As regulatory pressure mounts on use of broad-spectrum insecticides, alternatives such as pheromone mating disruption (PMD) are being considered for control of codling moth (CM), the main pest in California walnuts.

Joe Grant, San Joaquin County farm advisor, has been with IPM specialists and entomologists seeking to refine use of PMD aerosol dispensers, or puffers, in walnut orchards for the past five years.

At a recent Tri-County Walnut Day in Visalia, Grant talked about his work. “This technology works, but it is not a stand-alone technology,” he said, explaining that a critical feature is augmentation with some insecticides as needed during the initial two or three years of use.

Another key element involves introducing the puffer only in orchards where the CM population has already been suppressed by conventional sprays.

The basic concept is filling the air in an orchard with a synthetic pheromone to duplicate the scent given off by CM females to attract males for mating. In the resulting confusion, CM females produce fewer offspring.

Mating disruption of insect pests has been used in several crops, including apples and pears, but success in larger walnut trees, especially those with a history of large CM populations, has occurred only relatively recently.

Earlier designs of pheromone dispensers, including tubes and flake products, at 200 units per acre, Grant said, were effective, but material costs of $130 to $150 per acre and the costs of placing them 40 feet to 50 feet up in mature walnut tree canopies made them too expensive.

“In the late 1980s the breakthrough came with the development of aerosol puffers containing enough pheromone to last the entire season, some 200-plus days,” he said.

The dispensers release “plumes” of pheromone at 10 to 15-minute intervals during the evening and nighttime hours when CM is mating. One unit per each two acres costs around $70 per acre, and the method works best in blocks of at least 40 acres.

About four years ago, Grant and his University of California colleagues started evaluation and implementation trials with puffers in major walnut production areas of the state from Tehama County to Kings County. The 2,400 acres of trials now take in orchards of several varieties of several ages.

On 600 acres of numerous varieties of walnuts near Lockeford in San Joaquin County, Grant said puffers were installed in 2005. By 2008 no insecticide sprays for CM were needed, although some damage occurred along the edges of the puffer-treated blocks.

“In the first year of puffer use, we were seeing CM damage in the 2 percent to 2.5 percent range, which was half what the grower got with a conventional spray program the year prior.”

The grower, Grant recalled, seeing the initial reduction in CM numbers, chose to spray less in 2005. But the following season, he made more applications to lower the population. “As a consequence of lowering that population in 2006, he was able to spray less in 2007 and none in 2008.”

Grant cautioned that neighboring walnut growers should be notified of puffer use. The scent will shut down 1X pheromone-monitoring traps downwind, and CM-DA combo traps should be used instead.

As a result of the edge-effect, less control occurs with smaller, isolated orchards, and the puffer technology works best on blocks of 40 acres or more.

Grant also noted that where use of broad-spectrum insecticides is reduced, there is evidence of rising numbers of other pests, including walnut husk fly and aphids.

In a review of management of walnut insect pests, Robert VanSteenwyk, entomology specialist at UC Berkeley, said walnut husk fly is becoming a “bigger and bigger pest,” mainly because of withdrawal of organophosphates such as Guthion and Supracide.

Once a rarity in the southern San Joaquin Valley and confined largely to river areas in Northern California, the WHF has expanded its range as well as its emergence period.

It must have adequate carbohydrates and nitrogen to begin to oviposit, and walnut susceptibility to it depends on firmness and thickness of the husk, determined by variety and time of season.

“You can have flies out there for a considerable time doing no damage, and then overnight it can become a tremendous infestation,” VanSteenwyk said. Once oviposition occurs, he added, the pest is very difficult to control.

In a 2008 trial near Hollister with Bill Coates, San Benito County farm advisor, VanSteenwyk found that excellent control of husk fly was obtained with three applications of the following combinations of chemicals:

Assail + Dyne-Amic, Delegate + NuLure, Provado + Dyne-Amic, Baythroid + NuLure, Leverage + Dyne-Amic + NuLure, Provado + Delegate + NuLure + Dyne-Amic, and Assail + Delegate + NuLure + Dyne-Amic.

The event also included a report on new research on crown gall of walnut, caused by the bacterium Agrobacterium tumefaciens found in commercial walnut orchards throughout the state. The common Paradox rootstock is highly susceptible.

The persistent, soil-borne organism is able to transfer its own DNA into the host plant, disturb the host’s hormonal balance, and produce galls or tumors. These growths around the crown girdle the tree and claim a toll in vigor and production.

No effective controls for crown gall exist, although practices such as use of clean planting material, pre- and post-cold storage treatment of seedlings prior to planting, and care to avoid wounding of trees during cultural operations can reduce its incidence.

The common practice on infected mature trees is to surgically remove the galls, sterilize with flame, and apply a bactericide to check further spread of the disease.

Dan Kluepfel, USDA-ARS plant pathologist stationed at UC Davis, is leading a team of scientists searching for resistance to crown gall.

The group reasons that since wild relatives of cultivated walnut species contain resistance to insect, microbial, and other pests, they could potentially have resistance to crown gall.

Kluepfel said a number of wild black and butternut species adapted to California conditions are in the USDA-ARS National Clonal Germplasm Depository near Davis.

Their task is to screen the collection for sources of crown gall resistance that could be bred into improved varieties for nursery and production orchards.