, USDA-ARS plant breeder at Salinas, on Verticillium-resistant spinach germplasm. Other breeders there are continuing work on additional wilt-resistant lettuce germplasm.
Commercial laboratories are working on standardization of seed assays to detect the disease. Traditionally, two methods have been used, and the object is to learn which is the best and adapt it.
On another front, plant pathologists are investigating where the wilt resides on spinach seed, as well as any relationship between it and lettuce and spinach.