California Red Scale and Yellow Scale

California red scale and yellow scale are armored scales that are distributed throughout the citrus-growing regions of the state except in parts of the Coachella Valley where they are under an eradication program. The two species are difficult to distinguish by appearance. Yellow scale, however, is rarely found on mature wood of the tree whereas California red scale can be found on the wood as well as on fruit and leaves. Biologies and management tactics for California red scale and yellow scale are similar, but yellow scale is more easily controlled by natural enemies. In recent years, yellow scale has not been observed in California.

Female scales have a roundish cover, about the size of the blunt end of a nail. The cover is firmly attached to the leaf, wood, or fruit substrate when the scales are molting or reproducing; they remain under this cover throughout their life. When mature, they produce 100 to 150 crawlers.

Crawlers hatch and emerge from under the female cover at a rate of two to three per day. Crawlers move around to find a suitable place to settle and can be spread about by wind, birds, or picking crews. They settle in small depressions on twigs, fruits, or leaves and start feeding; soon after, a circular, waxy cover forms over their body. Midway through the second instar, females and males begin to develop differently. Males form an elongated cover while the female cover remains circular. The female molts twice, developing a concentric ring in the center of the waxy covering each time.

Adult male scales are small, two-winged insects that emerge from the elongated scale covers after four molts. They live about 6 hours and their sole purpose is to mate. The number of male flights, along with the number of generations per year for this insect varies according to the growing region in the state and the weather but is generally about four flights per year.

California red scales attack all aerial parts of the tree including twigs, leaves, branches, and fruit by sucking on the plant tissues with their long, filamentous mouthparts. Heavily infested fruit may be downgraded in the packinghouse and, if scale numbers are high, serious damage can occur to trees. Severe infestations cause leaf yellowing and drop, dieback of twigs and limbs, and occasionally death of the tree. Tree damage is most likely to occur in late summer and early fall when scale numbers are highest and moisture stress on the tree is greatest.

Management of California red scale varies according to location in the state and the other pests present in the orchard. Natural enemies can provide good control of California red scale in all regions of California except the Coachella Valley where it is under pesticide eradication. However, biological control tends to be easiest in the coastal areas and some inland districts of Southern California because milder weather in these regions allows the overlap of generations, which provides susceptible host stages for parasitism year-round.

In the San Joaquin Valley, many red scale and yellow scale populations developed high levels of resistance to organophosphates and carbamates during the 1990s. Growers shifted to using Aphytis releases, oil sprays, or the insect growth regulators pyriproxyfen (Esteem) and buprofezin (Centaur) for scale control. Where biologically based IPM is practiced, yellow scale is easily controlled by parasites and is not currently a problem, whereas California red scale continues to be a key pest. In the 2020s, numerous California red scale populations in the San Joaquin Valley developed resistance to the insect growth regulator pyriproxyfen. Spirotetramat (Movento) and mating disruption (Checkmate) are currently effective in controlling these populations of California red scale.

Augmentative releases of Aphytis melinus have been shown to be effective in controlling red scale, but this approach requires the minimization of the use of broad-spectrum pesticides for the control of pests such as Asian citrus psyllid, katydids, Fuller rose beetle, or citricola scale. Use the information in these Guidelines to choose the most selective tactic. Careful management of armored scales in the San Joaquin Valley may allow them to be managed by resident and augmented natural enemy populations.

In years of high summer temperatures, it can be difficult to control California red scale with insecticides because high temperatures result in an additional generation of scales. Each insecticide treatment generally controls only one generation, thus in a hot year, more applications of insecticides are needed. In addition, high heat and drought reduce the efficacy of natural enemies. Warm winter temperatures contribute to the problem by reducing overwintering mortality of young instars. Mating disruption, such as Checkmate CRS, can reduce the density of scales and so reduce the number of insecticide treatments needed. When used for several years in a row, it may reduce the population sufficiently to eliminate the need for insecticides. It is completely compatible with biological control, in fact, it aids Aphytis melinus efficacy because it extends the period that the scales are in the third-instar, virgin-female stage, the stage preferred for oviposition by this parasite.

The parasitic wasps, Aphytis melinus, A. lingnanensis (coastal areas), and Comperiella bifasciata (San Joaquin Valley), play an important role in controlling California red scale but their effectiveness depend on careful monitoring and use of selective insecticides for other pests. Several insect predators also feed on California red scale including the lady beetles Rhyzobius (=Lindorus) lophanthae, Chilocorus orbus, and Chilocorus cacti. To enhance the effectiveness of all natural enemies, use pesticides only when their need is indicated by careful monitoring, use the most selective insecticides available, and spray only portions of the orchard where scale numbers exceed the threshold.

