The California red scale problem:
California red scale, Aonidiella aurantii, is an important economic pest of California citrus. Armored scale cause not only cosmetic damage to the fruit, resulting in downgrading or rejection at the packing house, but also cause yellowing of leaves, defoliation, branch dieback, and possible tree death when pest densities are high. Fruit that is sent to the juice market does not bring in sufficient returns to pay for cultural and pest control practices that occurred earlier in the season. Thus, growers experience great losses when the fruit is juiced or culled. To prevent losses due to scale infestations, San Joaquin Valley California citrus growers have depended primarily on broad spectrum insecticides.
Insecticide resistance in California red scale:
Recently, populations of California red scale in various areas of the San Joaquin Valley have been found to have resistance to all of the currently registered organophosphates, chlorpyrifos (Lorsban) and methidathion (Supracide), and the carbamate carbaryl (Sevin). Whereas a single application of an organophosphate or carbamate was effective in reducing scale populations for 1-2 years when these insecticides were first introduced, by 1997, some citrus growers in the San Joaquin Valley were applying 3-4 applications of these insecticides per year and still not achieving control because of resistance.
Over 280 laboratory bioassays were conducted to detect organophosphate and carbamate resistance in armored scale in the San Joaquin Valley during 1990-97. The majority of these orchards, were located in Tulare and Kern counties. Of the Fresno, Tulare, and Kern county orchards tested, 61%, 75% and 59%, respectively, had at least low levels of resistance to one or more of the broad spectrum pesticides and 38-46% had very high levels of resistance requiring multiple applications of pesticides. Our sampling was biased towards locations with resistant scale, because Pest Control Advisors led us to problem orchards and so resistance was likely to be a problem. It is difficult to estimate the exact percentage of citrus acreage with a pesticide resistance problem. However, given the data that we have, I estimate that 35-40% of the 150,000 fruit-bearing acres of citrus grown in the San Joaquin Valley of California are affected by organophosphate and carbamate insecticide-resistance problems. These orchards required multiple applications of insecticides in 1997 and ended the season with a significant percentage of fruit that is downgraded in the packinghouse.
Field trials have demonstrated that if laboratory bioassays detect highly resistant scale, growers achieve only 1 generation of scale control or complete lack of control in the field. Scale have four generations per year in the San Joaquin Valley and so resistance problems result in multiple applications of broad spectrum insecticides in an attempt to reduce each generation. Even with multiple insecticide applications, fruit frequently become encrusted with scale and end the season with a significant percentage (up to 48%) of fruit that is downgraded in the packinghouse if the grower does not have a high pressure washer available to remove the scale. Even with a high pressure washer, heavily scale-infested fruit are not completely cleaned and may suffer some downgrading.
When resistance monitoring was first initiated in 1990, resistance appeared to be patchy. During the ensuing 6 years, resistance increased and developed into localized hot spots consisting of 50-150 contiguous orchards. Localized hotspots have been found in Fresno County (Orange Cove), Tulare County (Cutler, Orosi, Lindcove, Ivanhoe, Exeter, Lindsay, Strathmore, and Terra Bella) and Kern County (Richgrove and Edison). Other hotspots may exist, however, they have not yet been sampled. Not surprisingly, the areas of greatest resistance were where the scale have infested trees for a long period of time and where the pest management strategy has depended upon insecticides. Growers have relied upon organophosphate and carbamate insecticides since the late 1950s, and so it is not surprising that the scale have developed resistance to these insecticides.
The high cost of scale resistance to insecticides:
Armored scale insecticide applications in citrus are extremely expensive compared with other pests. This is because the once the first instar scale settle, the female scale do not move and so insecticides must be applied in high volumes of water (750 to 2000 gallons/acre) to penetrate the foliage and move the insecticide to the insect pest. In addition, spray rig speed must be kept to 1.5 miles/hour to keep the leaves of the citrus tree from forming a sheet and a barrier to spray penetration. The application cost averages $90/acre for a 1500 gal/acre ($0.60/gal of water), 1.5 mph spray application. The cost of the insecticide is $65-75/acre. Thus, the full cost of an armored scale spray averages $160/acre. In situations where resistance was a serious problem, growers were applying insecticides 2-3 times per season. Thus, scale control alone was costing $320-480/acre and these growers suffered losses because a percentage of the fruit was still encrusted with scale.
Alternative soft insecticide program:
The current alternative to broad spectrum insecticides, is biological control in combination with oil sprays (when needed). In southern California, where winters are mild, a group of natural enemies including the parasitoids Aphytis melinus and Comperiella bifasciata, as well as predatory beetles work together in conjunction with occasional oil sprays to provide effective control of armored scale. The parasitoids prefer to oviposit in 2nd- and 3rd-instar scale. Because of the extensive overlap of generations of scale in southern California, there are scale of the appropriate age-classes available for parasitism year round. In contrast, in the San Joaquin Valley, winter temperatures are more extreme and this causes the first 1 or 2 generations of scale to develop synchronously in the spring. This synchrony minimizes overlap of scale age classes, and consequently, there are periods during the early season when 2nd- and 3rd-instar scale are not available. This causes a delay in the build up of parasitoid populations until the 3rd generation of scale, which is often too late to prevent scale infestation of citrus fruit. In most years, the scale on the fruit is heavily parasitized by the end of the season (October) and what remains is easily washed off in the packing house. Thus, biological control of California red scale can be very successful in the San Joaquin Valley.
