CLOVIS, CA — Onion thrips, a serious onion pest that can also carry a viral disease affecting onions, and the development of resistance to pyrethroid insecticides by onion thrips has become a global problem. "We continue to have a problem with onion thrips, and we are very concerned about iris yellow spot virus," which is transmitted by the thrips, said Robert (Bob) C. Ehn, chief executive officer and technical manager of the California Garlic & Onion Research Advisory Board, here, in an interview with The Produce News.
One reason thatthrips continues to be a problem, he said, is that control has historically been largely by means of pyrethroid insecticides, but "we just about wore the pyrethroids out."
The search for suitable alternative methods for controlling the pest is ongoing. Those alternatives include not only other pesticide formulations but also biological controls and cultural controls, and the Garlic & Onion Board is involved in those efforts.
"I have a fairly large program on thrips," Mr. Enh said.
The board technically does not represent fresh or storage onion growers but only growers of processed onions, although on the garlic side it represents growers of fresh market as well as processed garlic. However, "the pests we deal with are the same," he said. "A processed onion is a [also] a dry bulb onion. It is just bred for higher solids."
Onions and garlic are both grown throughout a wide range of geographic areas in California, from the Imperial Valley on the Mexican border to the Oregon state line, with the harvest period for onions running from April through November. Central California, and in particular the west side of the San Joaquin Valley from Kern County to Fresno County, accounts for a major portion of the state's production, with the crop coming off predominantly from June through September.
"We represent 24,000 acres" of processing onions in California, which is "just a tat over half of the total production" in the state, the rest being storage and fresh market onions, he said. Both crops can be affected by the thrips and the virus.
The virus itself "will suck the juices out" of the plant and reduce yield, he said. But if it transmits the virus, the disease "will knock the plant over," and "you really don't have anything left to harvest.
One field of onions in the Imperial Valley three or four years ago "lost over 50 percent production from the yellow spot," he said.
The iris yellow spot virus is even more serious in seed crops which are particularly vulnerable because the disease causes the seed-producing umbel, or flower cluster, to drop.
Yellow spot, so far, has been much more of a problem in some other parts of the country such as Georgia and the Pacific Northwest than it has in Central California, Mr. Ehn said.
With thrips alone, losses to an onion field are generally less, causing a reduction of as much as 15 to 20 percent in yield, "but it doesn't affect the quality of the harvested crop," he said.