Intraspecific Variation in Levels of Pesticide Resistance In Field Populations of a Parasitoid, Aphytis melinus (Hymenoptera: Aphelinidae): The Role of Past Selection Pressures
JAY A. ROSENHEIM ANDMARJORIE A. HOY Department of Entomological Sciences, University of California, Berkeley, California 94720
FORUM:J. Econ.Entomol.79: 1161-1173 (1986) ABSTRACT Thirteen populations of Aphytis melinus DeBach, a key biological control agent of California red scale, Aonidie/la aurantii (Maskell),were collected from the citrus- growing regions of California. Each population's history of exposure to insecticides was estimated by determining history of insecticide use at both local (in-grove) and regional (county-wide) geographical scales. Concentration/mortality regressions for five insecticides widely used in citrus were estimated for the populations. For each chemical, substantial variability existed in the responsesof different populations. LC",,'swere correlated with both in-grove and county-wide pesticide use histories; patterns of variability were best explained by results of a multiple regression analysis that combined the influencesof these two histories. Roles of food limitation, migration, and host distribution in determining patterns and rates of evolution of pesticide resistance in arthropod biological control agents are discussed.
KEY WORDS Aonidie/la aurantii, Aphytis melinus, biological control, citrus, integrated pest management, intraspecific variation, pesticide resistance
PRACTITIONERSOF integrated pest management ecologically selective manner (Hull & Beers 1985). (IPM) in agricultural cropping systems may em- Another is the use of materials with a physiological ploy a diverse array of tactics to control noxious selectivity based upon either natural tolerance or organisms. Many of these tactics may be grouped evolved resistance of the biological control agent under the categories of biological control, chemi- (Mullin & Croft 1985). The evolution of resistance cal control, cultural control, and plant resistance. in beneficial arthropods through natural selection Most of these approaches are compatible; thus, bi- in the field and artificial selection in the laboratory ological control, cultural control, and the use of has recently been documented, and such resistant plant resistance may, with certain noteworthy ex- natural enemies have been successfully incorpo- ceptions (e.g., Campbell & Duffey 1979, Herzog rated into IPM programs (Croft & Strickler 1983, & Funderburk 1985), be combined to good effect, Hoy 1985a). as may chemical control, cultural control, and plant Aphytis melinus DeBach is the major biological resistance. This mutual compatibility does not, control agent of the California red scale, Aonid- however, often extend to biological and chemical iella aurantii (Maskell), in California and in many control. other citrus-growing regions of the world (Rosen The application of broad-spectrum insecticides & DeBach 1979), Although the degree of control frequently disrupts the action of arthropod biolog- exerted by A. melinus ranges from partial to com- ical control agents, with resulting pest resurgences plete in the different citrus-growing regions of and secondary pest outbreaks (Bartlett 1964, California, its effectiveness in all these regions is De Bach 1974) (Fig. 1). This disruption may be due severely impeded by the use of insecticides either to a direct toxic effect on predators and (DeBach et al. 1971, Bellows et al. 1985, Griffiths parasitoids, whose susceptibility to insecticides is et al. 1985). Chemicals commonly applied for con- often greater than that of the associated pest species trol of A. aurantii and other key pests of citrus not (Croft & Brown 1975), or to an indirect effect via under biological control, including the citrus thrips, starvation, emigration, or lack of hosts after the Scirtothrips citri (Moulton), and several lepidop- reduction of the pest population by the pesticide teran species, are toxic to A. melinus (University (Huffaker 1971, Newsom 1974, Powell et al. 1985) of California, Statewide IPM Project 1984; Morse (Fig. 1). This incompatibility represents a serious & Bellows 1986), resulting in the destruction of handicap in the development of sound IPM pro- resident populations and hindering programs of grams. augmentative releases of insectary-reared parasit- Attempts to overcome this handicap have taken oids. several forms. One is the use of chemicals in an Our study was done to investigate whether A.
1161 1162 FORUM: JOURNAL OF ECONOMIC ENTOMOLOGY Yol. 79, no. 5
I Pesticide is ecologically I I TYPE OF I selective to population, IPROXIMATE EFFECTI Ul TIMA TE EFFECTS ARTHROPOD 121; population is naturally I OF PESTICIDE I OF PESTICIDE APPLICATION POPULATION I tolerant, Q[ population I APPllCA TION I I has evolved resistance. I I
A)I TARGET PEST ~ DEATH 1----~.-11.Suppression of target pest population. I
DEATH 2. Possible resurgence of secondary pest.
B) SECONDARY PEST (Host/Prey of (C» 3. Collapse 01 biological control agent population due to lack of hosts or prey SURVIVAL" 't- (i.e. food limitation, assuming no alternate hosts). " / \',.1..•.. Possible resurgence 01 secondary pesl. ./ ' DEATH -~\'~'-'+I4. Probable outbreak of secondary pesl. I C) BIOLOGICAL CONTROL AGENT // "... SURVIVAL -----:•.---+15. Continued biological control.
