Parasitic wasps sought as SJV citricola control

Entomologists at the University of California, Riverside hope to augment populations of tiny, egg-parasitizing wasps as a biological control for citricola scale, a serious pest of citrus in the San Joaquin Valley.

Robert F. Luck, professor of entomology at UC, Riverside, and project leader, discussed the concept recently at a fall citrus meeting at Tulare, Calif. He has submitted a proposal for funding additional research on it to the California Citrus Research Board.

Citricola scale is an economic pest in citrus when a severe infestation reduces tree vigor, kills twigs and reduces flowering and fruit set.

Honeydew excreted by citricola accumulates on leaves and fruit and attracts sooty mold that hampers photosynthesis and degrades fruit quality.

Although citricola is controlled by oil or pesticide sprays, Luck and others reason that growers may need a biological alternative if certain pesticides, especially those associated with volatile organic compounds, are removed from the market and replaced with more expensive products in the future.

The basis of Luck's research is that citricola is rarely a problem in Southern California citrus, where it is naturally controlled by a number of parasitic Metaphycus wasp species. The parasites exist in the SJV but not in sufficient populations to control citricola there.

“One of the problems we have with citricola in the SJV,” Luck said, “is we don't have enough brown soft scale that is generally distributed in Southern California.”

He proposes to develop ways of encouraging higher “background” populations of brown soft scale to sustain Metaphycus wasps, which live only a month or so, during parts of the year when citricola stages are not vulnerable to them.

Brown soft scale has multiple generations each year, providing a constant food source for the wasps, which parasitize the scale's eggs.

On the other hand, citricola produces only one generation per year in the SJV. If wasp populations, sustained by brown soft scale, are numerous enough when the vulnerable early stages of citricola appear, Luck contends they can control citricola in the SJV just as they do in Southern California.

Control of ants, natural enemies of the wasps, is another element of the concept and helps keep the brown soft scale in check but still adequate to support the wasps.

Timing of releases of Metaphycus would be scheduled to fit with other practices such as releases of Aphytis wasps for control of red scale.

“In pest management,” Luck said, “we have to think about more than one pest. Using a control strategy for one thing and letting another go doesn't save money, it produces problems.”

In additional field research, Luck wants to confirm which Metaphycus species are most active against citricola and to develop a maintenance program for releases in late winter to early spring when scale numbers are low.

The experiment would be to place Yucca plants carrying brown soft scale parasitized by the wasps in citrus groves in Kern, Tulare and Fresno counties.

Jim Thompson, Extension specialist in postharvest operations at UC, Davis, spoke to the gathering about his Citrus Research Board-funded studies on a new method of identifying freeze-damaged oranges by their rind damage.

The centerpiece of the method is a device to detect ethanol, which appears on the rind within hours of a frost event and remains for weeks afterward.

It is the same compact, battery — operated technology used by law enforcement agencies to detect drivers under the influence of alcohol.

Ethanol is produced on the surface of the damaged fruit and fluoresces as yellow under an ultra-violet “black light.” Decay shows as solid spotting, peel-miner damage shows as trails, and freeze damage shows as speckling. The damage can be read from the rind fruit in about 15 minutes.

UC has a provisional patent on the method, which is best done in the temperature range of 50 to 70 degrees F. Gibberellic acid applied to the fruit does not create appreciable effects on the results.

Thompson said his investigations thus far show the ethanol method is objective and has an accuracy of 80 percent to 90 percent.

He said it is far superior to the California Department of Food and Agriculture's manual method of slicing and visually inspecting fruit samples for internal freeze damage.

In the CDFA test, he noted, if 15 percent of a batch is found to be damaged, the entire batch is rejected. It requires trained personnel, who are becoming fewer n number.

The active ingredient in the ethanol method, Thompson said, is a called tangeretin, a non-volatile compound that remains detectable on the rind for weeks following injury. It shows in high amounts on oranges, grapefruit and some tangerines, but since it has low levels on lemons and limes, it may not be applicable to them.

Another application is use of portable black-light lamps to inspect oranges in the field for early estimates of frost damage.

Thompson urged growers to ask their packinghouses to contact UC researchers to learn more about how the system could be easily incorporated in packing lines.

Human taste panel research is scheduled for next year to compare the system with the CDFA method.

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