Ants, dust, and a dense canopy all reduce the effectiveness of natural enemies.

• Control ants, particularly the Argentine ant in Southern California and the native gray ant in the San Joaquin Valley, because they disrupt red scale parasites.
• Minimize excessive dust that coats the leaves and fruit, including dust from manure mulches as well as whitewash and kaolin clays. If whitewash or kaolin clay is applied, delay application until the end of the season when Aphytis has completed its work. In addition, fine, talc-sized particles of ash from nearby brush fires can also disrupt biological control. Watering roads and washing trees can help solve these problems. Heavy fogs, drizzle, or rain can also help by either removing dust and ash particles or causing them to adhere to the leaf surface.
• Trees should be internally pruned and opened up so that the parasites can gain access to the scales.

Parasite Releases

Releases of mass-reared Aphytis melinus parasites can be useful in groves with insufficient biological control. Keep in mind that pesticide residues on leaves may have a detrimental effect on released Aphytis parasites. Test for possible toxicity by putting ten to twelve 1-year-old twigs with leaves in a gallon jar with Aphytis parasites for 24 hours and checking their mortality. If more than 35% have died, residues are too high for Aphytis releases. Also, prepare a control jar filled with untreated leaves for comparison of Aphytis vigor.

• In the San Joaquin Valley, recommended release rates are 100,000 parasites per acre per year for orchards undergoing the transition to an integrated pest management program. Begin releases about March 1, making releases of 5,000 to 10,000 parasites per acre every 2 weeks with the objective of releasing 50% of the parasites during the critical spring period, 25% more in summer, and 25% more in fall. Suspend releases when second- and third-instar scales are not available (normally mid-June to mid-July). Continue releases through mid-November. Concentrate later releases in areas in the block known to have higher red scale densities. Once a grove has moved through the transition period (2 to 4 years), the total number of parasites released per acre may be reduced to 50,000 to 70,000. A suggested release method is to hold the release cup upright and tap it to release a few Aphytis at every sixth tree in every sixth row.
• In Southern California, where natural Aphytis numbers are generally higher, so releases are often not necessary or need to be made only infrequently; one to four releases of about 10,000 per acre at 2-week intervals in April and May should be sufficient in these areas. Annual Aphytis releases in Southern California are needed in particular on young trees, on grapefruit and lemon trees, and in fall if biological control is not working effectively.

For more detailed information on natural enemy releases, see Natural Enemy Releases for Biological Control of Crop Pests.

Use biological control in the form of Aphytis releases, organically approved petroleum oil sprays (PureSpray Green),and Checkmate CRS mating disruption, as well as postharvest high-pressure washing in the packinghouse.

In the San Joaquin Valley, a number of populations of armored scale have been found to be resistant to the carbamate carbaryl (Sevin). Scales have not developed resistance to oil sprays or the insect growth regulator buprofezin (Centaur), but field observations indicate that resistance to pyriproxyfen (Esteem) has developed. In orchards where resistance is a severe problem, avoid using carbamates or pyriproxyfen, and instead release Aphytis melinus wasps, utilize mating disruption (Checkmate CRS), or apply buprofezin (Centaur), narrow-range oil, or spirotetramat (Movento).

Oil is the most selective pesticide available for control of armored scale insects. Oil only kills natural enemies that it contacts and slightly suppresses beneficial mites. However, the residues do not persist and Aphytis wasps can be released soon after application. As with all insecticides, use oil only when needed because oil sprays will eliminate the younger scale instars and thus synchronize development of the scale population. This makes parasitism by Aphytis more difficult, because they prefer to deposit their eggs in third instar scale and after an oil sprays, this stage may be absent for a period of time because their life cycle is about twice as fast as that of the red scale.

The insect growth regulators pyriproxyfen (Esteem) and buprofezin (Centaur) are safe for parasitic wasps, predatory mites, spiders, and lacewings but are quite toxic to vedalia beetles, which are needed for cottony cushion scale control. Spirotetramat (Movento) is very safe for parasitic wasps and vedalia beetles but is toxic to predatory mites.

The organophosphate and carbamate insecticides are the least selective insecticides. If adult Aphytis wasps are placed in a jar with leaves that were sprayed in the field with the dilute rate needed for California red scale control, carbaryl (Sevin) affects adults for 5 months after the application.

