HORTICULTURAL ENTOMOLOGY Chlorpyrifos Effect on Armored Scale (Homoptera: ) Populations in San Joaquin Valley

S.L.C. VEHRSI AND E. E. GRAFTON-CARDWELU

Department of Entomology, University of California, Riverside, CA 92521

J. Econ. Entomo!. 87(4): 1046-1057 (1994) ABSTRACT Portions of three citrus orchards infested with California red scale, Aoni-

diella aurantii (Maskell), and with yellow scale, A. citrina (Coquillett), were sprayed Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 either with a commercial rate of chlorpyrifos or left unsprayed. The three orchards in- cluded a chlorpyrifos-susceptible A. aurantii population, a chlorpyrifos-resistant A. auran- tii population, and a chlorpyrifos-resistant A. citrina population. The levels of efficacy of chlorpyrifos in the field trials confirmed the levels of susceptibility of scales in the laboratory bioassays. Susceptible A. aurantii were killed for 60 d after treatment with one application of properly timed chlorpyrifos. Chlorpyrifos-resistant A. aurantii were held at pretreatment levels for 30 d but increased in numbers similar to those of the unsprayed control plots by 60 d after treatment. There was no significant difference in percentage of parasitism between the sprayed and unsprayed treatments. Resistant A. citrina were unaffected by the chlorpyrifos treatment, both in the number of scales per plant structure as well as in the population structure. The percentage of parasitism was significantly greater in the unsprayed compared with the chlorpyrifos treatment in this population.

KEY WORDS aurantii, Aonidiella citrina, insecticide resistance

CALIFORNIA RED SCALE, ever, many growers now make 1-3 insecticide (Maskell), and yellow scale, A. citrina (Coquil- applications each year, with varying degrees of lett), were introduced into California in the late success. Furthermore, yellow scale, which virtu- 19th century (Hendrickson 1974) and are impor- ally had disappeared when California red scale tant pests of citrus in the San Joaquin Valley of was introduced into this region and when or- central California (Pehrson et al. 1991). Califor- ganophosphate and carbamate insecticides came nia red scale is found on leaves, twigs, and fruit, into use, has resurged in isolated areas (Carmean whereas while yellow scale prefers leaves and 1988). fruit. Both species are found on all varieties of Organophosphate and carbamate insecticides citrus and cause cosmetic damage to the fruit no longer provide adequate control of California and, in severe infestations, dieback ofleaves and red scale in Israel (Hefetz et al. 1988), and the twigs (Jeppson 1989). Currently, California red development of insecticide resistance among scale is the predominant armored scale species California red scale populations has been docu- on citrus in the San Joaquin Valley (Pehrson et mented in South Africa (Nel et al. 1979) and al. 1991). Australia (Abdelrahman 1973). This resistance, Organophosphate and carbamate insecticides in turn, has resulted in partial to complete failure have been used commercially for control of these insecticides in the field. Thus, insecti- in citrus since their development in the 1950s cide resistance is likely to occur in the United and 1960s (Carman 1977). It is a growing con- States as well. cern, among citrus growers and control ad- Our objectives in this study were to evaluate visors in the San Joaquin Valley, that armored how three San Joaquin Valley populations of ar- scale are becoming more difficult to con- mored scale insects respond to chlorpyrifos in trol with these broad, spectrum insecticides the field. Chlorpyrifos is the most commonly (Carmean 1988). When organophosphate and car- used insecticide for scale control in the San bamate insecticides first were used, one applica- Joaquin Valley because of shorter re-entry and tion every 2 yr effectively controlled the armored preharvest intervals (Bailey 1991). The three scale in most San Joaquin Valley orchards; how- scale populations were chosen based on labora- tory bioassay results, indicating differences in

1 University of California, Kearney Agricultural Center, 9240 susceptibility to chlorpyrifos and differences in South Riverbend Avenue, Parlier, CA 93648. insecticide treatment histories.

0022·0493/94/1046-1057$02.00/0 © 1994 Entomological Society of America August 1994 VEHRS & GRAFTON-CARDWELL: ARMORED SCALE POPULATIONS 1047

Table 1. Treatment histories for field trial sites indicating organophosphate and carbamate insecticides applied for 4-5 yr before the experiment

Site 1987 1988 1989 1990 1991 1992 No treatments (July) carbaryl" (May) dimethoateb No treatments Chlorpyrifosd (May) methidathionC Experiment 2 (Apr) chlorpyrifos (May) formetanate (Apr) formetanate (Apr) formetanate (May) formetanate Chlorpyrifos (Apr) formetanateC (May) naledf (May) dimethoate (Apr) naled (June) dimethoate Experiment (May) chlorpyrifos (June) carbaryl (June) dimethoate (May) methidathion (June) naled (June) carbaryl (June) dimethoate (July) carabaryl 3 No treatments (Oct) methidathion (Apr) methomylR (May) formetanate (May) formetanate Chlorpyrifos (new trees) (May) formetanate (May) naled (July) carbaryl Experiment (May) naled (June) methidathion (June) chlorpyrifos (Sept) chlorpyrifos Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021

" Or!(anophosphate insecticide applied for control of armored scale: Sevin 80S, Rhone Poulenc, Romford, Essex, England. /> Or!(anophosphate insecticide applied for control of thrips; Cygon 4EC, American Cyanamid, Wayne, NJ. c Orp;anophosphate insecticide applied for control of armored scale; Supracide 2EC, ClBA-Ceigy AG, Switzerland. d Or!(anophosphate insecticide applied for control of lepidopterous larvae or armored scale; Lorsban 4EC, Dow Elanco, Indianapolis, IN.

C Carbamate insecticide applied for control of thrips; Carzol 92SP, Nor-Am Agricultural Products, Wilmington, DE. IOrp;anophosphate insecticide applied for control of thrips; Dibrom 12EC, Valent, Walnut Creek, CA. R Orp;anophosphate insecticide applied for control oflepidopterous larvae: Lannate water-soluble liquid, Du Pont, Wilmington, DE.

