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Weather interrupts leafhopper controls

“Timing,” as the old saw goes, “is everything,” and it's borne out again with the recent wind and rain nightmare for the Curly Top Virus Control Program in California's San Joaquin Valley.

The longtime program exists to manage the western foothill populations of the vector of the virus, sugar beet leafhoppers (BLH), with strategic spraying before they can migrate from the drying slopes to sugar beets, tomatoes, melons, and other susceptible crops on the valley floor.

The program's aerial spraying of malathion on rangeland concentrations of BLH is augmented by roadside treatments in the valley. Growers contribute to the campaign by managing tumbleweed and other weed hosts of the BLH in their fields.

Program leader Bob Peterson in Fresno, wary after some 2001 curly top losses in sugar beets and tomatoes followed by a mild and dry winter that favored the pest, says this year he expected BLH movement to begin in mid-March, rather than the customary time during April.

Winds sprang up in foothill canyons and put a double-whammy on the program. First, they delayed spraying. Second, they rapidly dried down the native host forage that otherwise would have supported the leafhoppers longer before they migrated to growers' fields.

Although Peterson predicted the early move, he says he didn't foresee that the numbers of BLH would be as small as they turned out to be. “The dry winds desiccated the rangeland so fast that many nymphs did not hatch or did not have time to mature before their hosts were lost.”

Program surveys showed more than 60,000 acres in western Kings, Fresno, and Merced counties as needing spraying. Winds delayed spraying until March 15, then more winds prevented operations the following day, so that by March 17 only about 5,000 acres had been treated before the effort shut down again. A daylong soaker dropped three-quarters of an inch on the Panoche District of western Fresno County.

Rain not needed

Rain, said Peterson, was the last thing the program needed. Precipitation could revive rangeland forage and hold adult BLH longer. On the other hand, BLH remaining in the foothills could be expected to throw another generation distributed over a larger area, such as what happened last year. And more rain meant that weed control already done by the program and conscientious growers could be overcome by newly emerged weeds.

He urged growers to be alert to even the smallest amounts of new weed germination because BLH are attracted to the small, sparse growth.

Between spraying sessions, the program staff was continuing surveys and reallocating resources in an attempt to find any advantage.

“We do know that the largest BLH populations remain in western Fresno County, from the county line on the north tip of the Kettleman Hills, north to Little Panoche Canyon,” Peterson said.

On Friday, March 22 he and his crews were back at it, spraying along Interstate 5 in western Fresno County and threatened by sprinkles. “Between the wind and the rain holding us up, we just have to take it a day at a time,” he said. Meanwhile, more rain was forecast for that weekend.

How much rangeland spraying could be done, given the on-again, off-again weather, remained to be seen, but Peterson and local growers know well that as soon as the weather warms, virus-carrying leafhoppers will begin to depart the foothill forage and head for cropland.

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