Control of mites in almonds requires close monitoring

An Almond Day presentation at the Fresno Fairground opened with a look at the importance of monitoring in controlling spider mites and a talk on the sterile insect program that is a weapon in controlling navel orangeworm.

Monitoring is a major key in controlling spider mites in almonds, looking at pest populations and considering numbers of a voracious predator, the six-spotted thrip.

Kris Tollerup, a University of California area advisor at the Kearney Agricultural Research and Extension Center, said monitoring is just one part of an integrated pest management approach to managing the pest. It involves a combination of practices—biological, cultural, mechanical and chemical.

Tollerup talked of how to identify the pest, pointing out that newly hatched larvae are translucent with six legs and may have dark "food spots" on the back.

The spider mite feeds by inserting its stylophore into epidermal cells and removing their contents. That can result in damage that includes russeting or bronzing of leaves, some direct cosmetic damage to fruit, reduced photosynthesis and increased water stress.

Stippling can occur as mites feed, and there can be browning of leaves and defoliation. That defoliation may reduce fruit quality the following year due to reduced photosynthesis. There can also be sunburning of wood.

Tollerup raised the question of where the economic threshold lies for damage that would merit the costs of spraying. He said he believes the injury level threshold comes when there is 40% infestation—basically, when there are two mites per leaf.

Factors that can bring infestation include stressing of plants due to a lack of irrigation, along with nutritional deficiency and dust.

UC guidelines for sampling of spider mites on almonds call for sampling 15 leaves per tree on five trees at a minimum in a sampling area, examining leaves for mite adults, eggs and natural enemies.

Tollerup said predator enemies of mites fall into three categories: those that prey mostly on web-spinner spider mites; those that prey on mites and insects and pollen; and those that prey on mites, insects, pollen and leaf sap.

Natural enemies include the lacewing, six-spotted thrips and the spider mite destroyer.

Tollerup said early-season miticide sprays before mites show up show little benefit during research he has conducted.

He said six-spotted thrips are "voracious" and produce a full generation within two and a half weeks. If sprays of Abamectin come too early, he said, "you're blowing these thrips out."

He added that studies show some resistance to Abamectin. He said it is important to have the right equipment and get good coverage. He recommends checking nozzles, applying the material at 100 gallons per acre and driving at 2 miles per hour.

Abamectin is among miticides that kill immature and adult stages of the pest. Envidor is effective on all stages, but most effective against immature stages.

Etoxazole is a growth regulator that causes females to lay sterile eggs. It inhibits molting. Apollo is effective with contact on eggs.

Houston Wilson, assistant Cooperative Extension specialist at Kearney with the Department of Entomology at UC Riverside, talked of the research on the sterile insect program with help from Chuck Burks, research entomologist with the Agricultural Research Service of the U.S. Department of Agriculture.

Wilson explained that the idea of introducing sterile insects into wild populations is to promote mating that results in no reproduction and a population decline. The idea is to mass produce the target pest, sterilize with radiation or other means, release masses of the sterile insects over target areas and "overflood" wild populations with sterile insects.

Radiation was first used in insect sterilization in the 1910s. The concept of using the sterilized insects was finalized in the 1930s and 1940s; the first implementation came in the 1950s to combat the New World screwworm, a devastating livestock pest.

There were sterile insect releases through the southern United States and Central America that proved highly effective.

Flies used as sterile insects have cut down on population of the screwworm, the Mexican fruit fly, the Mediterranean fruit fly, the melon fly and the onion maggot. Sterile beetles have combated the sweet potato weevil and boll weevil. Sterile moths are used to lower populations of the codling moth and pink bollworm.

Work on irradiating the navel orangeworm moth came in the 1960s.

Notably in pistachios, NOW causes significant damage and brings concern over aflatoxin. Key tools in fighting NOW include sanitation by destroying mummy nuts, monitoring and well-timed sprays, timely harvest and mating disruption.

A USDA pink bollworm rearing facility in Phoenix holds some promise for the NOW fight. It has been operating since the 1960s, and since the bollworm was eradicated in 2018, there are efforts to repurpose the facility for NOW.

That repurposing was headed at the outset by the pistachio sector, USDA, California Department of Food and Agriculture and UC. This year, the Almond Board of California joined the effort and is doing field studies in California.

The program can produce 750,000 sterile NOW moths a day.

"That's a lot of moths," Wilson said. But there also a lot of orchards that can use those moths—among them almonds on 1.2 million acres, 300,000 acres of pistachios and 250,000 acres of walnuts in California.

Research is underway to determine what is the best "overflooding ratio," what's the best way to deliver the moths and how the sterile insect program can be integrated with existing integrated pest management tools.

Pilot programs are underway on 1,900 acres of pistachio and almond orchards in Kern County, with aerial releases five to six times a week from April 1 to Nov. 1. Additional field research is occurring at Kearney on two acres of pistachios, with weekly ground releases June 11 to Oct. 15, and in Kettleman City on 480 acres of pistachios and 160 aces of almonds, with weekly aerial releases July 1 to Oct. 15.

The studies have shown irradiated males rarely show up in pheromone traps and never in mating tables into which females are placed. But irradiated females do attract wild males.

Wilson said some of the poor performance could be due to shipping, which disrupts the period during which the insects are kept in the dark.

He said researchers are looking at the strains of moths and their radiation dose. They're also studying the degree to which moths may be damaged in aerial release.

"As they are dropped from airplanes, are wings being ripped off?" he wonders.

Researchers are also studying rearing conditions, shipping conditions, release methods and timing.

"We still have work to do," Wilson said.

Among objectives is to determine where release of sterile insects fits within the current IPM program.

(Dennis Pollock is a reporter in Fresno. He may be contacted at agcompollock@yahoo.com.)