Emily Symmes, Ph.D., IPM advisor, speaks to growers at the Walnut Conference in Yuba City.

It’s not normal for navel orangeworm to take a backseat to any pest, but that’s exactly what happened when Emily Symmes, an UC Cooperative Integrated Pest Management advisor, addressed growers at a walnut conference in Yuba City.

"Today I'm going to focus quite a bit on frosted scale,” Symmes told the crowd. “I talked to growers and PCAs who said in decades, we've never seen this level of frosted scale.”

This pest feeds on leaves and twigs, and, Symmes said, those feeding wounds can provide entry points for Botryosphaeria. Honeydew secreted by frosted scale can cover nuts. Full details on the pest can be found at the UC Integrated Pest Management website page.

Symmes said one reason that infestations are on the rise may be that “natural control has been less effective.” That natural control refers to parasites that naturally help knock down the frosted scale population.

The lifecycle of frosted scale can be seen below. As Symmes explained, the “frosted state” seen in spring, appears to be a waxy kind of bloom. Eggs develop in spring as trees wake up and orchards warm. Crawlers, which can be windborn, emerge from late April through the beginning of May. The insects feed on leaves all summer, and then in the fall, remobilize to woodier tissue. Over the winter, nicks around the pest are pinkish in color.

“When the crawlers emerge, there are copious amounts of them, they're incredibly dense,” Symmes said. “The fecundity is really extraordinary.”

Slides from Symmes depicting frosted scale at progressing stages in the lifecycle.

Growers should monitor frosted scale because natural parasites may keep the pest under control, meaning no action is needed.

Symmes advised growers to monitor and scout for the pest in the winter, during the dormant period.

“UC guidelines indicate that a treatment threshold of less than five nymphs per foot of last year's wood is a reasonable population,” Symmes said. “We wouldn't consider a treatment.”

She said that if growers see a lot of blackened nymphs, that's a good sign of natural control by parasites.

“If you see a higher level of parasitism, you wouldn't treat, to let the natural control take over,” Symmes said.

Growers can use double-sided sticky tape to monitor crawlers throughout the spring, looking for crawlers building up on the edge of the tape. To get a good look at them, Symmes recommended using a hand lens to see how many there are.

A flow chart from Symmes’ presentation that gives growers advice on frosted scale treatment can be seen below, and viewed in a larger format if downloaded.

A decision tree to help growers decide whether to treat Frosted Scale. Download or open the image in a new tab to see larger version.

Growers who do treated frosted scale should check the next winter to see how the population responded, Symmes said.

She also pointed out that walnut scale emerged later than normal in 2017, so growers should keep an eye on it in the field.

Frosted scale may have jilted navel orangeworm from the top of Symmes’ docket at the walnut event, but it’s not the only pest she told growers to watch out for.

She also covered codling moth and, of course, navel orangeworm.

“Walnuts for the most part came out much less scathed than our almond and pistachio crops,” Symmes said. “It rained, it rained, it rained and it was much harder to get in their and sanitize our nut crops...it's really important to sanitize all of these.”

Leaving nuts to remain in the orchard simply allows the navel orangeworm to build, according to Symmes.

“Other sources of damage allow for entry by navel orangeworm. Things like blight, if they haven't dropped to the ground, things like codling moth pressure can allow for buildup of navel orangeworm,” Symmes said.

Both navel orangeworm and codling moth benefitted from greater-than-normal heat, and degree days that were ahead of schedule in July, she said.

Levels of pest activity in 2017, from Symmes’ presentation.

“What we saw for codling moth is a lot of folks were surprised by really high spikes in activity,” Symmes said. “We saw an awful lot of damage in orchards from that first flight.”

The upshot? The bad year in 2017 means growers must be vigilant this season.

“We've had a very mild winter,” Symmes said. “It's really important that all nut growers go out and sanitize. Think about where any type of mummy or orchard debris might build up. The greater the destruction, the better navel orangeworm management.”

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