When I first began working as a farm advisor in the mid 1990s many citrus orchards in Kern County were heavily infested with California red scale. Dr. Beth Grafton-Cardwell, University of California Research and Extension specialist, was instrumental at that time in determining that this pest was resistant to the carbamate and organophosphate insecticides that were being used to control it.
Parts of entire orchards were defoliating in the worst cases and some growers were attempting to treat for this pest two or three times in a season, often, with unsatisfactory results.
In the late 1990s, two insect growth regulators (IGRs) were given emergency use exemptions and one was fairly-quickly fully registered for control of California red scale. This registration came about partly as a result of experiments conducted by others and myself in Kern County. At the time, many knowledgeable researchers and other professionals in the field of pesticides and scale control expressed concerns that California red scale would become resistant to repeated sprays of most chemicals and that the new IGR would not be an exception.
Previous experience in other countries had demonstrated that resistance to some IGRs occurred after a few years of use. However, the California red scale problem was so severe in some locations and the new registered IGR was so successful in controlling red scale (often a single spray would control scale for two years or more) that many growers understandably began using the material. I was pleased to see it registered myself. It was and is a useful tool to have in the arsenal.
The new IGR did such a good job that some fruit packinghouses have become much stricter on the amount of California-red-scale infested fruit that they accept for their premium markets. Growers relying primarily on beneficial insects such as Aphytis wasps to control California red scale in their fields and on the high pressure washers of the packing houses to clean up fruit, were soon, and in some instances still are, at a comparative disadvantage.
Some growers and pest control advisors who had been learning the ins-and-outs of a more biologically intensive integrated approach to pest control because they had no other effective option, began to rely instead almost exclusively on the new IGRs because it was easier. The onset of the glassy-winged sharpshooter further disadvantaged growers that were relying primarily on beneficial insects to control California red scale.
Mandatory sprays for glassy-winged sharpshooters were not beneficial to beneficial insect populations.
In my official travels around Kern County as a University of California citrus farm advisor, I have frequent opportunities officially and unofficially, to observe the degree of California red scale in many citrus orchards. I think it is fair to say that the IGR is not working as well in some orchards as it did originally. This observation should not come as a surprise to anyone. The emergency exemption that originally allowed this material to be used in California stated that it could only be sprayed once in two years to reduce the chance for resistance to develop in California red scale. Annual sprays are allowed under the current label.
The locations where this pesticide appears to have reduced efficacy is where it has been used the most and where we had some of the worst red scale problems in the mid 1990s. Generally, I see good scale control in orchards were growers and pest control advisors use pesticides only after carefully evaluating the degree of infestation (and potential infestation) of California red scale in relation to the activity (and potential activity) of the beneficial insects that are released or are naturally occurring. This information is developed through insect trapping, weather monitoring, close observation of fruit for California red scale life stages and the stages of its parasites, past experience with this pest, other pests of citrus and their natural enemies.
Insecticides, including IGRs, may be used in these fields, but only when biological control has been upset and recovery is not expected before harvest. The program to control glassy-winged sharpshooter has been so successful in Kern County, that while pesticide sprays are sometimes required for this pest, the frequency of application has been greatly reduced.
The possibility again exists to expect supplemental releases of beneficial insects like Aphytis wasps in combination with the naturally occurring beneficial insects to carry the brunt of red scale control. Relying solely on pesticides, any pesticide, may soon put the grower, and eventually the fruit packer, in the same unfortunate situation that we had in many orchards in Kern County in the mid 1990s.