Curly top threatens sugar beets

Growers having fields of sugar beets or fallow fields with Russian thistle have been alerted by the Curly Top Virus (CTV) Control Program to spray for sugar beet leafhoppers (BLH) before taking out the fields.

Bob Peterson, director of the state program, said prospects are for greater pressure, perhaps twice as much, from the virus next year.

That threat could emerge in the wake of acreage fallowed by potential land-use changes, driven by water shortages, on the west side of the San Joaquin Valley. Possibly as much as 200,000 acres of cropland in the 605,000-acre Westlands Water District could be taken out of production and returned to native habitat.

Peterson warned that BLH spread rapidly this summer through weedy fields across the valley. “You can find it about anywhere in the valley at a low level. During the past 10 years we've had less and less of it, but it can suddenly spring up on us.”

The reason for the increase is not known, although Peterson said it may be that some natural control is not as effective this year.

The program, charged with managing the overall population of BLH and bringing it down as much as possible, is budgeted for $1.47 million to treat about 112,000 rangeland and other non-crop acres in the current year.

But a mild winter, beneficial to the insect, could mean an additional 60,000 acres will need treatment in the spring, and next year's assessment-funded program budget is based on treating only 87,000 acres, or about $200,000 short of projected control needs.

Acreage shifts

Speaking to a recent gathering of sugar beet growers at Mendota, Peterson said the reduced budget is partly due to cropping patterns, namely shifts in sugar beet and tomato acreage from the Sacramento Valley to the San Joaquin Valley.

The program, however, he added, anticipates holding its present respective grower-assessment rates on the several participating crops. Examples are 4.4 cents per ton for sugar beets and 10.6 cents per ton for tomatoes.

CTV is devastating to several other crops, including melons, beans, and peppers, and is vectored by the BLH. The desert insect arrived in the western U.S. in the late 1800s, about the same time as one of its primary weed hosts, Russian thistle.

During World War I, large acreages on the west side of the San Joaquin Valley and other parts of California went into wheat production. At the end of the war, when wheat was no longer of strategic importance, the land was abandoned and taken over by Russian thistle, which supported massive populations of the leafhopper.

The following year, wholesale failure of the sugar beet crop occurred. During the 1930s to 1950s, Spreckels Sugar Co. entomologist Ed Schwing, after an extensive study charted the migratory patterns of the leafhopper, recorded the migratory pattern of the insect.

Meanwhile, the insect built up in prodigious numbers in western foothills several winters between 1920 and 1936, at times spilling into the Salinas Valley.

During winters, the leafhoppers, a large proportion of them female, feed on fillaree, peppergrass, and plantago to produce a spring generation. That generation migrates from drying hillsides into cropland on the valley floor.

One tactic by growers during the 1930s was to plant sugar beets as a catch crop. The leafhoppers moved in, the growers sprayed with pyrethrum or nicotine compounds to knock down the pests, plowed up the fields, and replanted sugar beets.

In 1943 the then California Department of Agriculture, took over control of BLH at a time when sugar was a strategic commodity and efforts had begun to establish an industry for tomatoes, another highly susceptible crop.

Foothill sprayings

After tracing BLH migratory paths, the state set up the foundations of today's program, spraying selected foothill canyons during the winter and spring.

The choice of materials, however, was DDT and diesel oil for both knock-down and some residual control. Today the material is malathion at eight ounces per acre, and it is effective for about three days.

As recently as 1990, losses to CTV infections along SJV foothills ranged to 25 percent and might have been greater if they had not occurred late in the season.

The program hopes to experiment with several imported wasps that parasitize BLH eggs, but state entomologists expect it will take several years for the beneficials to be cleared for introduction and become established.

Overwintering locations for egg-laying BLH include locations known as “hot spots,” where annual infestations can be expected, Peterson said. Between hot spots are locations which become critical every 10 years or so. Another type of site is entire hillsides of the western side of the SJV, and a fourth location is anywhere on the valley floor where any of 400 weed species hosts occur.

The staggering threat of infestation, and the challenge for control, are demonstrated by the numbers, Peterson said. A single female produces 250 to 350 eggs, half of them female. If only 50 females of a new generation survive, by the third generation, the population jumps to 15,000, and by the sixth generation it exceeds 19 million individuals poised to return to the foothills in the fall to renew the cycle.

“They have tremendous capacity for taking advantage of a food source. This year we saw good weather and host conditions for them,” said Peterson.

The program sprayed about 42,000 acres during last year's comparatively dry fall, but Peterson said the job this year is three times as great, 130,000 acres for spraying this fall from Kern County to Merced County.

Last winter, BLH moved for forage through the foothills without being forced to concentrate for easier control. Spring rains kept conditions ideal for trees and other plant reservoirs of curly top virus.

“We never got population concentration in so-called winter canyons in the Coalinga area, where early populations of females historically concentrate,” he said.

“Typically, they start depositing eggs early on sparse vegetation on the steep, sunny canyon walls, and we can spray a small area by air late in January or February to knock them down. They were spread out and in high numbers this year.”

He said they sprayed about 57,000 acres in the spring, where sweep net counts were running 100 plus per sweep.

One of the ironies of the BLH threat is they really don't like tomatoes. At the same time tomato growers are setting out transplants, many sugar beet growers are harvesting, and the BLH are displaced from beet fields to new crops.

Infect, don't feed

The insects move into a tomato field, sampling one plant and rejecting it and moving to the next and the next, all the while infecting many tomato plants with CTV without feeding on them.

The virus can be easily mistaken for other viruses. Symptoms of cucumber mosaic virus are especially hard to distinguish from CTV in the field, but Peterson said a new CDFA program in Sacramento for making laboratory identification of submitted samples will help.

e-mail: [email protected]

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