In the San Joaquin Valley, citrus growers use pheromone traps to monitor male scales during their first (May), second (June–July), and fourth (September–October) flight. Degree-days (DD)are used to estimate when these flights are occurring. To learn more about how to use degree-days to time insecticide applications, see Using Degree-Days to Time Insecticide Applications in Fruit and Nut Orchards. For assistance in calculating degree-days for California red scale in your location, see Degree-days: California Red Scale in Citrus.

The goal is to maintain California red scale numbers at levels that do not result in more than 10 scales per fruit at harvest. In the past, when an average of more than 1,000 scales were trapped during the fourth flight and fruit was infested with scales at harvest, a pesticide application was planned for the next season. However, this threshold was based on studies conducted during the organophosphate and carbamate era, and is not as reliable when other strategies are used. Because newer management tactics can decrease or increase the male numbers on trap cards, the threshold of 1,000 scales per flight developed in the 1980s is no longer a stand-alone tool for determining when treatments are necessary. It is critical to use other tactics, such as fruit and bin examination, to determine if treatments are needed.

• Pheromone cards overestimate scale numbers in Aphytis-release orchards, because Aphytis prefers to parasitize female scales and the male scale numbers can be very high when the female numbers are low.
• Pheromone cards underestimate red scale numbers when insect growth regulators or mating disruption are used because the males are more sensitive to these treatments than the females. In mating disruption situations: a threshold of 50 scales per flight is helpful in determining if mating disruption is effective.
• Pheromone cards are not reliable predictors of fruit infestation in orchards where spirotetramat (Movento) or imidacloprid (Admire) are used because these insecticides do not kill scale on the woody trunk or branches. Thus, while the number of scales can increase on these surfaces and, the trap cards are heavily infested, fruit can still be relatively free of scales.

Weekly Pheromone Trap Monitoring

Select five to six orchards that have a known population of red scales to monitor every week so that you can determine when flights are occurring and time your sprays.

Put out pheromone traps beginning in March before the first flight. Change the sticky cards weekly and the pheromone caps monthly through October. Use two to four pheromone traps per 10-acre block; add two traps for each additional 10 acres.

Pheromone Trap Monitoring by Flight

In the remaining orchards, use pheromone traps to determine areas of heavy scale infestation.

1. Hang the traps with a fresh lure just before the predicted 1st, 2nd, and 4th flights: for the first flight this is March 1, for the second flight it is at 1,100 degree-days after the biofix of the first male flight, and the fourth flight at 3,300 degree-days from biofix.
2. Use two to four pheromone traps per 10-acre block; add two traps for each additional 10 acres.
3. Remove traps at the end of each flight and count scales (or estimate based on counting the scales inside the squares [20%] and multiplying by 5).
4. Record results (PDF). These traps will tell you which areas of the block have heavy infestations.

Examining Fruit

In all orchards, whether Aphytis wasps are released or not, conduct visual inspections of citrus fruit once a month during August, September, and October. Walk around 20 trees in each quadrant of the block, and record the number of fruit examined along with the number of fruit with noticeable patches (10 or more) of scales (example form—PDF). Calculate the percentage of fruit with more than 10 scales.

Bin Counts

At harvest, look at the fruit on the surface of at least 10 bins from areas throughout the block, and count the number of uninfested and scale-infested fruit. Calculate the percentage of fruit with scale. At the same time, you can estimate the percentage of citrus thrips, katydid, cutworm, and peelminer-damaged fruit.

Detailed Evaluations of Parasitism in Aphytis-Release Blocks

In orchards where biological control agents such as Aphytis and Comperiella wasps are used to control scale, visually monitor all stages of scales on twigs, fruit, and leaves in August, September, and October.

1. Collect 10 scale-infested fruit (preferably from different areas of the block). Do not take more than one to two fruit per tree, avoiding trees in the outside rows.
2. Record the number of second- and third-instar red scales and the number of these that are parasitized (example form—PDF). To determine if a scale is parasitized, flip the cover over and search for Aphytis eggs, larvae, and pupae or Comperiella larvae and pupae.
3. Calculate the percentage parasitism by dividing the number parasitized by the total number of 2nd and 3rd instar scales examined. If biological control is functioning properly, you should see percent parasitism increase from just a few percent in August to a high percentage in October.

Guidelines for determining when parasitism is at sufficient levels vary by growing region, cultivar, and whether or not fruit are sent to a packinghouse that employs high pressure washers to remove scale.