Materials and Methods fornia red scale, and a potentially resistant pop- ulation of yellow scale. For our purposes, we Three populations were chosen for our study, considered populations which exhibited >80% based on the results of an insecticide resistance mean mortality at 10 ppm chlorpyrifos to be sus- survey conducted during the growing seasons of 1990-1991 (unpublished data). The survey was ceptible and those which exhibited :540% mean conducted by collecting scale-infested navel and mortality at 10 ppm chlorpyrifos to be resistant. Valencia oranges, Citrus sinensis (L.) Osbeck, Site 1 was a California red scale-infested navel from 34 commercial citrus orchards in Madera, orange orchard cultivar 'Washington' located at Fresno, Tulare, and Kern counties and perform- the University of California Lindcove Field Sta- ing laboratory bioassays with chlorpyrifos (Lors- tion, Tulare County, which had received 0-2 ban 4E [Emusifiable], DowElanco, Indianapolis, organophosphate or carbamate insecticide appli- IN). Green fmit was sampled during June through cations per year for 3 yr before the experiment October, corresponding to the second and third for control of thrips and scale insects (Table 1). generations of scale insects each season. The California red scale population exhibited Each piece of fmit was dipped in melted wax 86.0 ± 4.9% mortality when exposed to 10 ppm to cover 50% of the surface that included the of chlorpyrifos in a laboratory bioassay in June calyx to conserve moisture and prevent shrink- 1991 and was considered to be susceptible to age. The oranges were held at 27°C for 24 to 48 h chlorpyrifos. The trees in this site were ""2.5 m to allow a new cohort of first instars to emerge tall when this site was treated in August 1991. and settle. These newly settled nymphs, or white Site 2 was a California red scale-infested caps, were used for the bioassays, and a maxi- commercial Valencia orange orchard located in mum of 25 white caps were circled on each fmit Strathmore, Tulare County, which received 3-4 with a black felt-tip pen. Oranges with fewer applications of organophosphate and carbamate than 10 white caps were discarded. insecticides per year in the four preceding sea- A concentration of 10 ppm chlorpyrifos was sons for control of citrus thrips, Lepidoptera lar- used because it is the LCgg for a susceptible vae, and armored scale insects (Table 1). This population (Walker et al. 1991). This concentra- scale population exhibited 31.1 ± 16.8% mortal- tion and the water control were mixed to a total ity when tested with 10 ppm of chlorpyrifos in a volume of 500 ml, including 1 ml of 8% Triton B laboratory bioassay before the field experiment 1956, which was added as a wetting agent. Five in July 1992 and was considered to be resistant to individual oranges were dipped in each of the chlorpyrifos. The trees in this site varied in concentrations or the control for 10 s and then height, but those used for sampling were ""4.5 m held at 27°C for 10 d, at which time insects which tall when this site was treated in July 1992. had failed to begin the first molt were scored as Site 3 was a yellow scale-infested commercial dead. Valencia orange orchard located in Terra Bella, The percentage of mortality of scale insects Tulare County, where 1-4 organophosphate and exposed to 10 ppm of chlorpyrifos was used to carbamate insecticide applications had been detect a susceptible population of California red made per year in the four preceding seasons scale, a potentially resistant population of Cali- (Table 1). In October 1991, a narrow-range pe- 1048 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 87, no. 4 troleum oil and parasitoid release program was (stage 2) were omitted from the counts. In sites 2 initiated for control of armored scale insects. and 3, scales containing larvae or pupae of the This third scale population showed 39.5 ± 16.6% parasitoids DeBach or Compe- mortality when exposed to 10 ppm of chlorpyri- della bifasciata Howard were counted. Site 1 fos in a laboratory bioassay in July 1992 and was had insufficient live scale in the posttreatment considered to be resistant to chlorpyrifos. The evaluations to evaluate parasitism. The percent- b'ees used in this site were ""2.5 m tall when this age of fruit infested with at least one live scale site was treated in July 1992. also was determined. The field trials were performed using a ran- To target the crawlers and white caps, the most domized complete-block design consisting of susceptible stage (Abdelrahman 1973, Schoones four blocks, each containing an untreated control & Giliomee 1982), two methods of monitoring plot and a chlorpyrifos- treated plot of trees. The were used. Two-sided Scotch Tape (3M Corpo- Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 fields were blocked on the basis of evaluations ration, St. Paul, MN) was wrapped around a scale- before treatment of the armored pop- infested twig in each of four unsprayed trees at ulation. Each plot consisted of five rows of seven sites 1 and 2. Green twigs that had gray streaked trees at site 1, seven rows of 10 trees at site 2, and wood at the base and live adult female scale five rows of five trees at site 3. present were chosen. These tapes were changed Site 1 was treated with a commercial rate of weekly, the number of crawlers trapped on each 394 ml of chlorpyrifos in 9,463 liters of water per tape was counted, and their sums were recorded. hectare (10 pints of chlorpyrifos in 1,000 gallons The chlorpyrifos was applied in July or August of water per acre) on 5 August 1991 using an after the crawler numbers had continued to rise FMC 757 speed sprayer (Hoopeston, IL) driven sharply for 2 wk, indicating the emergence of the at 2.4 kph. The speed sprayer had eight size-5 second generation of scale insects. According to nozzles each with a two-hole spinner; 14 size-7 Walker et al. (1990), the best time to spray in the nozzles; and 12 size-8 nozzles, each with a three- second generation for optimal scale control is at hole spinner. Sites 2 and 3 were treated on 16 the first steep rise in crawler emergence. At site 'and 17 July 1992, respectively. Site 2 was treated 3, the tapes were wrapped around leaf petioles with 473 ml of chlorpyrifos in 14,194 liters of because yellow scale rarely infests the wood. In water per hectare (12 pints of chlorpyrifos in sites 2 and 3, the male scale flights also were 1,500 gallons of water per acre) with a FMC 727 monitored using pheromone traps. This helped speed sprayer driven at 2.4 kph. The speed predict when the emergence of crawlers would sprayer was set up with 64 size-5 nozzles, each sharply increase, which is usually 2-4 wk after a with a three-hole spinner. Site 3 was treated with peak in the second male scale flight (Walker et al. 473 ml of chlorpyrifos in 9,463 liters of water per 1990). hectare (12 pints of chlorpyrifos in 1,000 gallons The mean number of live scale in each life of water per acre) with the same sprayer and stage on each plant structure and the proportion speed as site 2 except 12 of the nozzles were of infested fruit in the control were compared turned off. with those in the chlorpyrifos treatment on each Samples were taken from five trees in the cen- of the sample days using analysis of variance ter of each plot at site 1 and from six center trees (PROC ANOVA, SAS Institute 1985). The pro- in each plot at sites 2 and 3 to avoid the effect of portion of infested fruit data was transformed insecticide drift from neighboring plots. Sam- arcsine (squareroot (X)) prior to ANOVA. The pling was done by randomly selecting five twigs proportion of susceptible stages of scale insects with five leaves each and five outer green fruit that were parasitized was determined at 60 d from each sample tree. Green twigs with gray after treatment. Stage-2 and virgin females are streaks at the base, indicating one year's growth, susceptible to A. melinus, and the proportion of were clipped from the main branch at their point parasitism was calculated as (A. melinus parasit- of origin. Each twig had its length trimmed to 20 ized scale)/(unparasitized stage 2 + unparasit- em, its side branches removed, and five leaves ized virgin females + A. melinus parasitized retained to make each sample uniform. Samples scale). Stage-2 through gravid female scale can were taken 16-23 d before treabnent with chi or- be found parasitized by C. bifasciata, and the pyrifos and at 30 and 60 dafter treabnent. The proportion of parasitism was calculated as (C. samples were brought back to the laboratory and bifasciata parasitized scale)/(unparasitized stage examined under a 60x power dissection micro- 2 + unparasitized virgin females + unparasitized scope. gravid females + C. bifasciata parasitized scale). Live armored scale insects were counted ac- Total parasitism of armored scale insects was cal- cording to plant structure (twig, leaves, fruit) and culated as (A. melinus parasitized scale + C. hi- life stage (stage 1, stage 2, virgin female, gravid fasciata parasitized scale)/(unparasitized stage 2 female). Stage 1 included crawlers, first-instar + unparasitized virgin females + unparasitized white caps, and first-molt scales. Stage 2 in- gravid females + A. melinus parasitized scale + cluded second-instar and second-molt females. C. bifasciata parasitized scale). These propor- Male scales old enough to be differentiated tions were transformed arcsine (squareroot (X)), August 1994 VEHRS & GRAFTON-CARDWELL: ARMOREDSCALE POPULATIONS 1049

