Reflective mulch may fight citrus psyllids that transmit HLB

08/06/2013 10:45:00 AM
Tom Burfield

With the help of some local citrus growers, the University of Florida is literally shining some light on Asian citrus psyllids in the hope of making them go away.

Researchers have found that reflective metalized polyethylene film used as mulch beneath newly planted citrus trees causes the dreaded psyllids to become disoriented and impairs their ability to find the host plant. The pests are vectors for huanglongbing—or citrus greening—which has devastated much of Florida’s citrus crop.

Phil Stansly, an entomology professor based at the Southwest Florida Research and Education Center in Immokalee, and graduate assistant Scott Croxton both have had experience with reflective mulch in combating whiteflies and thrips in vegetable fields and thought it might work with psyllids.

They found that the reflective mulch actually provides two benefits for citrus trees.

First, it helps keep the trees free of psyllids.

Second, the mulch—together with a fertigation system that goes with it—helps trees grow more quickly and come into production sooner than they would otherwise.

The first tests were conducted in single plots at the experiment station. Now, the university is testing the mulch on 15 acres, and there are up to additional 25 acres of commercial testing.

Multi-pronged test

In addition to evaluating the effectiveness of reflective mulch, the tests also looked at bare ground, which served as a control, and white-faced polyethylene mulch.

“White face doesn’t repel the psyllids at all,” Stansly says, although it does provide other advantages, such as weed control and moisture and fertilizer retention.

Although testing will continue for at least two more years, Stansly says positive results already have been detected.

“I think that by using the technology, you can delay the onset of greening, reduce the incidence of greening and accelerate tree growth,” he says.

In a summary of the first tests, Stansly and Croxton found less than half as many young shoots were infested with psyllids on young trees growing on reflective mulch compared with trees on white-faced mulch or bare ground.

More than three times as many psyllids were captured by sticky cards between trees on bare ground compared to those on reflective mulch.

In one test, the incidence of HLB was four times greater in trees growing on bare ground and white-faced mulch than on metalized mulch.

The researchers did not use systemic insecticides that typically are used to control psyllids. They believe the rates of infection would be even lower if they had.

Commercial test

The Packers of Indian River Ltd. in Fort Pierce has been testing reflective mulch for about a year on 8 acres of newly planted grapefruit trees, says Thomas Stopyra, technical adviser.

Four acres are in Fort Pierce and 4 acres are in Charlotte County.

The test is in its early stage, but Stopyra shared some preliminary observations.

Perhaps the most impressive findings to date have been the results of tree trunk measurements taken in early July.

Researcher Croxton found that trunks of trees grown with reflective mulch were 21 percent larger after one year compared with other trees, Stopyra says. He says similar results were found in a test at the research station.

Also, only about half the typical amount of fertilizer was used.

If that trend continues, Stopyra says, “That could be huge.”

However, Stopyra also found that the mulch gives off no reflection on cloudy days.

“It doesn’t confuse the psyllids, so they can still find the trees,” he says.

Stopyra says he is “pretty diligent” about using a trunk drench to protect the trees from psyllids, and so far, he has not seen any signs of psyllid damage.

A 10-acre trial

Oviedo-based A. Duda and Sons Inc. is cooperating with Stansly with its own 10-acre trial that began in July 2012, says Joby Sherrod, research and development manager.

It’s too soon to tell how much of an effect the reflective mulch will have on psyllids and citrus greening, but Sherrod says it is having a positive effect on tree growth.

“It certainly has an effect on the flushing habits of the trees,” he says.

Sherrod says he hopes that using reflective mulch will help reduce the psyllid populations early on, but he emphasizes that mulch alone will not wipe out the psyllids.

“It’s an additional cultural practice for us to evaluate," he says.

Duda continues to use chemical control in its regular grove operations; in the trial, there are both chemical and nonchemical treatments.

The benefits of the reflective mulch will be limited to about a tree’s first three years, Sherrod says. After that, the foliage will prevent the sunlight from reaching the mulch, eliminating its reflectivity.

He says he’s hopeful that the mulch will delay the tree’s exposure to greening by deterring psyllids from feeding on the tree. The earlier a tree is infected with greening, the more severe it affects the tree.

“Our goal is to try to prevent infections, at the very least, in the first couple of years,” Sherrod says. Ideally, the infection will be prevented beyond that time.”

Technology tweaked

Stansly already has improved the technology involved in laying the mulch.

In early tests, the mulch was laid flat.

“That’s really not a good idea because you get a kind of depression in the middle of it, and it accumulates dirt and becomes less effective,” he says.

He suggests raising the bed a few inches and allowing it to be crowned, creating a slope to allow water to roll off.

The mulch currently is being put down by equipment that was designed for use with vegetables in a 3-foot swath. But Stansly learned that the wider the mulch bed is, the more effective it is.

“We need to modify the equipment so we could have maybe a 6-foot bed,” he says.

The jury still is out on whether the process will be economically viable.

“There is a cost involved, and there are benefits,” he says. “All those costs and benefits need to be accounted for in a quantitative way before growers jump in feet first,” he says.

The mulch itself costs about $300 per acre, depending on what kind of spacing is used.

Stansly recommends using mulch that is stronger than the kind used for vegetables – at least 3 mil with a clear coating on top so that it can protect the aluminized layer.

The program requires drip irrigation, and growers need to provide freeze control, which requires a dual system that includes a microjet sprinkler. That will add $200 to the cost.

Stansly emphasized that the reflective mulch is not a standalone solution.

“When you use it together with insecticidal control, you get much better control of the insects,” he says.

The process can slow the spread of the disease at a critical time in the life of a tree when it is very susceptible to greening while at the same time accelerating its growth, he says.   

“We feel that the first two or three years are critical,” Stansly says. “If we can bring it into production a year earlier than otherwise, it would be well worth it.”



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