In some years parasitism is poor because of weather conditions and biological control alone is not sufficient to control armored scale in the San Joaquin Valley. Citrus growers may use Narrow Range 415 and 440 oil sprays in this situation. However, oil sprays can be phytotoxic to citrus resulting in lowered yield and therefore must be used infrequently and carefully. Oils can only be applied during July through September in carefully irrigated orchards when temperatures are below 90o F. NR 415 oil sprays are generally not as effective in controlling armored scale compared with organophosphate and carbamate insecticides. Finally, narrow range oil sprays can disrupt Aphytis biological control by killing the younger scale stages and synchronizing the scale age structure.
Successful biological control requires a large commitment on the part of the grower to eliminate broad spectrum insecticides for other pests of citrus such as citrus thrips, worms, and katydid. The released Aphytis wasps are very sensitive to these broad spectrum insecticides. The alternative selective insecticides are not always as effective in controlling pests compared with the broad spectrum insecticides. Thus, growers are tempted to use the harsher pesticides, however, these are just too toxic to the natural enemies.
When broad spectrum insecticide use is eliminated pests with poor biological control, such as citricola scale often become key pests. Citricola scale is very prolific, causes yield loss of the tree, and is not easily controlled by biological control agents such as parasitic wasps. Frequently, growers must use broad spectrum insecticides such as Lorsban to get this pest under control. Katydid is another severely damaging pest that is not easily controlled by biological methods. Thus, soft insecticide program growers experience some secondary pest problems that growers using broad spectrum pesticides do not.
Insecticide Resistance Management Program
To manage insecticide resistance in California red scale, we need to limit the total number of sprays of each insecticide. The best way to do this is to practice the basic principles of IPM; monitor pests, maximize the use of biological and cultural controls, spray only when pests reach economic thresholds, use the most selective pesticides available, and when insecticides are needed rotate between different chemistries of insecticides.
Rotation of insecticides with different modes of action is critical for managing insecticide resistance. The California red scale uses the same group of enzymes to detoxify the organophosphates, Lorsban and Supracide, and the carbamate Sevin and so these insecticides should be considered one group. Oils have a smothering effect and so would be a second group. The new insect growth regulator, Esteem (=Knack) (juvenile hormone mimic) has a unique mode of action and is considered to be an additional group available for rotation. Growers should have their scale tested for resistance to the organophosphates and carbamates. If a grower does not have resistant California red scale and is on a broad spectrum insecticide program, then he should rotate between the three major chemical groups 1) organophosphates and carbamates, 2) Insect growth regulators, and 3) oil. If the grower has organophosphate and carbamate resistant scale then only oil and insect growth regulators are available for rotation. Esteem should be used no more than once every other year to avoid developing resistant scale.
Growers used organophosphates and carbamates for more than 30 years before resistance was detected and initially used them only once every 1-2 years. Whitefly populations infesting cotton in Israel developed resistance to Esteem within 5 years in spite of the fact that resistance management programs were well-established. We expect California red scale to eventually develop resistance to Esteem and so it is critical that we rotate insecticide classes immediately after registration. Reduced use of the organophosphates and carbamates may eventually result in a decline in OP and carbamate resistance in the scale and increase effectiveness of these insecticides at some future date.
Growers that use a selective insecticide program in order to preserve Aphytis wasps and natural enemies for other citrus pests, generally use Aphytis and occasional oil sprays for scale control. At this point, Esteem appears to be relatively nontoxic to spiders, lacewings, predatory mites and parasitic wasps. Esteem is highly toxic to the predatory vedalia beetle. These predatory beetles are critical for cottony cushion scale control. Thus, use of insect growth regulators improves parasitism of red scale by wasps (compared to organophosphates and carbamates) but hinders biological control of cottony cushion scale. The switch from organophosphates to Esteem has also released citricola scale from control. By changing pesticide practices, San Joaquin Valley citrus growers have traded California red scale and citrus thrips for citricola scale and cottony cushion scale as key pests. See our web page describing the outbreaks of cottony cushion scale that followed the use of insect growth regulators in San Joaquin Valley citrus: (http://www.uckac.edu/citrusent/vedainfo.htm).
Integrated Pest Management for California Red Scale
(For details, see IPM for Citrus Publication 3303 and UC IPM Pest Management Guidelines # 28)
Resistance Management
Test the California red scale for susceptibility to organophosphates and carbamates to ensure field sprays will be effective
Rotate between insecticide groups, do not use the same product back to back
Good application technique is essential to maximize control including thorough coverage (750-1500 gal/acre), good nozzle selection, careful nozzle placement, proper adjuvants, and slow speed (1.5 mph).
Rotate and Limit Use of the Same Insecticides
Group 1: Lorsban, Supracide, Sevin
Group 2: Esteem (=Knack)
Group 3: Oil