100 r-:.,.------, Site 1 Results and Discussion Site 1. The total number of crawlers trapped in 80 CAS crawlers r.... the two-sided tapes increased from three indi- , , viduals on 24 July to 22 individuals on 5 August , , .' Chlorpyrifos ! . 1991, indicating emergence of the second gener- ation of California red scale crawlers (Fig. 1). The chlorpyrifos treatment was applied as soon as a sharp increase in crawler emergence was ," . detected. However, the peak in crawler emer- 20 :.. ."~~. .' .....:.:. ..•.../...... , gence also occurred at this sharp rise in numbers, so the application was ..,2 wk after optimal tim- ..' , Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 7/10 7/24 8/5 8/21 9/4 9/17 10/2 ing according to Walker et aI. (1990).

Collection date The population sample before treatment on 17 July (-19 d after treatment) estimated a mean Fig.1. Total number of California red scale crawl- total scale population of 0.92-1.00 live scale per ers trapped on two-sided tape in four unsprayed trees per week in site 1. Chlorpyrifos was applied to the twig, 0.19-0.33 live scale per five leaves, and treated trees on 5 August 1991 when crawler emer- 0.00-0.05 live scale per five fruit (Fig. 2). Cali- gence was peaking. fornia red scale moves to the interior of the tree in the winter and gradually moves back out to the exterior as the generations progress in the spring. Therefore, higher infestations of California red and the means were tested with general linear scale occur on the twigs compared with the model analyses to detect differences between leaves and fruit in the early part of the growing the control and chlorpyrifos treatments (PROC season. There were no significant differences be- GLM, SAS Institute 1985). tween treatments (P 2= 0.455) in the pretreatment

Site 1 Ii.: Stage 1 scale Stage 2 scale Vlrgin9 scale Gravid9 scale Total scale CI) Co 0.8 0.8 1 3 0.6 0.6 -:E m 0.6 2 W tJ) ~ 0.4 0.4 + po 0.2 1 0.2 0.2 .. CI) 0.2 - ". --- ~ 0 en 0.4 0.4 0.6 1.5 "C III CI) GI 0.3 0.3 > 1 ell "- GI 0.2 0.2 ~ ..J '2 It) 0.1 0.1 0"- ~ ~ 8 30 (.) 6 15 6 '3 0 -•.. 20 IL 4 10 4 - It) 0 10 C 2 5 2 C 0 ~ -19 +30 +60 -19 +30 +60 CI) -19 +30 +60 -19 +30 +60 :E Days after treatment untreated chlorpyrlfos ------Fig. 2. Mean number of live California red scale in each life stage and the total population found on twigs, leaves, and fruit sampled in the site 1 'Washington' navel orange field experiment. Bars surrounding means represent standard errors. Negative days after treatment = days before treatment. 1050 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 87, no. 4

Table 2. Fond P values for ANOVA performed on data from each life stage on each part of tree sampled in site 1, susceptible California red scale field trial. 1991

Sample unit Date (DAT)" 1 Twig 5 Leaves 5 Fruit F P F P F P Stage 1 17 July (-19) NCb NC NC 4 Sept (+30) NC 0.71 0.404 NC 2 Oct (+60) 4.12 0.050 NC NC Stage 2 17 July (-19) NC NC NC 4 Sept (+30) NC NC NC Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 2 Oct (+60) NC 6.94 0.013 6.90 0.013 Virgin female 17 July (-19) 0.14 0.713 0.57 0.455 NC 4 Sept (+30) 12.15 0.001 NC NC 2 Oct (+60) NC NC 19.95 0.000 Gravid female 17 July (-19) 0.01 0.908 0.20 0.660 NC 4 Sept (+30) 12.49 0.001 9.59 0.004 6.43 0.016 2 Oct (+60) NC NC 13.49 0.001 Total scale 17 July (-19) 0.04 0.845 0.39 0.535 NC 4 Sept (+30) 15.82 0.002 10.38 0.003 6.53 0.015 2 Oct (+60) 11.30 0.002 11.38 0.002 12.09 0.001

Degrees offreedom = I for each ANOVA. " Days after treatment (DAT), negative DAT indicates pretreatment sampling. b NC, not calculated because of zero variance in one or both of the means. sample for any stage of scale (Table 2). There chlorpyrifos treatment, the numbers of scale were primarily virgin and gravid females present clearly had increased in the control well beyond on twigs and leaves at that time (Fig. 2), indicat- those in the treated trees. The mean total popu- ing recent maturation of the first generation, lation had increased from 1.00 :!: 0.27 to 2.81 :!: which is further illustrated by the low number of 0.59 live scale per twig in the control while crawlers trapped on 17 July (Fig. 1). The mean the chlorpyrifos treatment had decreased from percentage of infested fruit was ::::;1.0:!: 1.0% on 0.92 :!: 0.18 to 0.06 :!: 0.01 live scale per twig. All this date (Table 3). life stages were represented equally in the con- On the twigs, at 30 d after treatment, the virgin trol at 30 d after treatment, whereas only low and gravid females were significantly higher in numbers of virgin and gravid females were found the control compared with the chlorpyrifos treat- in the chlorpyrifos treatment. Because first and ment (P ::::;0.002) (Fig. 2; Table 2). Although second instars are the two most susceptible stage-l and -2 scale numbers could not be com- stages of the scale insect life cycle (Abdelrahman pared statistically because of zero variance in the 1973), chlorpyrifos should have the greatest im-

Table 3. Percentage of fruil infesled (±SEM) with one or more live armored scale insecls

Percentage of fruit infested Site no. Treatment Pretreatment 30 DAT 60 DAT Control 1.0 ± 1.0 NA" 78.8 ± 4.0 Chlorpyrifos 0.0 ± 0.0 NA 6.5 ± 1.8 NCb F = 76.62, P = 0.003 2 Control 10.8 ± 4.5 50.8 ± 7.8 47.5 ± 6.2 Chlorpyrifos 13.3 ± 4.5 13.3 ± 4.6 15.8 ± 4.7 F = 0.45, P = 0.504 F = 32.03, P < 0.001 F = 36.16, P < 0.001 3 Control 10.8 ± 3.4 38.3 ± 5.3 19.2 ± 3.9 Chlorpyrifos 6.7 ± 2.9 10.8 ± 3.8 15.8 ± 4.5 F = 1.4, P 0.243 F = 18.21, P < 0.001 F = 0.50, P = 0.482

Means within each cell (each sample date) were compared using ANOVA.F and P values are given for each analysis; degrees of freedom = 1 for each ANOVA. " NA, samples not taken on this date. b NC, not calculated because one of the means had zero variance. August 1994 VEHRS & GRAFTON-CARDWELL: ARMORED SCALE POPULATIONS 1051

pact on these stages. At 60 d after treatment, each 35.000 1,000

of the life stages on the twigs was higher in "0 Q) a. number in the control population than in the a. ~ chlorpyrifos treatment with mean total popula- ID 25.000 tions of 1.80 ± 0.42 and 0.01 ± 0.01 live scale per ~ twig, respectively (Fig. 2). Low numbers of only Q) ~ stage-l scales were found in the chlorpyrifos "0 treatment, while all stages were present in the ~ 15.000 '" control. ~'" OJ On the leaves at 30 d after treatment, each of U the stages was present in the control, but only ~ 5.000 stage-l and gravid females were found in the I- Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 chlorpyrifos treatment (Fig. 2). The number of individuals in stages 2 through adult were higher in the control on this date, and there were sig- Fig. 3. Total number of California red scale crawl- nificantly more total live California red scale in ers trapped on two-sided tape in four unsprayed trees the control (P = 0.003) (Table 2). The mean total per week in site 2. Chlorpyrifos was applied to the population had increased from 0.19 ± 0.06 to treated trees on 16 July 1992, when crawler emergence 0.79 ± 1.14 live scale per five leaves in the con- was peaking. trol compared with the chlorpyrifos treatment, for which the mean total population had de- creased from 0.33 ± 0.16 to 0.03 ± 0.02. At 60 d selves in the scale covering and produce more after treatment, stages 1 through gravid female wax (Abdelrahman 1973). This may explain the were higher in number on the leaves in the con- survival of gravid females on the twigs and trol compared with the chlorpyrifos treatment, in leaves 30 d after the chlorpyrifos treatment (Fig. which there were low numbers of stage 2 and no 2). However, the offspring of these females did other stages detected (Fig. 2). The mean total not survive on the chlorpyrifos residues; there- population had increased to 1.36 ± 0.38 live fore, the population on the leaves and twigs did scale per leaf sample in the control, which was not recover from the insecticide application and Significantly higher (P = 0.002) than scale densi- nearly was eliminated during the 60 d after treat- ties of 0.01 ± 0.01 in the chlorpyrifos treatment. ment. On the fruit at 30 d after treatment, each of the Site 2. Crawlers trapped in the two-sided tapes scale stages was represented in the control where- increased from 235 individuals on 22 June to 921 as low numbers of gravid females were the only individuals on 13 July, indicating peak emer- stage found in the chlorpyrifos treatment (Fig. 2). gence of the second generation of California red The control trees had significantly more total scale crawlers (Fig. 3). The chlorpyrifos was ap- California red scale per fruit sample than the plied on 16 July 1992. chlorpyrifos trees (P = 0.015) (Table 2), and the The population samples before treatment on mean total population in the control had in- 30 June (-16 d after treatment) revealed the creased from 0.05 ± 0.05 to 8.55 ± 3.38 live scale mean total scale population was 1.38-2.03 live per five fruit whereas the chlorpyrifos population scale per twig, 0.80-1.17 live scale per five had increased from 0.00 ± 0.00 to 0.10 ± 0.07 leaves, and 1.25-2.37 live scale per five fruit live scale per fruit sample. At 60 d after treat- (Fig. 4). There were no significant differences ment, each of the stages was abundant and sig- between means of any scale stage within each nificantly higher in the control trees (P :s 0.013) plant part (P :2: 0.123) (Table 4) except on the where the mean total population was 28.65 ± twigs where the chlorpyrifos treatment started 7.89 live scale per fruit sample compared with with significantly higher numbers of stage 2 Cal- the chlorpyrifos treatment, which had low num- ifornia red scale (P = 0.021). Virgin and gravid bers of stages 2 through gravid female and a females as well as increasing numbers of crawl- mean total population of 0.50 ± 0.31 live scale ers trapped on two-sided tape (Fig. 3), indicating per fruit sample. the beginning of the second generation, were At 60 d after treatment, the percentage of in- present on twigs and leaves at this time. The fested fruit was significantly higher in the control percentage of infested fruit was 10.8 ± 4.5% in than in the chlorpyrifos treatment (P = 0.003) the control and 13.3 ± 4.5% in the chlorpyrifos (Table 3). In the control, the percentage of fruit treatment, with no significant differences at this infested with one or more live scale insects had time (P = 0.504) (Table 3). Valencia trees have increased from 1.0 ± 1.0% to 78.8 ± 4.0% com- mature fruit and green fruit on the tree at the pared with the chlorpyrifos treatment, which in- same time. The scale insect population is able to creased slightly from 0.0 ± 0.0% to 6.5 ± 1.8%. increase throughout the year and readily move Mature gravid females and molting scale in- onto the green fruit as well as the leaves and sects are more tolerant to insecticides than other twigs, in contrast to navel trees where the fruit armored scale stages because they encase them- usually is harvested several months before the 1052 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 87, no. 4

