San Joaquin Valley citrus growers cannot be too aggressive in their battle against the Asian citrus psyllid (ACP).

University of California citrus expert Elizabeth Grafton-Cardwell wants citrus growers and their crop consultants to take a more active role in sampling and treating for the tiny pest.

“I’m already getting a lot of push-back from PCAs because they say they’re already over-burdened and can’t do the sampling,” Grafton-Cardwell told a lunchtime audience in San Bernardino.

Growers and PCAs are told not to rely on the yellow sticky cards used by regulatory agencies to sample for ACP and other pests as an indicator of ACP infestation. Grafton-Cardwell cites the Dinuba, Calif. find from earlier this year as a classic example of how poorly the traps do.

The Dinuba find discovered hundreds of psyllids – a breeding population of all ACP life stages – on two small tangerine trees in a residential yard while sticky cards on two adjacent properties had one psyllid each in them.

University experts are suggesting growers and PCAs walk the borders of their orchards and sample 50 trees – 10 from each side and 10 trees in the middle of the grove – with either a tap-sample method or a sweep net.

Tap sampling requires a squirt of Dawn detergent mixed in a half-liter bottle of water and a white, plastic surface upon which to knock debris from a branch onto with a large stick or piece of PVC pipe. Count the number of winged adult psyllids collected on the clipboard.

A better method of sampling, according to Grafton-Cardwell, is a sweep sample. This is not the typical sweep netting practice used to collect flying insects, but a method wherein an entire citrus branch is stuffed into a net and the branch vigorously shaken.

According to Grafton-Cardwell, two-to-three times more ACP adults can be collected in a net using this method, meaning it is much more effective at determining the number of adult psyllids on a particular branch.

Visual sampling with hand lenses will still be necessary to inspect for nymphs and eggs.

Treatment methods

The San Joaquin Valley (SJV) of California is still in an eradication mode related to the ACP. That designation could soon change with additional psyllid finds.

Grafton-Cardwell expects the flush that comes about with the coming cooler weather could reveal what she already suspects: that the SJV already has a significant population of psyllids just waiting to be discovered.

“I think we have an established population and we’re just keeping it really low right now because we have weather conditions that are not as conducive to the psyllid,” Grafton-Cardwell said.

“Another reason our numbers might be low is because citrus growers spray for other pests with products that are effective against the psyllid,” she continued.

Chemical treatment guidelines may need to be rethought for some growers in certain growing regions, Grafton-Cardwell says. The combination of two broad-spectrum pesticides tends to work best, she said.

While there are products licensed for use with certified organic crops, Grafton-Cardwell says they are ineffective in an eradication approach. However, the softer chemicals could be utilized in an area-wide treatment approach if everyone uses the same chemicals at the same time because the softer compounds still have an ability to kill psyllids on contact.

For those going with a conventional treatment approach, timing is critical, Grafton-Cardwell says.

For example, a systemic product does not work well in the winter months because of the colder temperatures. At this time it’s perfectly acceptable to do a single-pesticide application.

“I’d prefer it to be a pyrethroid as those are better in the cold weather,” she said.

“At other times of the year I’d like to see two insecticides applied for the eradicative approach,” she continued. “I’d prefer to see growers start with a pyrethroid or some other broad-spectrum products to knock down the population then follow up with another product.”

The University of California’s IPM website has updated treatment recommendations.

She also recommends waiting until summer to apply Imidacloprid if the intent is to kill psyllids.

Area-wide treatment

Citrus experts say a new area-wide approach that is being talked about won’t begin in the San Joaquin Valley until more psyllids are discovered. This is one reason experts recommend an aggressive sampling approach: to determine just how impacted commercial citrus is with the ACP.

Without that aggressive sampling approach, a hidden ACP infestation may go undetected, and therefore not be addressed in a timely manner.

The goal of area-wide treatment is to capitalize on what studies show to be a more effective than the current 800-meter treatment approach following a psyllid find.

While area-wide control methods can be more costly because they can multiple treatments, Grafton-Cardwell expects the overall effectiveness of this approach to be greater.

Experts like Grafton-Cardwell have recommended the area-wide approach in Ventura County for some time now because of the rapid growth in psyllid finds and the discoveries of psyllids in traps at area packing houses. Still, growers and the citrus industry in the Ventura region continue to move ahead with an eradication-style approach to treatment.

A greater presence of organic orchards and organic farming practices in that region is making the more chemically-reliant approach used by much of the San Joaquin Valley much harder there as well.

Regardless of management practices – whether organic or conventional – Grafton-Cardwell recommends the citrus industry come up with a standardized sampling method for detecting psyllids and quick adoption of area-wide management because of its benefits in both systems.

Even the use of softer Ag chemicals and those approved for organic production can have a lethal impact against the ACP if used over a wide area all at the same time, Grafton-Cardwell says.

All of this is still seen as a method to “buy time” as researchers across the United States look for more early-detection methods to discover HLB and ultimately a cure for the disease.

So far there are some early detection methods that seem to work in lab experiments, and some other methods that Grafton-Cardwell says seem to prolong the life of trees infected with HLB in a research setting.

Dodging bullets

California officials have been sampling for some time now the psyllids caught in traps, as well as live adults and nymphs captured in southern California and elsewhere for the deadly HLB bacteria. Aside from the 2012 HLB-positive tree in Hacienda Heights, no other tree samples or pest samples have tested positive for HLB.

This has stymied researchers because it took less than 10 years for all of Florida’s citrus groves to become infected with HLB once the first case of citrus greening was discovered in 2005.

According to UC Riverside Entomologist Mark Hoddle there are still many unknowns related to HLB infection in the ACP.

For instance, Hoddle says one lab study suggests about 3 percent of psyllids are infected with the bacterium that causes HLB by the mother when they are laid as eggs.

According to Hoddle, researchers do not know whether that occurs also in nature or is simply an artifact of lab studies.

What researchers do know is that the most potent way for the ACP to get HLB is as a nymph feeding on infected plant material. Once it has acquired the bacterium it is infectious for life, Hoddle says. Uninfected adults that acquire the HLB-causing bacterium as adults don’t transmit the bacterium as readily as adults that were infected as nymphs, he said.

“I don’t think we really understand the reason behind this,” Hoddle said.

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Another factor in California’s ability to stave off the HLB disease may be in its regulatory inspections and cleanliness requirements related to nursery stock, according to Silvie Robillard, an ACP/HLB liaison with the California Department of Food And Agriculture’s Citrus Pest and Disease Prevention Program.

Those standards, according to Hoddle, were not in place in Florida when HLB was first discovered in 2005. The sale of infected citrus plants in commercial and retail settings throughout Florida, coupled with Florida’s early thought that they did not have the disease allowed HLB to spread throughout the entire state in under a decade.

While the disease is not suspected to pose a human-health risk, the effects of it in the bittering of fruit is a marketing concern that some say has already begun to impact Florida’s famed juice industry with inconsistent flavored products already being discovered in grocery stores.