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CDFA Plant Health

Curly Top Virus: Program Specifics

Fall control operations in the San Joaquin Valley are the culmination of monitoring the BLH population on Russian thistle (Salsola sp.). Beginning in June, Russian thistle is mapped where it is growing on fallow ground, oil fields or rangeland. Maps are updated weekly and the BLH populations are monitored with sweep net surveys.

By mid to late September, the largest populations of BLH's have been delineated. The overwintering generation will consist of adults that migrate from the Russian thistle to the hills on the west side of the San Joaquin Valley. Adults seek out sunny, south-facing slopes where they produce the spring generation of BLH. A percentage of the overwintering BLH carry BCTV to winter annuals where the disease multiplies and is carried back to cultivated crops by the spring generation of BLH. The only difference between the spring and winter treatments are the time of year and the phase of the BLH life cycle that is targeted. In winter, the adult female is targeted prior to egg deposition, whereas, spring operations target adults and nymphs of the first spring generation.

Once CTVCP personnel, entomologists and Agricultural Pest Control Specialists (APCS), determine that the probability of achieving maximum populations reduction is high, pretreatment counts of the BLH population are made and control operations are started.

BLH control is accomplished by insecticidal application. Malathion is mixed with buffered water at a rate of 7.7 oz. of 95% malathion in 120.3 oz. of water equaling one gallon, which is applied to each acre treated. Malathion is routinely sampled by the CDFA, Center of Analytical Chemistry, to assure quality and absence of contaminants.

Aircraft, usually fixed-wing, are used to apply malathion. Supervisors are in constant contact with the pilot by radio to give directions, where needed. A sophisticated globial positioning System (GPS) unit on the Contractor's aircraft keeps application swaths in alignment making flag-persons unnecessary.

Fixed Wing Aircraft
Fixed Wing Aircraft

Concentrated malathion and water are transported to the aircraft loading site as near to the control area as practical. Mixing is accomplished by metering water, buffered to a pH of 6.5, into a mix tank then metering the prescribed ratio of malathion into the mix tank under agitation. The aircraft is loaded by connecting a hose with a drip proof connector between the mix tank and the helicopter. Each load is metered and checked against the known area treated to assure proper application rate. The aircraft's spray boom system is calibrated under the supervision of the CDFA supervisor on site before application is started and periodically rechecked during the course of treatment operations.

Aircraft and pilots are under contract to CDFA and meet or exceed all FAA standards. The CDFA also requires that pilots, licensed as a journeyman agricultural pilot, have a minimum of 1,000 hours in the type and model aircraft being used.

Within 72 hours after application is completed in an area, post-treatment checks are made to assure positive depopulation of BLH has been achieved. Both pre and post-treatment surveys in Russian thistle are conducted by using a heavy-duty sweep net with shallow net bag of CTVCP design. The net frame consists of a 15" round hoop constructed of 3/16" steel attached to a hardwood handle 7/8" round by 25" long. During survey, the net is vigorously swung horizontally in order to contact the Russian thistle plant in such a manner as to enter the foliage several inches and sweep through with sufficient velocity to dislodge BLH and collect them in the attached net bag. The bag is 16" deep and 15" in diameter, constructed to form a shallow cone. Once captured, the BLH's begin moving from the base of the net toward the open top where they are counted as they attempt to exit. The single net sweep method is directly related to actual counts from enclosed trap studies conducted over several decades.

If during actual pretreatment survey, counts on Russian thistle averaged 100 BLH per net sweep and post-treatment counts taken 72 hours after treatment averaged three BLH's per net sweep in the same area, the population is considered to have been reduced by 97%. A 97% reduction is considered excellent control since malathion at 7.7 oz. per acre cannot fully penetrate the canopy of moderate sized (24"-30") Russian thistle. Most treatments result in a 90 percentile plus mortality, which is good. BLH’s typically move to the outer perimeter of the plants where contact with the malathion is assured.

Survey of BLH populations in winter/spring is a slightly different operation in that the host plants are small, usually only an inch or so above ground and normally on south-facing slopes. A different net and sweeping technique is used. The net bag is the same, but the 15" flexible hoop is made of flat stainless steel attached to a 30" handle. In sweeping, the net is held against the ground and swiftly moved in a horizontal arc approximately 150° from side to side. As it passes over the tops of host plants, BLH attempting to escape, are caught in the cone of the net. Both pre and post-treatment surveys are conducted and daily evaluations of populations are made. Growers of susceptible crops are alerted to the threat level posed by BCTV infection in various areas.

Young Filaree and Popcorn Flower
Young Filaree and Popcorn Flower