Site 2 ..: stage 1 scale stage 2 scale Virgin? scale Gravid? scale Total scale Q) 10 2.5 1.2 3 Co 3 U) 2 ~ 2 - 2 UJ ~ 1.5 V) •.• 1 1 + 1 0.5 ". ... +------I '1•••• --., -Q) C6 o O.S 0.8 Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 en 1.5 "0 : 0.6 0.6 2 Q) > 1 •.• : 0.4 CO ...J 1 _ ... J '2 It) 0.2 ...... ·0.5 , 'i--. •..o :: C6 o 30 15 60 20 10 40 10 5 20

-16 +30 +60 -16 +30 +60 -16 +30 +60 -16 +30 +60 -16 +30 +60 Days after treatment untreated chlorpyrlfos Fig. 4. Mean number of live California red scale in each life stage and the total population found on twigs, leaves, and fruit sampled in the site 2 Valencia orange field experiment. Bars surrounding means represent standard errors. Negative days after treatment = days before treatment. new crop is set. This is likely why the percentage This is further evidence of poor efficacy of this of infested green fruit and the mean number of chemical. scale per five green fruit were higher in site 2 On the leaves at 30 d after treatment, each of than site 1 before treatments were applied. the stages was significantly higher in the control Each of the stages on the twigs was signifi- compared with the chlorpyrifos treatment (P ~ cantly higher in number in the control compared 0.040) (Table 4), and all stages were present in with the chlorpyrifos treatment by 30 dafter both treatments (Fig. 4). The mean total popula- treatment (P ~ 0.036) (Fig. 4; Table 4). The mean tion increased from 0.80 ± 0.26 to 4.19 ± 1.24 total population had increased from 1.38 ± 0.24 total live California red scale per five leaves in to 8.21 ± 1.64 live scale per twig in the control the control and decreased in the chlorpyrifos while the chlorpyrifos treatment had decreased treatment from 1.17 ± 0.33 to 0.39 ± 0.11 by 30 d from 2.03 ± 0.46 to 1.10 ± 0.27 live scale per after treatment. Thus, there was Significant sup- twig. All life stages were represented in the con- pression of California red scale through 30 dafter trol as well as in the chlorpyrifos treatment. This treatment on the leaves. By 60 d after treatment, failure to reduce the number of young instars however, only virgin female California red scale well below the levels of virgin and gravid fe- were significantly higher in number on the leaves males, in contrast to the response of the armored in the control compared with the chlorpyrifos scale insects at site 1, suggested poor efficacy of. treatment (P = 0.019). The mean total popula- chlorpyrifos in this experiment. At 60 dafter tions were 1.94 ± 0.36 live California red scale treatment, the numbers of virgin and gravid fe- per leaf sample in the control and 1.35 ± 0.37 in male California red scale were significantly higher the chlorpyrifos treatment (Fig. 4) with no signif- on the twigs in the control than in the chlorpyr- icant difference (P = 0.280), and all stages were ifos treatment (P ~ 0.009). There were no or only present in both treatments. slightly significant differences between stages 1 On the fruit at 30 d after treatment, the control and 2 (P = 0.072 and P = 0.495, respectively), trees had significantly more stage-l and virgin suggesting that the third generation was emerg- female California red scale compared with the ing and overcoming the chlorpyrifos residue. chlorpyrifos treatment (P ~ 0.014) (Table 4). The August 1994 VEHRS & GRAFTON-CARDWELL: ARMORED SCALE POPULATIONS 1053

Table 4. F and P values for ANOVA performed on data from each life slage on each parl of lree sampled in sile 2, chlorpyrifos.resislant California red scale field lrial, 1992

Sample unit Date (DAT)" 1 Twig 5 Leaves 5 Fruit F p F P F P Stage 1 30 June (-16) 0.07 0.409 2.47 0.123 NCb 17 Aug (+30) 4.70 0.036 5.99 0.019 6.64 0.014 14 Sept (+60) 3.41 0.072 2.13 0.152 1.36 0.251 Stage 2 30 June (-16) 5.73 0.021 0.83 0.366 0.68 0.415 17 Aug (+30) 7.10 O.Oll 4.51 0.040 2.49 0.122 Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 14 Sept (+60) 0.47 0.495 4.83 0.033 0.01 0.943 Virgin female 30 June (-16) 0.20 0.654 0.40 0.532 0.41 0.525 17 Aug (+30) 4.70 0.036 5.14 0.029 6.95 0.012 14 Sept (+60) 10.88 0.002 5.93 0.019 0.16 0.695 Gravid female 30 June (-16) 1.03 0.316 0.01 0.928 0.35 0.555 17 AUld+30) 7.16 O.Oll 6.36 0.016 2.90 0.096 14 Sept (+60) 7.45 0.009 2.13 0.152 0.29 0.590 Total scale 30 June (-16) 2.03 0.162 0.60 0.427 0.41 0.526 17 Aug (+30) 8.15 0.007 6.59 0.014 3.83 0.057 14 Sept (+60) 7.60 0.009 1.20 0.280 0.28 0.597

