“I’m down in Bakersfield and spider mites for us are huge,” said David Haviland, a University of California Cooperative farm advisor for Kern County. “Everything that makes a tree stress and work hard, that’s what we’ve got, and that’s what makes spider mites go crazy.”
Some spider mite presence isn’t cause for total panic, Haviland said.
“The science says the trees can take some hits,” he said. “There is a tolerance for mite damage.”
In fact, Haviland said, a key driver of spider mites populations is how much biocontrol there is in a given year.
“The mites are in the orchard every year,” Haviland said. “The key is what is out there eating them.”
Predatory mites, one traditional source of biocontrol, have been missing from orchards for five years, Haviland said.
“The orchards are getting greener and greener, so why are predatory mites still not there?” he asked. “This guy is eating them. Sixspotted thrips. It eats spider mites, that’s it.”
Another powerful mite predator is the spider mite destroyer — essentially a tiny black ladybug.
Spider mite destroyers eat a lot of spider mites but late in the season, Haviland said. They’re about a third of the size of traditional ladybug.
For good control of spider mites, it’s important not to put pyrethroids out in April and May, Haviland said.
If spread too soon, the pyrethroids kill natural enemies or kill the mites those enemies will feed on, limiting the predatory populations too early in the season.
“You can go out one day and there’s no mites, and you go out the next day, and they’re full. It can happen so fast,” Haviland said. “When you go from zero to infinity in a week, it’s because you have no predators.”
The UC system has forms that allow growers to estimate a threshold that indicates spider mite problems, he said, and growers can go to UC advisors for help calculating that threshold.
“There are a lot of miticides, and they’re very very good,” Haviland said. “Ten years ago that was not the case. For me the key factor of miticide efficacy is not the product you choose, but how you apply it.”
Two miles per hour is a speed limit to obey spraying miticide, he said, referring to one grower who bought a radar gun to check his crews.
“If you’re driving fast, these products don’t work,” Haviland explained. “You have to actually displace the air in these trees…that means two miles per hour.”
Haviland said with the most successful biocontrol from predators, sprays can be reduced.
He showed slides of dramatically reduced populations of spider mites. The cause? Booming populations of sixspotted thrips.
“You can see how quickly this can happen in all three cases,” Haviland said to growers. “All of our data show the populations of thrips are doubling about every three to four days. The reason the mites are dropping so fast is those thrips are doubling.”
That’s good news for growers, he said.
“We have many orchard that used to do two or three sprays, that now are just using one,” he said. “Almonds up and down the state should be zero to one spray orchards, year in and year out, with more than one spray the exception.”