Armored scale insecticide resistance challenges San Joaquin Valley growers

Elizabeth Grafton-Cardwell o Wing Ouyang o Rebecka Striggow o Stacy Vehrs

Organophosphateand carbamate alifornia red scale and yellow the year. Biological control utilizes insecticides have been used to scale have been important eco- natural predators of crop pests to limit treat citrus problems for nomic pests in California citrus for their growth and spread. more than 40 years. From 1990 to more than 80 years. Scale are tiny in- In the San Joaquin Valley, where 1998, we documented California sects with a dark, circular, waxy cover- winter and summer temperatures are red scale and yellow scale ing over their bodies. They live on the more extreme and the pest citrus resistance to these insecticides. twigs and leaves of the plant and the thrips (Scirtothrips citri) have been con- Armored scale resistance is found rind of citrus fruit, giving the fruit an trolled by broad-spectrum insecticides, on an estimated 40% of 163,000 unattractive, scaly appearance. biological control has been more diffi- acres of citrus in the San Joaquin California red scale, cult to accomplish. Broad-spectrum in- aurantii (Maskell), and yellow scale, secticides applied for the pest citrus Valley. Citrus growers have Aonidiella citrina (Coquillett), cause thrips kill the parasitic wasps and responded by either increasing cosmetic damage to the fruit, resulting predators needed for biological control their use of natural enemies, in downgrading at the packinghouse. of scale. Consequently, San Joaquin especially the parasitoid wasp In addition, scale can cause yellow Valley citrus growers have depended , or by applying leaves, defoliation, branch dieback and primarily on insecticides to control ar- newly registered growth tree death when the pest density is ex- mored scale. regulator or neonicotinoid tremely high. In Riverside, Orange, From the 1950s until the mid- to late insecticides. While the California Los Angeles and San Bernardino coun- 1990s, San Joaquin Valley growers used, red scale problem is, for the ties, growers have been very success- almost exclusively, organophosphate moment, greatly reduced, ful in controlling scaIe with biological and carbamate insecticides to control outbreaks of cottony cushion control methods (Luck et al. 1986). scale, citrus thrips and most other pests scale are occurring because the This is due, in large part, to year- of citrus. In the 1980s, many populations new insecticides are highly toxic round uniform temperatures, which of citrus thrips became resistant to these to the predatory vedalia beetle. allow preferred stages of scale to be insecticides (Morse and Brawner 1986). available for parasitism at all times of In the early 1990s, growers noticed that

20 CALIFORNIA AGRICULTURE, VOLUME 55,NUMBER 5 Using the fruit-dip bioassay technique, scientists found numerous populations of California red scale that were resistant to chlorpyrifos, methidathion and carbaryl.