Degrees of freedom = 1 for each ANOVA. " Days after treatment (DAT), negative DAT indicates pretreatment sampling. b NC, not calculated because of zero variance in one or both of the means. mean total population increased from 2.37 ± 1.75 chard was distributed evenly throughout the dif- to 80.96 ± 42.01 live scale per five fruit in the ferent plant structures (twigs, leaves, and fruit), control (Fig. 4), which was significantly higher illustrated by similar numbers of total scale in- than the population in the chlorpyrifos-treated sects on each plant structure with a relatively trees (P = 0.057), which had increased from high percentage of infested fruit at the pretreat- 1.25 ± 0.64 to 2.92 ± 1.53. All stages were ment sample date (10.8 ± 4.5 to 13.3 ± 4.5% present in both the control and chlorpyrifos treat- infested fruit). By the end of the field trial, the ment. At 60 d after treatment, the numbers of live numbers of scale insects per five infested green scale insects had decreased in the control and fruit in the chlorpyrifos treatment greatly in- increased in the chlorpyrifos treatment, and creased, but the percentage of fruit that were there were no Significant differences in any stage infested remained fairly constant (15.8 ± 4.7%). (P ~ 0.251). The mean total population in the This suggests that the chlorpyrifos applied in control was 34.58 ± 14.50 live California red July did exhibit one form of efficacy-it reduced scale per fruit sample and that of the chlorpyrifos the movement of the second generation of crawl- treatment was 23.33 ± 14.70. ers onto the fruit. However, 60 d after treatment, At 30 d after treatment, the percentage of in- there was still at least one more generation of fested fruit had increased from 10.8 ± 4.5% to scale emerging on the fruit, resulting in greater 50.8 ± 7.8% in the control which was signifi- than 15% infestation. All stages were found in cantly greater than that of the chlorpyrifos treat- the chlorpyrifos treatment on all parts of the ment (P < 0.001), which remained relatively con- plant throughout the experiment. This contrasts stant at 13.3 ± 4.6% (Table 3). At 60 dafter with site 1, where newly emerging scale were treatment, the percentage infested fruit re- not able to survive the residues. In site 2, the mained Significantly higher in the control (P < numbers of each stage of California red scale 0.001), with 47.5 ± 6.2 % of the fruit infested were reduced for 30 d in the chlorpyrifos treat- with at least one live California red scale com- ment, but, by 60 d after treatment, they had in- pared with the chlorpyrifos treatment with creased to pretreatment levels on the leaves and 15.8 ± 4.7% infested fruit. fruit. This illustrates that chlorpyrifos did not Because site 2 showed resistance in laboratory control effectively California red scale in site 2. bioassays (31.1 ± 16.8% mortality at 10 ppm Parasitism was investigated as a possible cause chlorpyrifos), survival of some scales in the field of population decline in the control population was likely to occur after a treatment with this between 30 and 60 d after treatment in site 2 insecticide. Initially, the population in this or- (Fig. 4). There were no differences in parasitism 1054 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 87, no. 4

40.000 200 Site 3 IChlorpyrifos and the chlorpyrifqs population did not. It is pos- sible that one or more insect predators were

"C YS males feeding on the California red scale in the control 8:" 30.000 150 "C plots. Aphytis melinus and C. bifasciata were ~ Crawlers Q. ID "Q. able to survive residues on treated twigs, leaves, iii jg E and fruit, suggesting some tolerance to chlorpyr- ~ 20.000 ifos residues exists in these species. ~ £1 Site 3. The total number of yellow scale crawl- -.; ers trapped increased from three individuals on 7U'" 10,000 ...o 22 June to 179 individuals on 17 July (Fig. 5). Chlorpyrifos was applied on 17 July 1992.

The pretreatment population samples on 24 Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 5122 615 6119 713 7117 7131 8114 8128 June indicated a mean total population of 0.04- Collection date 0.06 live scale per twig, 2.74-6.08 per five leaves, Fig. 5. Total number of yellow scale crawlers and 0.71-1.21 per five fruit (Fig. 6) and no sig- trapped on two-sided tape in four unsprayed trees per nificant differences between treatments (P ~ week in site 3. Chlorpyrifos was applied to the treated 0.176) (Table 5). On the leaves, most of the pop- trees on 17 July 1992, when crawler emergence was ulation was represented by virgin and gravid fe- peaking. male yellow scale at this time, which corre- sponds to the low number of crawlers trapped on between the control and chlorpyrifos treatment this date (Fig. 5). The percentage of infested fruit on the twigs (2.4 ± 1.4% and 8.8 ± 4.7%, respec- was 6.7-10.8%, with no significant difference be- tively, N = 36, F = 2.52, P = 0.122), leaves (4.9 ± tween the control and chlorpyrifos treatment 1.7% and 6.3 ± 3.1%, N = 29, F = 0.05, P = (F = 1.40, P = 0.243) (Table 3). 0.826), or fruit (10.2 ± 2.7% and 6.5 ± 2.8%, N = On the twigs at 30 d after treatment, there were 33, F = 1.70, P = 0.203) by 60 d after treatment. no significant differences between treatments in Thus, parasitism does not explain why the con- the number of scale per twig in any stage (P ~ trol population declined on all parts of the tree 0.295) (Table 5). The mean total population had

Site 3 ... Stage 1 scale Stage 2 scale Virgin ~scale Gravid ~scale Total scale Q) 0.06 a. 0.8 0.06 0.12 0.3 ~ .Ql 0.6 0.04 .. - , . 0.08 ' W ~ 0.04 , . 0.2 . ,, .. 0.4 (J) •... . f 0.02 + 0.02 0.2 0.1 , ", --Q) CO () 10 10 4 8 en III , '. 8 2 ,,' ~ 3 " 6 ~ t1l 6 0 2 4 ~ 0.5 4 Q) It) >- 1 2 2 15 ... 2 Q) 2 .0 3 6 1.5 E ±! 1.5 .. ::J 2 4 ::3 u: 1 . 1 0.5 Z It) 2 c: 0.5 0.5 CO Q) -23 +30 +60 -23 +30 +60 -23 +30 +60 -23 +30 +60 -23 +30 +60 ~ Days after treatment untreated chlorpyrifos .... --.-- .. --.

Fig. 6. Mean number of live yellow scale in each life stage and the total population found on twigs, leaves, and fruit sampled in the site 3 Valencia orange field experiment. Bars surrounding means represent standard errors. Negative days after treatment = days before treatment. August 1994 VEHRS & GRAFTON-CARDWELL: ARMORED SCALE POPULATIONS 1055

Table 5. F and P values for ANOVA performed on dala from each life slage on each pari of lree sampled in sile 3, chlorpyrifos-resistanl yellow scale field lrial, 1992.

