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Clean Up Time - Dormant Sprays

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While many growers, farm advisors and pest control advisors (PCAs) lean more and more to non-chemical controls for most almond pests, most continue to believe that one of the most efficient and economical pest control treatments is a dormant spray.

Mario Viveros, Kern County farm advisor, considers the dormant spray "the cornerstone of a good pest management program in almonds." A dormant spray can control a multitude of pests with a minimum of risk to trees, people, and beneficial insects, he says. A grower has the convenience of a wide window of opportunity for applying the spray and there is a broad range of materials available for use.

According to Viveros, one of the chief advantages of the dormant spray is its timing. "It is applied during the most susceptible developmental stage of our target pests," he notes. "It is done when no leaves are on the trees, which allows better spray coverage, and it is done when it is least disruptive to beneficial insects."

In the opinion of Steve Sibbett, Tulare County farm advisor, the dormant spray "is essential for controlling San Jose scale, peach twig borer (PTB), and mites that overwinter in orchards." He says, "It is the most important, least disruptive, and easiest to apply of all pest control measures."

Scale and Mites

The annual dormant spray takes on even more importance now that the populations of scale and mites seem to be increasing in many almond growing areas. Viveros, who has seen heavy infestations of San Jose scale this year, ties these increases to ineffective or missed applications of dormant sprays, as well as the possible development of insecticide resistance, and the use of low rates of dormant oils.

"I have seen orchards heavily infested with San Jose scale whose dormant spray was skipped two to three years," he says. This is a high price to pay, he notes. "This insect sucks plant juices from twigs and limbs. It injects a toxin, which results in loss of tree vigor, growth, and productivity. When the infestation is severe, fruiting and scaffold wood will be killed within one to three years," he explains.

Control of San Jose scale can be obtained with a properly applied dormant spray, he says. "Eighty percent of San Jose scale overwinter in the black cap stage, which is susceptible to dormant spray. It is the same dormant spray that would control PTB."

Oil alone will control low to moderate infestations of San Jose scale, he says. However, to control heavy infestations, oil plus an organophosphate insecticide is required.


A prime target of dormant sprays is PTB. This pest overwinters as a small worm in cells called hibernacula under the thin bark of limb crotches in one- to four-year-old wood, Viveros explains. When the trees bloom in late winter, PTB worms leave their hibernacula and feed on flower buds or young foliage. They can damage several clusters of leaves or buds before settling down and mining into the interior of twigs.

Based on that behavior of the PTB, Viveros advises applying a dormant spray late in winter when the worm is actively feeding, but before beehives are placed in the orchard. "The best spray is be one that can penetrate and also fume inside the hibernacula," he explains. The oil in a typical dormant spray helps the insecticide penetrate the hibernacula and kill the PTB, and the oil is also more effective than the synthetic insecticide in killing mite eggs, he adds.

The Right Mix

Viveros says that the recommended rate for a narrow range oil (superior, supreme or dormant) is four to eight gallons per acre mixed with 80 to 100 gallons of water. Research has shown, he says, that the best organophosphate insecticides for PTB have been Supracide, Lorsban, Diazinon, and Dibrome. He advises reading and following label directions when using any pesticide.

Never apply a dormant spray to water-stressed trees, Viveros cautions. "The grower needs to make sure that there is at least three feet of soil moisture."