The answer to the questions how many angels can fit on the head of a pin and how many glassy-winged sharpshooters (GWSS) does it take to reduce citrus yields are very similar.

A bunch.

University of California, Riverside entomologist Raymond Hix has never researched the angel-on-a-pinhead issue, but he has determined that it takes 1,000 or more GWSS to reduce navel and Valencia orange yields.

GWSS populations of that level on citrus groves anywhere near a grape vineyard would send the grape grower screaming through the vineyard because it takes only one Pierce's disease-carrying GWSS to infect a vine and potentially destroy a vineyard.

That is why federal and state governments are pouring millions into controlling GWSS in areas where both citrus and grapes are grown.

However, Hix told a Citrus Research Board growers educational seminar in Dinuba, Calif., that the feds and state government will not be in California forever to control GWSS and citrus growers need to know how many GWSS it takes to cause damage.

Pierce's disease does not infect citrus, but there is a disease threat for California citrus. It is citrus variegated chlorosis (CVC). It is found primarily in Brazil, Argentina, Uruguay and Costa Rica. If the disease ever reached California, the combination of the bacteria and the vector will cause serious damage to the citrus industry. In South America, CVC has caused severe economic losses.

Even without a diseases threat, high levels of GWSS feeding can reduce yields and size distribution, according to Hix.

In the second year of Hix's study, the yield reduction at population levels of more than 1,000 GWSS per tree was 50 percent. The effect of GWSS feeding was cumulative over the two years of the study as the magnitude of the changes increased in magnitude and significance from the first to the second year of the study.

To control such high levels, the systemic Admire can be applied in May through drip irrigation systems. It takes three to six weeks for the systemic pesticide to control GWSS. Hix said high population levels in July and August are the most damaging.

If the citrus with high levels of GWSS are adjacent to disease-susceptible grapes or almonds, Admire should go on in March or April to control movement of GWSS to those crops.

e-mail: hcline@primediabusiness.com