Stopping destructive leafhoppers is goal of research

Processing tomato growers are supporting numerous efforts to find management practices that could make their plants less attractive to the insect pests that transmit the sometimes-devastating curly top virus.

Beet leafhoppers overwinter in the foothills near the Central Valley, sometimes in epidemic numbers, contract the virus from weeds or other plants, and then carry the curly top virus to nearby tomatoes, which are sometimes stunted and die.

There may be ways to cut down on pressure from this disease by making the tomato plants less attractive to the insect vector.

One trial showed that, when pressure is heavy, a simple spray application of kaolin clay to the base of the plants can reduce the number of leafhoppers in the vicinity of the crop.

Another study is showing that organic soil, properly managed, may produce tomato plants that are less attractive to the pest that carries curly top virus.

And a team of researchers at the University of California, Davis, is looking at the possibility that reducing symptoms of stress from transplanting can also make tomato plants less attractive to beet leafhoppers.

"It's been shown that plants that are stressed are more attractive to insect vectors," said Clare Casteel, UC Davis assistant professor of plant pathology. "If you inhibit the stress response to transplanting, you should make the plants less attractive to leafhoppers."

The first study on tomato transplants in a test field on the Davis campus last year showed that ethylene inhibitors reduced the number of insect pests attracted to the plants.

"When plants have stress, they release hormones; one of them is ethylene," said Aurelie Bak, UC Davis postdoctoral researcher. "We found that when they release ethylene, they are more attractive to pests, and have lower defenses."

Numerous commercially available ethylene inhibitors, like Retain and Smart Fresh, are already widely used to slow down senescence of a wide variety of fruit and flower crops, and could be sprayed or applied through drip lines.

One goal is to have the inhibitor last long enough to outlast the stress reaction to transplanting, but not so long as to interfere with the ripening process.

That sweet spot looks to have been achieved last year in California Tomato Research Institute-sponsored ethylene inhibitor trials on tomatoes in test plots on the Davis campus.

"We saw an impact to susceptibility to being attacked by insects, but not to ripening," Casteel said. "We saw a decrease of insect populations, especially of leafhoppers."

This year, the researchers are taking the trial to a commercial processing tomato field in the San Joaquin Valley, in conjunction with UC Cooperative Extension farm advisor Brenna Aegerter.

In addition to measuring the impact of ethylene inhibitors on leafhopper and thrips populations and strikes, the researchers also hope to learn if there can be a potential economic benefit from reduced insecticide use.

A related study of soil management systems to produce more resilient plants began after a UC researcher saw that a Sutter Basin conventional processing tomato field was hit hard by curly top while an organic field just across the road was relatively unscathed.

Tests then showed that given a choice between the conventional and organic plants, the leafhoppers overwhelming struck the conventional tomatoes, which prompted an ongoing study of why the organically managed plants were less attractive.

"We are far from having a soil management program in place, but we just know that organically-grown plants are less attractive," said Amelie Gaudin, UC Davis assistant professor of agroecology.

Gaudin said she is continuing the study of how soil management can produce more resilient tomato plants. With continuing financial support being provided by the growers' research institute, she will publish an analysis of the main drivers after this year.

Yet another CTRI-financed study shows it may be possible to reduce leafhopper strikes by making the ground look less attractive to the pests.

Various mulches, including spray-on kaolin clay, successfully reduced the attractiveness of tomato plants to leafhoppers in a 2016 southern San Joaquin Valley trial, but those results were not repeated under relatively light pest pressure last year.

"Use of silver reflective mulch is a proven method of repelling many insect pests away from crops," said Joe Nunez, UCCE farm advisor based in Bakersfield. "Surround, kaolin clay, was used as a spray-on reflective mulch onto the top of the tomato beds."

These mulches made a dramatic difference in 2016 in the number of leafhoppers captured near the tomatoes, he said.

"There were significant differences among treatment means for total leafhopper counts and total beet leafhopper counts," Nunez said.

Kaolin clay sprayed on the ground at the base of the plants reduced leafhopper trap counts by well over 40 percent, while the more expensive silver reflective mulch cut down the pest counts by 60 percent.

"The silver reflective mulch had a highly significant effect in repelling the amount of beet leafhoppers away from the tomato plants," Nunez said. "But the Surround as a spray-on mulch and the green dye turf paint were also able to significantly repel beet leafhoppers away from the tomato plants."

Plants protected by kaolin clay on the ground produced better than 15 percent more tomatoes by weight, and the reflective mulch plots yielded even more than that, he said, but in the relatively small plots the differences were not statistically significant.

Nunez tried the kaolin clay again in 2017 on larger tomato plots, hoping that he would see a greater impact. But because the pest pressure was light last year, the mulch did not reduce leafhopper trap catches.

(Bob Johnson is a reporter in Davis. He may be contacted at bjohn11135@aol.com.)