San Joaquin Valley

Effective biological control of California red scale is achieved if by mid- to late October more than 70% of the third-instar female scale are parasitized either by Aphytis or Comperiella. A good proportion (50%) of large second-instar females and second-instar males should also be parasitized.

Interior Southern California

If parasitization with Aphytis melinus is poor (e.g., by the end of September, monitoring reveals more than 15 to 20% healthy, unparasitized third-instar female scale), an insecticide application is recommended. If parasitization is good (e.g., by late September to early October there is almost no survivorship of third-instar female scale and parasitism of second-instar male and female scale is greater than 50%), an insecticide application is not required unless infestations by live scale reach 25 to 40% of the fruit. These thresholds can be increased as high-pressure washers are used more frequently in citrus packinghouses to remove scale from fruit.

Coastal Area

Biological control of California red scale on oranges is complete and insecticide applications are generally not required. Biological control of California red scale on lemons is substantial, but occasionally an insecticide application is required to reduce scale numbers. Maintain a few pheromone traps all year in key areas to determine when red scale flights are occurring and when to apply an insecticide. In orchards with California red scale on scaffold limbs, visually inspect fruit to determine if insecticide applications are necessary, paying special attention to the presence of mature females (which indicates they have escaped parasitism). Generally, if parasitization is adequate, spray only if more than 15% of the fruit is infested. If parasitization is low, reduce the threshold to 10%.

Mating Disruption

Checkmate CRS
Mating disruption prevents or delays mating of males with females which has two consequences: unmated females do not produce crawlers and it improves biological control. Mating disruption keeps females in the unmated, third-third-instar stage and this is the preferred stage for parasitism by Aphytis melinus.

Dispensers are applied to trees at a rate of 180 per acre prior to the first or second flight of male scales. Distribute the dispensers evenly throughout the orchard. Application prior to the first flight has been demonstrated to provide season-long protection in most situations. Hang the pheromone dispensers at the 6-foot or higher level inside the canopy on 1-inch-thick branches within the tree row. Prune prior to hanging the dispensers to avoid dispenser losses. Place monitoring traps inside the orchard. A threshold of greater than 50 scales per flight is helpful in determining if the disruption is effective. Mating disruption will likely not be effective in very high red scale density situations. Insecticides must be utilized to bring the scale population down ahead of mating disruption or treatments must be applied combined with mating disruption over several years to achieve good results in these situations.

Insecticide Treatments

Carbamates
Time carbamate insecticide sprays to treat the crawler stage, which peaks about 555 degree-days (accumulated above a 53°F threshold) or about 1 to 3 weeks after the peak in the male flight. For assistance in calculating degree-days for California red scale in your location, see Degree-days: California Red Scale in Citrus. Optimal treatment timing varies from year to year because of temperature, but usually occurs in May (first generation) or July (second generation).

An even more reliable method of timing carbamate applications is to monitor for crawlers by wrapping sticky tape around 1-year-old branches (about 0.5 inch diameter) that have both gray and green wood and are infested with live female scales. Always back up pheromone trap count decisions with inspection of twigs, leaves, and fruit for female and immature scale.

Insect Growth Regulators
Apply pyriproxyfen and buprofezin sprays after crawlers have completely emerged and become white caps because these insect growth regulators will kill the scale when it tries to molt to the next stage. Optimal timing for insect growth regulators is the second generation of scale (June–July) in order to protect vedalia beetle during the time it is controlling cottony cushion scale (February–May).

Lipid Synthesis Inhibitors
Make a foliar application of the systemic spirotetramat (Movento) between the second male flight (1100 DD after the first male flight) and the third male flight (2200 DD). The systemic action of Movento takes some time, but it is active against all scale stages so precision of application timing is not as important as application technique.

Oils
Oils can be effective against California red scale if coverage is thorough. They also have the advantage of being relatively less damaging to natural enemies than other insecticides. However, special care must be taken to avoid applying dilute applications of oil at times when it can damage fruit and leaves or reduce numbers of natural enemies. Oil applications after October 1 carry some risk of increasing frost damage. To avoid phytotoxicity and reduction in yield, time oil sprays according to the following guideline:

For more information on monitoring and management of California red scale, see UC Ag Experts Talk: California Red Scale.

• Citrus red scale pheromone trap monitoring form (PDF)
• Citrus red scale fruit count monitoring form (PDF)
• Degree-day calculator
• Degree-day table
• Using degree-days to time insecticide applications