This spring’s flush of mustards and other weeds is a likely harbinger of heavy pressure from yield- and quality-robbing onion thrips on San Joaquin Valley onion fields, warns Rich Coviello, Fresno County farm advisor.
Both onion thrips and western flower thrips (WFT) have a wide range of hosts, from various weeds to cereals and broadleaf crops, but the onion species, which can claim half a crop, is considered more damaging to onions than WFT.
Onion thrips is also suspected of vectoring a "new" disease to the SJV, iris yellow spot virus.
Coviello told those at the recent California Onion and Garlic Conference at Five Points that with ample vegetation the thrips build high populations and as soon as mustards and other hosts dry down, adults and nymphs will head for onion fields.
The sucking insects first attack undersides of leaves and in the whorl of the onion plant, creating an abraded effect. Later, as populations swell, they move on to exposed leaf surfaces. Severely damaged foliage has a silvery appearance.
"You can find both species, but in recent years I’ve seen more onion trips on onions around the west side of Fresno County. You need a 20X magnification lens to tell the difference. One distinguishing feature is onion thrips have spines farther back from the head than WFT," he said.
Another distinction is populations of WFT tend to appear early and then level off as springtime temperatures climb. From then through the growing season, as bulbs develop, onion thrips are most active. Their scarring of leaves of fresh onions is a serious problem.
Coviello said the higher the thrips populations, the higher the percentage of under-sized bulbs. In his small plots to evaluate insecticides at Five Points he saw harvestable onion yields slip from 135 pounds in treated portions to 60 pounds in untreated controls.
"There may be some discussion in other onion-growing areas about whether thrips cause significant damage, but I think this demonstrated that they can be damaging around here," he added.
In approaching management of onion thrips, Coviello said a defined threshold for treatment has not been worked out. However, by sampling at least five plants from four parts of a field, an average count of 30 thrips per plant has been used successfully for dry bulb fresh market and drying onions.
For processing onions, when weekly population sampling, as detailed in University of California Management Guidelines for thrips, reveals 500 to 600 cumulative thrips days, or the equivalent of 20 per plant, significant losses can occur.
Applications of synthetic pyrethroid insecticides have shown the best longevity, while organophosphates and carbamates have been less effective, Coviello said.
Thrips resistance to organophosphates has been documented in other states and is suspected in California, so use of alternating classes of chemicals is recommended.
Organic materials may give some protection for a few days, but Coviello said "the aerial flotsam of thrips" coming off drying crops and weeds is overwhelming to most of these materials.
He said there is anecdotal evidence that sulfur, either wettable or dusting, gives some control of onion thrips, although precautions to avoid contact with watermelons or other surrounding, susceptible crops need to be taken.
Regardless of the material applied, thorough coverage is vital, since most thrips activity is in protected portions of the plant.
Grant Poole, Los Angeles County farm advisor, said his trials at Lancaster screening several chemicals demonstrated that aerial applications of Lannate gave the best performance against thrips. The insects, he added, are beginning to show resistance to Warrior, a compound used in the area for the past five years.
Another speaker at the conference, Joe Nunez, Kern County farm advisor, said reports been made of a new onion disease identified as iris yellow spot virus (IYSV).
It was reported in Brazil in 1981 and has also been found in Israel, as well as several onion-growing regions in the U.S., including Oregon and Washington, Utah, Colorado, and New Mexico.
Nunez said the pathogen turned up in Idaho seed-onion fields in 1991 and although it was not lethal, it did reduce bulb size. Two strains of the virus have been identified.
ISYV, a topovirus, is vectored by various thrips species, including onion thrips, and also goes to garlic and other alliums. The thrips commonly move from drying grain and bean fields into onions.
Symptoms are straw-colored lesions on leaves. Some lesions have a distinct green center surrounded by yellow-tan borders, giving an "eye" effect. Others are concentric rings of alternating green and yellow-tan tissue.
Management strategies in other states, such as Colorado, have included sanitation and destruction of onion debris and volunteers, use of clean seed, crop rotation, selection of varieties less susceptible to thrips, and thrips control.
Marita Cantwell, Cooperative Extension vegetable specialist at the University of California, Davis, told the conference that "fresh-cut," value-added onion products need special care from the field through processing.
Fresh-cut vegetables made up nearly a third of all pre-packaged produce retail sales in 2003.
"We are making these products more convenient for the consumer, but we have to realize they are more perishable after they are sliced or chopped," she said.
Fresh-cut onions can be stored for about 21 days if they are cut with sharp blades and maintained at 32 to 41 degrees with the proper atmosphere.
Growers, she said, need to know the shelf life is reduced if fresh-cut onions are damaged, either by bruising or by improper storage, before or during the cutting process. If damaged, they become more translucent and contract microbial growth sooner.
"The industry has quite sophisticated processing, but one of the main problems now is consistency of raw material quality, and we’ve found a lot of variability in handling that leads to physical damage."
She said she and other researchers are identifying the different variables in onions by variety, chemical composition, and they ways they are grown.
"We’ve started a project to look into what happens with each of these, we have an annual workshop for fresh-cut vegetables and fruit at Davis, and we are publishing the information on the Web at email@example.com."