!/ Only pesticides thet ere ellectlve In killing the target pest are employed, hence the,e Is no ·yes· option. (Some IPM programs do, howeve" use allective chemicals at 'ates Intanded to kill only a p,oportion 01 the target peat populetion.)
Fig. 1. Some proximate .and ~ltimate effects of pesticide applications on the ecology of arthropod parasitoidj host and predator jprey relatIonshIps:secondary pest resurgences and outbreaks due to the direct effects of pesticide toxicity and t~e indirect effects ?f food limitation on biological control agents. (For purposes of explanation, we present scenanos ~here populat.lOnsshow either 100% mortality or 100% survival; these outcomes represent the two ends of a contmuum of pOSSibleresponses.Outcomes that are intermediate to both the proximate and ultimate effects listed may occur.) melinus has evolved increased levels of resistance number of generations (ca. 0-4) required for pop- in response to the selective pressures exerted by ulations to adapt to laboratory conditions and at- the use of insecticides. We determined if variabil- tain a normal rate of increase varied. During this ity existed in the resistance levels of different field period of adaptation the effective population size populations of A. melinus, and ascertained if a may have been substantially less than the total adult relationship existed between these resistance levels population. In addition to the 11 colonies collected and the previous insecticide use in the localities as described above, 2 colonies, 1 collected in where the colonies were collected. Finally, we Ventura County in February 1983 (population 10), considered the roles of food limitation, migration, and the other a long-term laboratory colony (pop- and host distribution in determining patterns and ulation 13), were provided by T. S. Bellows and rates of the evolution of pesticide resistance in A. R. F. Luck (Division of Biological Control, Uni- melinus. versity of California, Riverside), respectively, for testing. Voucher specimens from each colony were confirmed as A. melinus by D. Rosen, Hebrew Materials and Methods University, Rehovot, Israel. Colony Collection and Maintenance. In Octo- Colonies were maintained in the laboratory at ber 1984, 11 populations of A. melinus were col- 26 ± 2°C, 70 ± 10% RH, and a photoperiod of lected from the major citrus-growing regions of 16:8 (L:D) on a uniparental strain of oleander scale, California (Fig. 2) by two techniques: I) citrus Aspidiotus nerii Bouche, that was grown at 24 ± fruits bearing parasitized A. aurantii were col- 1°C on pink banana squash, Cucurbita maxima lected, and 2) trap fruits (lemons infested with A. Duchesne, under constant darkness. This reaTing aurantii in the laboratory) were placed in citrus technique was adapted from that described by trees to elicit oviposition by A. melinus. Both tech- DeBach & White (1960). Undiluted honey was niques were used at each collection site. In sites provided in cages as a carbohydrate source for A. with heavy infestations of A. aurantii, the collec- melinus. tion of scale-bearing citrus fruits generally yielded Bioassays. Adult male and female A. melinus the greater number of parasitoids, whereas in sites (0-48 h old) were collected for testing with pes- with light infestations use of the trap fruits was ticides by placing squash bearing parasitized scale the more effective approach. The number of field- in an emergence cage similar to that described by collected parasitoids used to initiate each colony Abdelrahman (1973). The cage consisted of a sealed varied from 42 to 1,560 (average = 415). The plastic garbage can with 12 holes drilled in the October 1986 ROSENHEIM& Hoy: PESTICIDERESISTANCEVARIATIONIN A. melinus 1163
repeated on at least three different days for a total of 4-18 replicates per concentration. Data were analyzed by probit analysis with the POLO computer program (Russell et al. 1977). Hypotheses of parallelism (equal slopes) and equality (equal slopes and intercepts) were tested with likelihood-ratio tests (Savin et al. 1977). Pop- ulations were considered to have different toler- ances if the hypothesis of equality was rejected (a= 0.05). Pesticide Use Histories. To investigate the re- lationship of past exposure to insecticides to ob- served population resistance levels, we investigat- ed each population's history of insecticide exposure. (Population 13, a laboratory colony, was not in- cluded in this analysis.) This was done indirectly by assessing 1) the past use of insecticides in the grove from which the colony was collected (the local or in-grove pesticide use) during the 5-year period (1980-84) and 2) the past use of insecticides in citrus groves in the surrounding areas (the re- gional or county-wide pesticide use) for the same period. Fig. 2. Distribution of commercial citrus in Cali- In-grove and county-wide pesticide use histories fornia. Each dot represents 405 ha. Numbered circles were obtained from individual growers and the are collection sites for A. melinus colonies. California Department of Food and Agriculture, