Did You Know Thrips Can Be an Almond Growers’ Ally?

closeup of a six-spotted thrips

Six-spotted thrips weren’t a factor when growers were commonly using harder chemicals in the orchards, such as organophosphates.
Photo by Jack Kelly Clark

In the past half-dozen years, California almond orchards have seen a drastic change in spider mite behavior, and that change should dictate a change to growers’ IPM programs. University of California Cooperative Extension Entomology and Pest Management Farm Advisor David Haviland, who has spent the last 15 years working on spider mite management, says the change is due to the successful development and implementation of softer chemistries. Their implementation meant “harder” products, such as organophosphates (OP) and pyrethroids, weren’t being used as much for growers’ toughest pest, navel orangeworm (NOW).

“The orchards have become almost OP- and pyrethroid-free,” he says. “The insecticides used for NOW, SJS (San Jose scale), PTB (peach twig borer), and fire ants are all products considered soft and safe.”

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Due to the changes in pesticide spray programs, six-spotted thrips are becoming more prevalent. In the past, significant numbers of spider mites emerged in April, May, and certainly by June, causing growers to spray. Now he’s seeing a shift. It’s not until late June or July after hull split for the mites to appear in significant numbers.

“As a result, almond growers that used to spray all their acres in April or May with a miticide haven’t had to do it in recent years,” he says. “Six-spotted thrips are feeding on the mites in April and May.”

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What’s interesting is the mites’ emergence hasn’t changed. It’s just that they are getting eaten. It’s gotten so adults can be difficult to find.

Researchers are finding the mites’ eggs in crotches of trees, but they soon disappear. It’s the same story, says Haviland, being told over and over again for the last several years. They don’t see the thrips eating mite eggs. They just see leaves with stippling, indicating the mites’ presence, but then nothing. Biocontrol is working.

“So in past if you read UC guidelines about spider mite management, they would say to monitor every week,” he says. “But the guidelines now need to change, because now they need to focus on whether six-spotted thrips are present.”

Start By by Trapping
Haviland uses a yellow strip trap from Great Lakes IPM that is attached to a binder clip they hang from a tree limb with a small unfolded paper clip. That specific yellow sticky trap works better than other cards, he says, even better than other yellow cards.

avid Haviland

David Haviland

He encourage growers and Pest Control Advisers (PCAs) to hang these in orchards and check on them a week later so the level of six-spotted thrips can be determined. Currently there is no official threshold for mites, though they are working on ratios, as in number of thrips per card compared to mites per leaf. That research is ongoing, and for now they recommend seeing if they are present, and how many there are compared to the previous week.

“It’s not the raw number,” he says, “but the trend in population that is important.”

There is typically a two- to three-week lag time between spider mites and six-spotted thrips. Thrips populations can quadruple weekly. Once they show up they can absolutely take over and dominate the spider mites in a very short period of time.
“In fact, entomology researchers get frustrated with six-spotted thrips because they destroy miticide trials,” he says. “They come in and just eat up all the mites. Both public and private researchers have had that experience many times.”

Haviland said when the initial switch to softer chemistries was made, growers wanted to spray for spider mites, but many didn’t have the equipment for spraying on hand or couldn’t spray because it was too close to harvest. But then came the six-spotted thrips to the rescue.

“There’s almost no exception to the rule that once spider mites occur, six-spotted thrips will follow; it’s just a question of when,” he says. “In most cases the six-spotted thrips take care of (the mites), but in some cases trees get defoliated before (the thrips) do their jobs. That’s why constant monitoring is needed to tell whether a miticide is needed before the thrips can finish the job.”

Haviland says there’s no doubt that more miticides are being used than needed. But in defense of pest control advisers, most PCAs have gotten burned by spider mites. And they can hit when it’s difficult to spray, especially at hull split.

“Even the presence of mites would trigger a treatment by a PCA who knew this was his last opportunity to spray,” he says. “In hindsight many of those treatments weren’t needed, but recommendations are not made in hindsight.”

Chart showing natural enemies' impact on spider mites in Kern County, CA

Graphic courtesy of University of California Cooperative Extension

Free Rides Aren’t All Free
The most common time to apply a miticide is at initiation of hull split, because growers are already making applications of insecticides for NOW. In many areas that’s the last opportunity to make a spray. But Haviland says in other areas with high NOW pressure, another pesticide application for NOW is done two to three weeks later, normally in the second half of July.

“People can be more prone than normal to recommend a miticide at hull split because it’s their last shot,” he says. “But if you’re going to make another NOW spray anyway, it’s better to hold off and watch the mites and spray them if necessary.”

If it sounds like there’s a fair amount of judgment involved, it’s because there is. Ideally you watch mite populations weekly and determine whether you’re reaching a threshold. You also have to evaluate all beneficial predators, obviously including six-spotted thrips, taking into account how long you have until harvest and the opportunities for “free rides” on other sprays. Such important factors as weather patterns and the stress levels of the trees come into play.

If you do decide to spray a miticide, Haviland emphasizes it’s not so much the miticide you decide on, but how you apply it. With miticides, coverage matters, which means slowing down the spray rig. Never go beyond 2 MPH. Also, you need plenty of air and sufficient water volume, some say as much as 200 gallons per acre.

In any case, growers and PCAs must realize biocontrols are for real, and beneficial predators need to be taken into account when making decisions.

Haviland says he’s trying to learn more about six-spotted thrips’ behavior. The only previous research he can find is either 100 years old or done in laboratories — no published work on field behavior. He hopes to show through his research — largely funded by the Almond Board of California — what predator/pest ratios work well.

“If our data can be used to predict mite populations, it can be used for treatment decisions, and while we’re not there yet, we’re working on it,” he says. “Unfortunately, there’s no magic app you can push on your phone that can tell you when to spray. That’s why Pest Control Advisers exist, and really good ones are greatly appreciated.”

Which Way Did They Go?
In their efforts to understand six-spotted thrips, Haviland says he and his colleague Stephanie Rill have asked the simplest questions, like: Where do they winter?

“We have looked at grassy fields, orchards, vegetable crops – you name it – and we were never able to find the six-spotted thrips in the winter,” he says.

It’s not like no research has been done. Haviland said they found a source in the literature from decades ago that described them as predators. Those researchers went to all kinds of efforts to find the overwintering thrips.

“All those efforts failed completely,” he said. “We thought that was interesting that we, and the researchers a century ago, both shared the same frustration.”

Haviland said they now believe the six-spotted thrips overwinter as adults in low populations tucked away somewhere tough to find. They reappear in spring, but there’s still no evidence where they come from. All that’s known is they hibernate during winter and re-emerge around April.

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