fore, rotating these insecticides would not reduce resistance. California red scale. Resistant populations of armored scale were found by our bioassay in most of the citrus-growing region along the foot- hills of Fresno, Tulare and Kern coun- ties (fig. 1).During the 1950s and 1960s, California red scale eradication districts in Fresno, Tulare and Kern counties used parathion (an organo- phosphate) to eradicate the pest. After the eradication effort ended in the 1970s, many growers continued to use California red scale and yellow scale cide, carbaryl (Sevin). We began our parathion, as well as chlorpyrifos, were becoming more difficult to control. research using a fruit-dip bioassay methidathion and carbaryl as they be- When these pesticides were first intro- technique. We circled the first-instar came available. With this history of in- duced, a single application of an organo- scale (the first stage that settles down secticide use, it is not surprising that phosphate or carbamate was effective in and forms a white waxy covering on we found resistant populations in reducing scale populations for 1 to 2 its back) with a black pen, dipped four many orchards in the San Joaquin Val- years. But by the mid-l990s, some citrus to six scale-infested green (underripe) ley in the 1990s. growers were spraying three to four ap- citrus fruit into a specific concentra- Yellow scale. Although yellow plications per year and still not achiev- tion of an insecticide, and waited 10 scale was less common than California ing satisfactory control. The increase in days to see if the were alive or red scale, it seemed to develop a insecticide use escalated the economic dead (Grafton-Cardwell and Vehrs greater frequency or intensity of resis- and environmental costs. At the same 1995). We tested each insecticide sepa- tance than red scale. From 1990 to time, the marketability of the fruit de- rately. Because armored scale insects 1994, eight of the 100 orchards tested clined because scale insects were en- do not move around, it is difficult to were infested primarily with yellow crusting the fruit. tell if they are alive or dead, so we scale. All eight populations exhibited Armored scale insects have demon- waited 10 days for them to molt into resistance to organophosphates and strated their ability to develop resis- the second instar. Those individuals carbamates, and most had high levels tance to many of the insecticides used that molted were considered to be of resistance. Knowing that yellow to control them. As early as 1912, Cali- alive and had survived the chemical scale had a greater tendency to be re- fornia red scale resistance to hydro- treatment. We conducted a total of 148 sistant to insecticides than California cyanic acid was detected (Quayle bioassays for methidathion, chlor- red scale and realizing that yellow 1938).In the 1970s, California red scale pyrifos and/or carbaryl resistance scale is more easily controlled with populations in some citrus-producing from 1990 to 1994 (an average of 30 or- natural enemies, growers with yellow areas of South Africa developed resis- chards per year). scale rapidly shifted to biological con- tance to organophosphate insecticides, We found many populations of scale trol. Since 1994, it has been difficult to including dimethoate, parathion and with resistance to one, two or all three find yellow scale in the San Joaquin methidathion, and to a carbamate in- insecticides. Usually, if the scale was re- Valley. secticide, methomyl (Nel et al. 1979). sistant to one insecticide, it was at least somewhat resistant to all three (Grafton- New colorimetric resistance test Insecticide resistance Cardwell and Vehrs 1995). Tests with Mapping resistance using the fruit- Fruit-dip bioassay. In 1990, we be- synergists (chemicalsthat block the en- dip bioassay procedure was problem- gan testing armored scale populations zymes the insect is using to resist the in- atic. Because the bioassay required a from various orchards of the San secticide) suggested that the insects large number of scale-infested green Joaquin Valley for resistance to two or- were using the same mechanism to re- fruit from each orchard, we were lim- ganophosphate insecticides, chlor- sist all three pesticides. The same syner- ited to orchards with heavy infesta- pyrifos (Lorsban) and methidathion gist reduced the scale’s ability to resist tions. This biased our sampling to- (Supracide), and a carbamate insecti- poisoning by all three insecticides; there- ward locations with resistance. We

CALIFORNIA AGRICULTURE, SEPTEMBER-OCTOBER 2001 21 Fig. 1. San Joaquin Valley citrus distri- bution of insecticide susceptible (81% to 100% mortality), slightly resistant (46% to 80% mortality) and highly resistant (45% or less mortality) armored scale, 1990- 1994. Resistant populations of armored scale were found in most of the citrus- growing regions tested. We tested the resistance to chlorpyrifos, methidathion and carbaryl using a fruit-dip bioassay.

were also limited to testing when the scale-infested fruit was available, Au- gust through October. Growers often sprayed their orchards with insecti- cides from May to July, so we often could not use the bioassay to help them make decisions about which pes- ticide would be most effective in the current season. The fruit-dip bioassay requires 10 days to determine results, giving growers a very slow response time. In 1995 we began to develop a rapid biochemical test for detecting insecticide-resistant scale. Our re- search with synergists and electro- phoresis demonstrated that resistant California red scale survived the insec- ticide sprays by using esterase enzymes to bind and detoxify the insecticides en- tering their bodies (Grafton-Cardwell et al. 1998). In electrophoresis, an electri- cal current is used to sort enzymes in the ground-up insect. We adapted a colorimetric test used for detecting in- secticide resistance in mosquitoes, in which the chemicals in the dish react with esterase enzymes in the crushed body of the insect (Dary et al. 1990). We placed a portion of a crushed third-instar female scale in each test well of a microtiter plate. This plate has a reader that measures the optical density (color) of the fluid in the plate wells. We used known amounts of alpha naphthol (chemical standard) to estimate the levels of esterase enzymes in the test insects. We found that resistant scale had higher esterase enzyme levels; the liquid in the wells of the plate turned darker for insecticide-resistant scale than susceptible scale. This new method allowed us to test individual third-instar female scale during more months of the year and from any part of the tree, broadening the types of orchards we could sample. This method also required

