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.