Researchers look for ways to control cucumber beetle

After years of trials from the Central Coast to the Sacramento Valley, University of California researchers say they are beginning to get a handle on cucumber beetles, which are most destructive to smooth-skinned melons but can also reduce stand establishment of many green vegetables.

The key tool developed for the studies is a kairomone-baited sticky trap that pest control advisors can use as an easy and economical monitoring tool. The floral scent that attracts the pest is available commercially, and the container is a plastic cup covered with yellow sticky tape.

"We used this yellow trap successfully in our study of the number of generations, and in many of our other studies. The floral attractant trap still works fine when you have a full-flowering field. The lure is available commercially, and the trap is just a plastic cup. This trap does tell us what is going on in the field. We think the trap might be a cheap, easy monitoring tool for PCAs," said Larry Godfrey, UC Cooperative Extension entomologist.

The California Melon Research Board made the cucumber beetle its No. 1 insect pest priority a decade ago, when PCAs were using ever-increasing amounts of carbamate and pyrethroid insecticides and still seeing significant damage to the surface of smooth-skinned melons like honeydew and watermelon.

But the most widespread cucumber beetle can feed on many crops.

"The spotted cucumber beetle feeds pretty much on anything—lettuce, leafy greens, peppers, eggplants and other crops. They can feed on seedlings or blossoms. Cucumber beetles have been around forever. They are an omnipresent pest; they are not picky about what they feed on," Godfrey said.

In areas where beetle infestations, are reducing stand establishment, seed treatments look to be a viable option.

"We had about twice as many seedlings where we used the Cruiser seed treatment, and the plants were bigger. The larval stages feeds on roots. The striped cucumber beetle feeds on cucurbits, and the spotted cucumber beetle will feed on a lot of crops," Godfrey said.

The cucumber beetle can be found anywhere in the Central Valley as far south as Fresno County—the desert is too hot for this pest—and in warmer areas of the Central Coast like San Benito County.

Another promising management technique might be to use alfalfa as a trap crop, because the cucumber beetle is drawn to this host but does little or no damage to the crop.

"Alfalfa is a good host, and we might be able to use it as a trap crop. The pest likes alfalfa, and it doesn't really do significant damage," Godfrey said.

The first attempt to test this promising theory in the Winters area was, however, unsuccessful, as management of alfalfa as a trap crop did not protect a nearby honeydew field.

"When alfalfa next to honeydew melons was cut, the cucumber beetles moved to the honeydew, we could document that with our traps. But when the grower left a strip of alfalfa uncut, it didn't help. I would like to try that again, because I think there is promise for alfalfa as a trap crop," Godfrey said.

Researchers are also continuing their study of a parasitic fly that might be able to help control cucumber beetles.

The greatest challenge is finding a way to reduce the cosmetic damage this beetle causes by feeding on the skin of smooth melons.

"They damage the surface of smooth-skinned melons, like honeydew and watermelon," Godfrey said.

They are particularly damaging to the bottom side of the melons, where they are well shielded from insecticides.

Researchers tried the spinosad Success, the carbamate Sevin and the pyrethroid Warrior in a trial on honeydew melons on the UC Davis campus.

"There was no effect on yield. The beetles aren't really affecting yield, they are causing damage to the surface," Godfrey said.

The pyrethroid was the most effective material, dropping damage to the surface of the topside of the melons from 10 percent to 4 percent. But even with the pyrethroid applications, damage to the skin on the bottom of the melons was more than 10 percent.

"The soil we were working with was pretty cloddy, which gave the beetles entry points to the bottoms of the melons," Godfrey said.

The results of the trial were particularly discouraging for organic growers.

"We saw that Success didn't work to control this pest. Surround and Grandevo also gave little control," Godfrey said.

The sticky traps have helped researchers learn where this pest is at different times of the year, and how fast it multiplies.

"We wondered where they were in the winter, and found every time we sampled, they were in alfalfa. We also know they are in the foothills in the winter. The first thing we did was look at the life cycle of these two species and found there are roughly four generations a year," Godfrey said.

There are two cucumber beetle species found in California, the western spotted and the western striped. While the western striped usually feeds on cucurbits, the western spotted attacks a wide variety of crops.

The spotted cucumber beetle can complete its entire life cycle on alfalfa, melons, maize, tomatoes and beans, and, probably, also on many other crops, he said.

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