22 CALIFORNIA AGRICULTURE, VOLUME 55, NUMBER 5 only 1 day to complete, so test results could be returned to the grower quickly. From 1995 to 1997 we were able to conduct 280 tests of scale using the colorimetric test and increase our testing from 30 bioassays per year to nearly 100. We also tested 34 of these scale populations with chlorpyrifos and methidathion in the standard fruit-dipping method to verify the colorimetric test’s reliability. The tests disagreed with each other in only two cases, resulting in 94% agreement. (We didn‘t expect the two tests to agree perfectly, because we were testing dif- ferent scale individuals and different stages of scale with the two bioassay methods.) Combining data from the fruit-dip bioassay and the colorimetric test for 1990 to 1998, the highest percentage of orchards with resistant scale occurred in Tulare, which had the greatest cit- rus acreage, followed by Fresno, Kern and Madera counties (table 1).While A new colorimetric test for armored scale resistance can obtain results in 1 day, as opposed to 10 days for the fruit-dip bioassay. our sampling is somewhat biased to- ward orchards with resistance prob- acre for a 1,500-gallon-per-acre ($0.60 blocks bearing fruit for the first time lems, we estimate that 40% of the per gallon of water), 1.5-mile-per-hour had resistant scale (31% to 40%). 163,000 acres of citrus in the San spray application. The cost of the pes- Because we found that resistance Joaquin Valley have armored scale ticide is $65 to $75 per acre. The full was becoming a uniform problem in populations with some level of resis- cost of one armored scale spray aver- this region, we designated it a “hot tance to chlorpyrifos, methidathion ages $160 per acre. In areas where re- spot.” Hot spots have been found in and/or carbaryl. When a California sistance was a serious problem be- Fresno County (Orange Cove), Tulare red scale population has a high per- tween 1995 and 1997, growers applied County (Orosi, Exeter, Lindsay, centage of resistant individuals (45% insecticides three times each season. In Strathmore, Porterville and Terra or less mortality), insecticide control some orchards, scale control alone was Bella) and Kern County (McFarland using these pesticides lasts for no costing $480 an acre, making citrus and Edison) (fig. 1). Other hot spots more than 30 days (Vehrs and growing highly unprofitable. may exist that have not yet been de- Grafton-Cardwell 1994), which is less Spreading resistance. When our tected. The new biochemical colori- than or equal to the duration of one resistance monitoring was first initi- metric technique helped us better de- scale generation. If the insecticide is ef- ated in 1990, resistance appeared to be fine regions of greatest scale resistance fective for only 30 days, the grower is patchy. In 1996 we tested scale from so we could help growers make the forced to spray repeatedly for each of each of 30 citrus orchards within a decision to stop the use of organo- the four scale generations. 1-mile-by-1-mile section of cropland in phosphate and carbamate insecticides. Treatment costs. Armored scale Tulare County (fig. 2). This section insecticide applications in citrus are was farmed by eight growers who Grower response to resistance extremely expensive compared with used different pest management prac- As a result of the high cost of pesti- other pest control methods (Haney et tices through the years and farmed cide resistance in armored scale, doz- al. 1992). Insecticides targeted to ar- other crops in addition to citrus. The ens of citrus growers began to increas- mored scale must be applied in large purpose of the test was to determine if ingly rely on biological control. These volumes of water (750 to 2,000 gallons resistance was unique to one or more growers release the parasitoid wasp per acre) to penetrate the foliage and growers or was becoming an areawide Aphytis melinus DeBach and protect move the insecticide to the insect pest. problem. We found that scale in every other natural pest enemies such as the In addition, the spray-rig speed must orchard tested showed at least a low parasitoid wasp Comperiella bfasciata be very slow, 1.5 miles per hour. If it is percentage of resistant individuals (a (Howard), predatory beetles and any faster, the leaves form a sheet and minimum of 13%) and that some or- lacewings. The registration of the se- prevent penetration of the pesticide. chards had 100% resistant individuals. lective insecticides abamectin (Agri- The application cost averages $90 per In addition, we found that young Mek) and spinosad (Success) in the