Sample unit Date (OAT)" 1 Twig 5 Leaves 5 Fruit F p F P F P Stage 1 24 June (-23) NU 0.02 0.895 1.01 0.322 17 Aug (+30) 0.61 0.439 0.02 0.881 0.06 0.804 14 Sept (+60) 0.00 1.000 om 0.929 0.07 0.791 Stage 2 24 June (-23) NC 0.24 0.626 1.89 0.176

17 Aug (+30) NC 1.29 0.262 0.09 0.770 Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 14 Sept (+60) 0.93 0.340 1.98 0.166 2.47 0.123 Virgin female 24 June (-23) 0.15 0.703 1.52 0.225 0.04 0.884 17 Aug (+30) 0.00 1.000 0.09 0.763 0.19 0.667 14 Sept (+60) NC 3.49 0.069 NC Gravid female 24 June (-23) 0.16 0.690 0.76 0.387 0.30 0.584 17 Aug (+30) 1.13 0.295 0.25 0.619 0.01 0.914 14 Sept (+60) 0.15 0.699 0.28 0.599 0.87 0.356 Total scale 24 June (-23) 0.19 0.664 0.64 0.428 0.71 0.405 17 Aug (+30) 3.06 0.088 0.01 0.922 0.20 0.656 14 Sept (+60) 0.32 0.575 0.71 0.405 1.07 0.307

Degrees of freedom = 1 for each ANOVA. " Oays after treatment (OAT), negative OAT indicates pretreatment sampling. /> NC, not calculated because of zero variance in one or both of the means. increased from 0.04 ± 0.02 to 0.09 ± 0.04 live By 30 d after treatment, the percentage of in- yellow scale per twig in the control and from fested fruit in the control increased from 10.8 ± 0.06 ± 0.03 to 0.26 ± 0.09 in the chlorpyrifos 3.4% to 38.3 ± 5.3%, which was significantly treatment. All life stages were represented in greater than the chlorpyrifos treatment, which both the untreated control and the chlorpyrifos increased from 6.7 ± 2.9% to 10.8 ± 3.8% (P < treatment. At 60 d after treatment, there were no 0.001) (Table 3). This suggests some effective- significant differences between the control and ness exerted by chlorpyrifos; however, it did not chlorpyrifos treatment (P ~ 0.340), and the mean last. At 60 d after treatment, the control and total populations had decreased to 0.05 ± 0.03 chlorpyrifos treatment had 19.2 ± 3.9% and and 0.09 ± 0.06 live yellow scale per twig, re- 15.8 ± 4.5% infested fruit, respectively, with no spectively. significant difference (P = 0.482). There were no significant differences among The low number of scale insects found on the any of the stages on the leaves and fruit at 30 and twigs throughout the field trial in site 3 is typical 60 d after treatment (P ~ 0.069) (Table 5), and all of yellow scale, that rarely infests wood (Fig. 6). stages were present in the untreated and treated The leaves and fruit are more useful in evaluat- trees (Fig. 6). By 30 d after treatment, the mean ing the impact of chlorpyrifos on this population. total population on the leaves increased from The data indicate that chlorpyrifos did little to 2.74 ± 0.71 to 6.58 ± 2.68 live yellow scale per control the numbers of scale in this population five leaves in the control and from 6.08 ± 4.17 to and prevent movement onto fruit. By the end of 6.92 ± 1.20 in the chlorpyrifos treatment. At 60 d the field trial, there were no differences in total after treatment, the mean total population had populations on any plant structure or in percent- increased to 7.04 ± 2.07 live yellow scale per leaf age of infested fruit. The control population was sample in the chlorpyrifos treatment and de- declining and had fewer individuals than the creased to 3.83 ± 1.18 in the control. At 30 dafter chlorpyrifos treatment population on all parts of treatment, the mean total populations on the fruit the tree. Stage-l and -2 yellow scale were able to had increased from 1.21 ± 0.45 to 4.42 ± 1.61 survive on the residues in this orchard just as in live yellow scale per five fruit in the control and site 2, which enabled the population to increase from 0.71 ± 0.43 to 5.37 ± 1.53 in the chlorpyr- on the leaves and fruit. ifos treatment. At 60 d after treatment, the mean Parasitism was investigated as a possible cause total population had decreased in the control to of population decline in the control population 2.67 ± 1.94 live scale per fruit sample whereas 60 d after treatment (Fig. 6). The percentage of the population had remained constant at 5.37 ± susceptible stages parasitized by A. melinus at 1.77 in the chlorpyrifos treatment. 60 d after treatment was not significantly differ- 1056 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 87, no. 4 ent between the control and chlorpyrifos treat- result in more infestation and denser colonies on ment on the leaves (10.8 ± 3.8% and 7.6 ± 3.8%, the fruit. At site 3, there were no significant dif- respectively; N = 29, F = 0.02, P = 0.898) or fruit ferences between the control and treated areas (26.0 ± 13.1% and 13.0 ± 11.0%, respectively; on the leaves or fruit at 30 and 60 d after treat- N = 17, F = 0.44, P ~ 0.519). No calculation was ment. The grower's economic threshold thus possible for the twigs because there were few or would be exceeded when the younger individu- no yellow scale or parasites on this part of the als matured in the treated and untreated trees, tree. A. melinus was able to survive and parasit- which had 15.8 ± 4.5 and 19.2 ± 3.9% fruit in- ize yellow scale equally well on treated trees fested with all stages, respectively. Also, another compared with the control trees. In contrast, the generation of scale was ready to emerge. The percentage of susceptible stages parasitized by C. main difference in response of the scale insects

