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Growers urged to monitor orchards for pests to determine if treatment is necessary.

Lee Allen, Contributing Writer

April 15, 2020

3 Min Read
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A spider mite is on a plant.National Pesticide Information Center/Oregon State University

May is the traditional miticide month for California almond growers, but a University of California Integrated Pest Management advisor is suggesting that pesticide applications could cause more harm than good.

While Kris Tollerup at the UC’s Kearney Agricultural Research and Extension Center recommends almond farmers monitor their orchards for spider mites and six-spotted thrips to determine if treatment is necessary.

He adds, “There’s no beneficial effect from preventative sprays for spider mite populations because the mites show up late in the season and their population density remains well below an economic level. Their natural enemy — six-spotted thrips, the so-called spider mite destroyer — will likely show up and suppress the mite population before any damage occurs.”

While over half a million acres of California almond orchards received a preventative miticide in May during the 2017 growing season, this strategy “runs counter to sustainable integrated pest management practices,” he notes.

“The sprays adversely impact the natural enemies of spider mites and are based on calendar application, not on the monitoring and economic thresholds that the US Statewide IPM program has determined help reduce pesticide applications,” he says.

Tollerup worked with large growers in Kern County and the Almond Board of California to compare the effectiveness of preventative miticide spray with plots that were simply monitored for pests and natural enemies.

80,000 acres not treated

Surveys conducted after his trial results showed some 80,000 acres of almonds were not treated with miticide spray in May 2018 and again in May of last year, a change in strategy that resulted a farmer savings of $2.2 million.

In addition to the dollar savings, Tollerup calculated a reduction of 880,000 pounds of carbon dioxide emissions due to the reduced use of diesel tractors and motor-driven application equipment associated with miticide spraying.

Asked about reactions from long-time growers to his no-spray recommendation, his response alluded to an analogy of a foreign object in the punchbowl. 

“I understand why these preventative spraying practices are done, so that growers can sleep at night — ‘early spraying is done, that should hold us until hull split’ — but you’ve got to trust those natural enemies,” he said.

“These are tough trees and they can take a tremendous amount of injury before responding with defoliation or reduced yield. These growers are savvy and know my recommendation of not spraying just as a preventative, but waiting until there is a mite problem, really isn’t a novel idea — even though it will save them money on labor, equipment, and chemicals.

“This idea may run converse to what growers have been doing most of their life as part of their annual program, but preventative miticide spraying really isn’t necessary unless there is an obvious problem. Trust the natural enemies to suppress mite problems.”

Suggestions from the pest control folks at University of California include biological controls such as predator releases and oil sprays as organically acceptable ways of managing spider mites. Cultural control methods include watering roadways and maintaining good ground cover to mitigate dusty conditions in orchards. 

Prevention of water stress is a given as this condition results in higher mite numbers and makes trees more susceptible to infestation and damage.

Under the category of “Monitoring and Treatment Decisions” is this — “Monitor for mites weekly from May through August. Treatment is not necessary after August as mites begin to migrate off trees for overwintering.

“Prior to July 1, focus monitoring on hot spots, areas that develop mites first (like dusty or water-stressed orchard areas). After July 1, monitor the entire orchard, dividing it into sample areas that can be treated separately.”

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