Many citrus growers are again finding that California red scale is slowly but surely becoming a more difficult pest to combat chemically in many orchards in the southern San Joaquin Valley.
The efficacy of chemical control for this pest has had its ups and downs. California red scale, an armored scale pest of citrus, became established in California citrus prior to the 1940s. At least since the early 1950s and into the late 1990s, high volume, dilute sprays of carbamate and organophosphate insecticides were the primary method of control.
Despite, and perhaps due to, very active chemical control programs for this pest the organophosphate and carbamate chemicals that had been effective and recommended were no longer working in many orchards by the late 1990s.
Beth Grafton-Cardwell, University of California Extension specialist, demonstrated that in orchards with the worst problem, California red scale had developed high levels of resistance to the organophosphate and carbamate insecticides. Entire quadrants of citrus trees were defoliating in the late 1990s in the southern San Joaquin Valley, where the high heat and long summers usually provide enough heat units to provide 4 generations of this pest annually.
Many citrus packinghouses installed high-pressure washers and literally blew the scale off the fruit. High-pressure washers were part of the answer, although the pressure could damage rind quality, especially later in the season, and many fruit were beyond help.
Another option that has been available for controlling scale and other pests of citrus is through an integrated pest management strategy that relies heavily on biological control. This strategy attempts to minimize the use of pesticides and focuses on preserving existing and introducing missing insects and mites that will biologically manage citrus pests.
At the heart of biological pest management for California red scale in the San Joaquin Valley is the supplemental release of Aphytis melinus, a parasitic wasp. Along with a naturally occurring parasitic wasp, Comperiella bifasciata, and other predators of California red scale like the nymph of the green lacewing, the use of high pressure washers, and the use of insecticides such as petroleum oils if necessary, very effective control of California red scale can be obtained and high quality fruit can be packed and sold.
Unfortunately, sometimes the use of biological control will result in a more scale showing up at the packinghouse. More scale on the fruit, even if they are dead scale bodies that wash off fairly easily, are not usually as easily accepted at most packinghouses. In the late 1990s, packinghouses had little choice but to accept fruit with even fairly high levels of scale, since so much chemical-resistant scale existed in citrus orchards.
In about 2000, insecticides that were based on insect growth regulators came onto the market. The use of these materials dramatically reduced California red scale populations, even in orchards that were almost totally defoliated by this pest. The principal product contained pyriproxifen as the active ingredient. When pyriproxifen was first sprayed, a single properly timed treatment of this material would control California red scale for two years or more.
California red scale control was so effective that many packinghouses, generally, began requiring that incoming fruit to be relatively scale free. By demanding fruit that has very little scale, pressure is put on growers and pest control advisers to avoid using biological control agents, like parasitic wasps, and to use materials like the insect growth regulator. Unfortunately, near total dependence on a single chemical almost ensures that the targeted pest will build resistance to that chemical eventually.
To get high levels of control of California red scale in 2005, some orchards were being treated annually or more often, with insect growth regulators. No new effective insecticides are expected to appear for control of this pest in the near or middle term.
California red scale appears to be a bigger problem in some orchards than in others. The orchards with the worst problems — in areas where some of the first California red scale appeared in the San Joaquin Valley and where resistance to organophosphate and carbamate insecticides first developed — also appear to be some of the first orchards where resistance to pyriproxifen is developing.
So what do we do?
Greater dependence on biological control and more judicious use of insecticides is probably required. Greater dependence on biological agents mandates that orchards be monitored carefully for numbers of beneficial insects and mites, level of California red scale, and the times in insect development when the scale are most susceptible to control. If insecticides are necessary, alternating use among classes is recommended if possible. In this case though, most California red scale remains resistance to organophosphate and carbamate insecticides, so alternating insect growth regulators with these is not an option.
Other growth regulators are available but have a similar action to pyriproxifen and will probably not help much with resistance management. Petroleum oil sprays can be effective, and since they smother the insect, resistance is not a problem. Total dependence of spray oil usually requires annual treatments for adequate control. These treatments also suppress other beneficial insects and mites necessary for maintaining low levels of other pests of citrus.
Spray oil control is not usually nearly as complete as occurred with the insect growth regulators at the height of their effectiveness.
Some orchards have been treated judiciously with petroleum oils, organophosphate, and carbamate insecticides for decades and resistance is not a problem. In these orchards, insect growth regulators are now being used in the same manner and provide a workable alternative to the organophosphates and carbamates.
Judicious use of pesticides is usually closely associated with depending more on biological control agents. Many growers have obtained good results for decades by relying primarily on biological control, and in packing their fruit with packinghouses that appreciate the fine line growers walk between scale control and chemically-resistant scale. Often pressure washers play an important role in these operations, to clean up that portion of the fruit that may come in with California red scale attached.
Packing houses might consider accepting higher level of scale infestation on fruit, realizing that cleaning up perhaps a few more slightly-infested fruit now, is preferable to dealing with the high levels of badly infested fruit that were seen in many orchards six or seven years ago.
Those orchards where red scale control is almost totally dependent on pyriproxifen, and where resistance to organophosphate and carbamate insecticides remain high, may soon be facing the kind of fruit infestation and tree defoliation in some blocks that was seen in the 1990s. Experiments in orchards in the southern San Joaquin Valley, where resistance to the old insecticides remains high, have shown that effective biological control can be instituted in a relatively short period of time. People knowledgeable on how to do this are available in the citrus industry.
Making the switch to a more biologically-oriented program in blocks that have scale that are beginning to show resistance to pyriproxifen is probably prudent.