CALIFORNIA AGRICULTURE, SEPTEMBER-OCTOBER 2001 23 the organophosphate and carbamate tance to pyriproxifen in California red insecticide resistance problem. Several scale in South Africa is suspected. We insecticides have been registered or expect California red scale in San are nearing registration for scale con- Joaquin Valley to develop resistance to trol in California citrus. Pyriproxifen pyriproxifen and buprofezin fairly (Esteem) is an insect growth regulator rapidly, within 5 to 10 years. Resis- registered in 2000 that mimics juvenile tance to imidacloprid in armored scale hormones. Buprofezin (Applaud) is an is also likely to develop. unregistered insect growth regulator Furthermore, nearly every new in- that inhibits chitin synthesis. Both of secticide has an impact on one or more these insecticides inhibit the scale’s natural enemies when it is first intro- ability to molt, thereby preventing fur- duced. Pyriproxifen, buprofezin and ‘ther growth. The soil-applied formula- imidacloprid are highly toxic to preda- tion of imidacloprid (Admire), regis- tory beetles (Grafton-Cardwell 2000, in tered in 2001, is a neonicotinoid that press, unpublished data). In some inhibits scale’s ability to feed. These crops this is not a concern, because new insecticides could help slow resis- beetles do not play an important role tance to organophosphates and car- in pest management. In citrus, how- bamates, increase worker safety, re- ever, the vedalia beetle (Rodolia duce armored scale densities, improve cardinalis) has been controlling cottony tree health, and increase fruit market- cushion scale for nearly 100 years. ability. When insecticides such as DDT and Field efficacy data for these new organophosphates were first intro- products (Grafton-Cardwell and duced in the 1940s and 1950s, vedalia Reagan 1995,1998), as well as our data beetle was eliminated and cottony documenting the extent of organo- cushion scale outbreaks occurred in a phosphate resistance, helped the citrus number of orchards. industry obtain an ”emergency” Sec- Unhappily, the San Joaquin Valley late 1990s for the control of citrus tion 18 registration to apply pyri- is experiencing the loss of vedalia thrips helped reduce the broad- proxifen and buprofezin in Fresno, beetle again because the beetles are spectrum organophosphate, carbamate Tulare and Kern counties in 1998 and highly sensitive to insect growth regu- and pyrethroid residues that are toxic 1999. Federal Section 18 registrations lators and neonicotinoids (Grafton- to the natural enemies of scale. New allow growers to use an unregistered high-pressure washers in most pack- chemical for specific uses that are inghouses in the San Joaquin Valley deemed critical. Growers requested also helped to disinfest scale-infested two pesticides so they could rotate in- fruit at the end of the season (Walker secticides from different chemical et al. 1996). classes and delay the development of Even when growers reduce or resistance in scale insects. The intro- eliminate broad-spectrum pesticides, duction of these insecticides resulted these orchards still have their share of in a 71% reduction of organophos- problems. Several pests with few op- phate and carbamate use in citrus be- tions for biological control in the San tween 1997 and 1999. The reduction Joaquin Valley, which were previously totaled more than 700,000 pounds of easily controlled by organophosphates pesticide active ingredient per year. and carbamates, have become serious Insecticide drawbacks. While problems. These include katydid and many San Joaquin Valley growers citricola scale. However, because looked to insect growth regulator and many of the natural enemies of scale neonicotinoid insecticides as a solution now have some resistance to organo- to their organophosphate and carbam- phosphates, growers have found that ate resistance problems, use of these they can occasionally use low rates of Fig. 2. Percentage of resistant California newer insecticides has its drawbacks. red scale individuals in 30 citrus orchards chlorpyrifos for katydid and/or Sweetpotato whitefly in Israel has be- in a 1-mile-by-1-mile section of cropland; citricola scale control and only mildly gun to develop resistance to both this area was designated a “hotspot” for disrupt biological control of scale. pyriproxifen and buprofezin in cotton scale resistance. Percentage resistance is New insecticides. Other growers based on the colorimetric test that detects despite the implementation of a resis- individuals with 0.5 nanomoles or more of believe that the registration of new in- tance management program esterase activity per microgram body secticides could provide a solution to (Horowitz and Ishaaya 1994). Resis- weightlminute.