bifasciata was significantly higher in the control at these three sites is that the newly emerged Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 than in the chlorpyrifos treatment on the leaves crawlers were not able to settle on the residues (13.4 ± 3.9% and 4.5 ± 2.0, respectively; N = 40, in site 1, whereas, in sites 2 and 3, early instars F = 4.77, P = 0.036) and fruit (26.3 + 11.9% and were able to settle and mature in the chlorpyrifos 4.6 ± 4.6%, respectively; N = 24, F = 3.96, P = treatment, and the growth of the population was 0.061). Thus, C. bifasciata activity may explain suppressed for only 30 d or not at all. the decline of the yellow scale population in the Field efficacy of chlorpyrifos was evaluated us- control. The differences in percentage parasitism ing twigs, leaves, and fruit. In the early season, by C. bifasciata between treated and untreated California red scale numbers were low and were trees suggest that this population of the parasi- found primarily on twigs and leaves. As the sea- toid was less tolerant of chlorpyrifos residues son progressed, California red scale population compared with the A. melinus population, which differences between the control and the chI or- showed no differences in densities between pyrifos treated trees could be measured on all treated and untreated trees. plant structures. Yellow scale were found pri- The economic threshold for armored scale in- marily on the leaves and fruit in Valencia or- sects on fresh market citrus fluctuates from year anges throughout the season. Thus, efficacy of to year. When citrus is more abundant, the chlorpyrifos in reducing yellow scale could be threshold is lower and the packinghouses are measured on either of these plant structures. more conservative. If fresh citrus is in short sup- The results of this experiment indicate that ply, a greater amount of scale-infested fruit is armored scale populations, which exhibit 31.1 ± accepted at the packinghouses. Usually growers 16.8 - 39.5 ± 16.6% mortality at 10 ppm chlor- try to maintain less than 10% of the fruit infested pyrifos in the laboratory, are not controlled effec- with adult female armored scales in expectation tively by this insecticide in the field. Sites 2 and of a conservative market. Clusters of armored 3 were treated more extensively with insecti- scales are more likely to be detected by fruit cides than site 1 (Table 1). The data also confirm inspectors in packinghouses than are scattered that chlorpyrifos laboratory bioassays may be individuals. used to detect resistance levels that result in At site 1, California red scale was almost un- poor field efficacy. This suggests that surveys to detectable on the twigs, leaves, and fruit with determine the extent of chlorpyrifos resistance only one application of chlorpyrifos. The popu- and a resistance management program are needed lation could not move onto the navel oranges, for armored scale in the San Joaquin Valley. and the percentage of infested fruit was within the economic threshold (6.5 ± 1.8% fruit infested with one or more California red scale of any Acknowledgments stage). The armored scale insect populations in sites 2 We are sincerely grateful to Mike Spry for helping us locate the commercial Valencia orchards used in this and 3, in contrast, were not sufficiently con- trial. Our thanks to Greg Walker for his helpful com- trolled by chlorpyrifos. At site 2, the California ments on the manuscript. We are grateful for the sta- red scale population was maintained at the cur- tistical advice provided by Carol Adams. We also thank rent level or decreased slightly for the 30 dafter the Citrus Research Board and the Western Regional treatment sample; however, by 60 d after treat- Pesticide Impact Assessment Program for funding in ment, the population in the chlorpyrifos treat- support of this project. ment had increased on the leaves and fruit to the same level as the control. Although the percent- age of fruit infested was smaller in the chlorpyr- References Cited ifos treatment (15.8 ± 4.7% infested fruit) than in Abdelrahman, I. 1973. Toxicity of malathion to Cal- the control (47.5 ± 6.2% infested fruit), it was not ifornia red scale,Aonidiella aurantii (Mask.)(Hemip- within the economic threshold. Growers nor- tera: Diaspididae). Aust. J. Agric. Res. 24: 11-1l8. mally would make a second insecticide applica- Bailey, J. B. 1991. Citrus treatment guide. Cooper- tion, given a situation such as this, because an- ative Extension Div. Agric. Nat. Res. Publ. 2903. other generation of scale would emerge and University of California, Oakland. August 1994 VEHRS & GRAFTON-CARDWELL: ARMORED SCALE POPULATIONS 1057

Carman, G. E. 1977. Chemical control of scale in- to insecticides. J. Entomol. Soc. South Afr. 42: 275- sects on California citrus. Proc. Int. Soc. Citricul- 281. ture 2: 468-474. Pehrson, J. E., D. L. Flaherty, N. V. O'Connell, P. A. Carmean, L. 1988. Integrated pest management in Phillips & J. G. Morse. 1991. Integrated pest the San Joaquin Valley of California, pp. 1269- management for citrus, 2nd ed. University of Cali- 1273. In R. Goren & K. Mendel [eds.], Proceedings fornia, Statewide Integrated Pest Management Pro- of the Sixth International Citrus Congress, Tel Aviv, ject, Div. Agric. Nat. Res. Publ. 3303. University of Israel, March 6-11. International Society of Citri- California, Oakland. culture, Rehovot, Israel. SAS Institute. 1985. SAS user's guide: statistics. Hefetz, A. S., B. Kronenberg, B. Peleg & 1. Bar-Zakay. SAS Institute, Cary, NC. 1988. Mating disruption of the California red scale Schoones, J. & J. H. Giliomee. 1982. The toxicity of Aonidiella aura7ltii (Homoptera: Diaspididae) pp. methidathion and citrus spray oil to mature and 1121-1127. In R. Goren & K. Mendel [eds.], Pro- immature stages ofOP-resistant and susceptible red ceedings of the Sixth International Citrus Congress, scale, A07lidiella aurantii (Mask.) (: Di- Downloaded from https://academic.oup.com/jee/article/87/4/1046/873469 by guest on 01 October 2021 Tel Aviv, Israel, March 6-11. International Society aspididae). J. Entomol. Soc. South Afr. 45: 1-13. of Citriculture, Rehovot, Israel. Walker, G. P., D. C. G. Aitken, N. V. O'Connell & Hendrickson, R. M., Jr. 1974. Extinction of the yel- D. Smith. 1990. Using phenology to time insec- low scale, Aonidiella citrina (Coq.) (Homoptera: ticide applications for control of California red scale Diaspididae) by competitive displacement in south- (Homoptera: Diaspididae) on citrus. J. Econ. Ento- ern California. Ph.D. dissertation, University of Cal- mol. 83: 189-196. ifornia, Riverside. Walker, G. P., C. B. Richards, W. G. Jones & D.C.G. Jeppson, L. R. 1989. Biology of citrus insects, mites, Aitken. 1991. Toxicity of five insecticides used and mollusks, pp. 1-87. 171 W. Reuther, E. C. Cala- to control California red scale (Homoptera: Dias- van, & G. E. Carman [eds.], The citrus industry, vol. pididae) against susceptible red scale strains. V. Div. Agric. Nat. Res. University of California, J. Econ. Entomol. 84: 17-24. Oakland. Nel, J.J.C., L. DeLange & H. van Ark. 1979. Resis- Received for publication 15 July 1993; accepted 16 tance of citrus red scale, Aonidiella aurantii (Mask.), March 1994.