As if the glassy-winged sharpshooter alone weren't enough of a hazard for the multiple crops it attacks, treatment for it with broad-spectrum materials invites secondary pest problems in citrus, according to a University of California entomologist.
Beth Grafton-Cardwell, of the Kearney Agricultural Center at Parlier, says materials such as organophosphates, carbamates, and pyrethroids, and the systemic Admire, gave the best control of GWSS in her research and that of the Kern County IPM Demonstration Project.
“Thus,” she said, “pesticides applied for GWSS will be broad-spectrum and will disrupt natural enemies of other citrus pests, potentially causing secondary pest outbreaks.”
She reported on her work for the Citrus Research Board in the board's 2000 annual report after screening several materials for GWSS, armored scale, soft scale, fire ants and other pests of citrus, along with products selectively favoring natural enemies of the pest group.
85 percent kill
In the GWSS research, partially funded by the Kern/Tulare GWSS Task Force, she found “soft” pesticides such as Bts, Esteem, Success, Veratran, and Agri-Mek had little effect on the insect.
She applied Admire through irrigation, saw an 85 percent kill, and observed it took three to four weeks to kill GWSS nymphs. However, the systemic's effect lasted longer than those of the broad-spectrum insecticides she evaluated.
“Admire was applied in April and remained effective in reducing egg-laying and adult survival through September. The foliar treatments tended to lose their effect on GWSS after about two months,” she said.
Products Assail, foliar-applied, and Admire, soil-applied, she said, “seem to be the best neonicotinoids for California red scale.”
In her 1999 trials she applied Admire through microjet sprinklers, and achieved the greatest control with a two-hour pre-irrigation, a two-hour release of insecticide, and a four-hour post-irrigation.
Also during the 1999 season she compared controls for citrus red mites and found that the greatest suppression was with oil and Nexter treatments.
Trials for control of citricola scale showed that Lorsban, Actara, Provado, Assail, and 1.4 percent oil reduced the scale with a 75 to 92 percent kill. Grafton-Cardwell plans to continue these tests to measure long-term efficacy of the materials.
In another project for the board, she provided more data on the role of aphids in transmission of citrus tristeza virus in the San Joaquin Valley. Aphid traps in three Fresno County and seven Tulare County orchards have been in place since 1996.
30 species in citrus
“To date, we have identified more than 30 species of aphids flying into citrus,” she said, adding that aphid traffic is greatest during the spring and fall growth flushes, during which the amount of disease is also greatest and most likely to be spread.
Colonizing aphids, particularly cotton, but also spirea, green peach, and black citrus, are known to be capable of transmitting the disease and are more common in Tulare County than the western portion of Fresno County.
That, she said, accounts for the greater incidence of tristeza, which varies year to year by temperature and tree growth, in Tulare County.
Detecting and removing infected trees remains the most effective means of reducing the disease and preventing introduction of more severe strains of it.
Ahead is more study, directed by molecular biologists, to monitor cotton aphid and spirea aphid to learn if different tristeza isolates occur in different trees, whether the cotton aphid transmits something different from the spirea, and whether the aphids carry something different from what is carried by graft transmission.