24 CALIFORNIA AGRICULTURE, VOLUME 55, NUMBER 5 References Dary 0, Georghiou GP, Parsons E, Pas- teur N. 1990. Microplate adaptation of Gomeri’s assay for quantitative determination of general esterase activity in single insects. J Econ Entomol 83:2187-92. Grafton-Cardwell EE. 2000.Citrus: Inte- grating biological control and insecticide treatments for cottony cushion scale and other scale pests. In: Hoddle MS (ed.). Proc of California Conference on Biological Con- trol; July 11-12, 2000; Riverside. p 47-50. Grafton-Cardwell EE. In press. Citrus IPM in California: Regional differences and their effects on management in the mil- lennium. Proc of International Society of Citri- culture, 9th Int Citrus Congress: December 3-7, 2000.Orlando, FL. Grafton-Cardwell EE, Ouyang Y, Sake J. The vedalia beetle feeds on cottony cushion scale. The beetle is highly susceptible to 1998. Insecticide resistance and esterase en- new insect growth regulators and neonicotinoid insecticides registered to control zyme variation in the California red scale armored scale. As a result. outbreaks of cottony cushion scale have occurred in the San (Homoptera: ). J Econ Entomol Joaquin Valley. 91:812-9. Grafton-Cardwell EE, Reagan CA. 1995. Cardwell 2000). Growers hadn’t seen cessfully managed using both natural Selective use of insecticides for control of ar- much of cottony cushion scale in re- and augmentatively released natural mored scale (Homoptera: Diaspididae) in San cent years, so they didn’t remember enemies (Haney et al. 1992). However, Joaquin Valley California citrus. J Econ Entomol 88:1717-25. how bad it could be. there will always be destructive native Grafton-Cardwell EE, Reagan CA. 1998. In the case of pyriproxifen, the citrus pests, such as katydids and California red scale insecticide efficacy trial, beetle is sensitive for several miles citricola scale that lack effective bio- 1996. Arthropod Manag Tests 23:62-3. Grafton-Cardwell EE, Vehrs SLC. 1995. around spray areas. When growers logical control. In addition, newly in- Monitoring for organophosphate- and first began to use this insecticide in cit- troduced pests such as glassy-winged carbamate-resistant armored scale in San rus in 1998, vedalia beetles disap- sharpshooter and citrus peelminer that Joaquin Valley citrus. J Econ Entomol peared from all San Joaquin Valley ci t- do not yet have adequate biological 88:495-504. Haney PB, Morse JG, Luck RF, et al. rus orchards for 9 months. The result control will probably require insecti- 1992. Reducing insecticide use and energy was dozens of cottony cushion scale cide treatments. costs in citrus pest management. UC IPM outbreaks that could be controlled The ongoing challenge for the UC Pub 15,Statewide IPM Project, UC DANR, Oakland, CA. 62 p. only with malathion or methidathion, integrated pest management program Horowitz AR, lshaaya I. 1994. Managing two organophosphates that are highly will be to understand the impact of resistance to insect growth regulators in the toxic to parasites needed to control ar- new insecticides on both pests and sweetpotato whitefly (Homoptera: mored scale. Problems with cottony natural enemies, rotate insecticide Aleyrodidae). J Econ Entomol 872366-71. Luck RF, Morse JG, Moreno DS. 1986. cushion scale have decreased since chemistries to forestall resistance, use Current status of integrated pest manage- 1998 and 1999, probably because less insecticides in ways that do not seri- ment in California citrus groves. In: Cavalloro acreage is being treated and the ously disrupt key natural enemies, R, Di Martino E (eds.). Integrated Pest Con- beetles seem to be less sensitive to and develop biological control and trol in Citrus Groves, Proc of Experts’ Meet- ing; Mar 26-29, 1985;Acireale, Italy. Boston, pyriproxifen than they were the first nonchemical solutions to myriad inter- MA: AA Balkema. p 533-44. year. However, cottony cushion scale related citrus pests. Morse JG, Brawner OL. 1986.Toxicity of will continue to plague citrus growers pesticides to Scirtothrips citri (Thysanoptera: Thripidae) and implications to resistance as the use of pyriproxifen, buprofezin management. J Econ Entomol 79565-70. and imidacloprid continues (Grafton- E. Grnftoiz-Cnrd7i~e/~is IPM Specinlist aizif Nel JJC, De Lange L, van Ark H. 1979. Cardwell in press). Resenrch Entonzologist, Depnrtnzent of En- Resistance of citrus red scale, Aonidiella tonzology, UC Rizierside, stntioized at UC aurantii (Mask.), to insecticides. J Entomol Resistance and pest control SOCSouth Afr 42:275-81. Kennzty Agriciiltiiml Center; Y. Oii!ynizg is Quayle HJ. 1938.The development of re- Because of pesticide resistance and Stnf Resenrclz Associnte, R. Striggoul is sistance to hydrocyanic acid in certain scale secondary pest outbreaks, new insecti- Lnborntory Assistnizt nrzd S. Vehrs, 11oil10 insects. Hilgardia 11:183-210. Porterville high school science tencher, is Vehrs SLC, Grafton-Cardwell EE. 1994. cides will be only a temporary solution Chlorpyrifos effect on armored scale to the armored scale problem in San former Stnff Resenrch Associnte, UC (Homoptera: Diaspididae) populations in San Joaquin Valley citrus. Greater reliance Kenrizq Agricziltiiral Center. This project Joaquin Valley citrus. J Econ Entomol on biological control and other 7uns siipported byfiiizd~fiointhe Citrzis Re- 8717046-57. Walker GP, Morse JG, Arpaia ML. 1996. nonchemical methods will be neces- senrcli Board. The niithors thank the nzniz!y Evaluation of a high-pressure washer for sary for long-term control of citrus growers and pest coiztrol nduisors idzo postharvest removal of California red scale. J pests. California red scale can be suc- helped iden fa$/ scnle-iizfested citriis orclmrds. Econ Entomol 89:148-55.

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