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.) and other fall host plants. 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 BCTVCP personnel, Environmental Scientists and Agricultural Pest Control Specialists (APCS), determine that the probability of achieving maximum population reduction is high, pre-treatment counts of the BLH population are made and control operations are started.

Fixed wing aircraft are used to apply malathion. Supervisors and field staff are in constant contact with the pilot by radio to give directions, where needed. A sophisticated Global Positioning System (GPS) unit on the Contractor's aircraft keeps application swaths in alignment making flag-persons unnecessary.

Fixed Wing Aircraft
Fixed Wing Aircraft

BLH control is accomplished by insecticidal application. Malathion is mixed with buffered water at a rate of 7.7 oz. of 96.5% malathion in 120 oz. of water equaling one gallon, which is applied to each acre treated. 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 aircraft. 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 fall host plants are conducted by using a heavy-duty sweep net with shallow net bag of BCTVCP 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 once, horizontally, in order to contact the fall host plants 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 pre-treatment survey, counts on Russian thistle and other fall host plants 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%. BLH’s typically move to the outer perimeter of the plants where contact with the malathion is assured. Most treatments result in a 90 percentile plus mortality.

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. Spring host plants include filaree, peppergrass, and Plantago sp. A different net and sweeping technique is used compared to the sweeping technique used in the fall. 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 for ten sweeps. As it passes over the tops of host plants, BLH attempting to escape, are caught in the cone of the net. 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. Both pre- and post-treatment surveys are conducted and daily evaluations of populations are made. Treatment conducted in the spring is more important in controlling the disease in susceptible crops, subsequent crop damage, and monetary loss to the industry. Growers of susceptible crops are alerted to the threat level posed by BCTV infection in various areas.

Winter treatment is not often conducted. Winter treatment was conducted primarily because the BCTVCP used a helicopter for aerial treatments, which was a much slower application process than using the fix wing aircraft. Since 2004, and the use of the fix wing aircraft, winter treatments have only been conducted three times (2004, 2006, 2015).

Young Filaree and Popcorn Flower
Young Filaree and Popcorn Flower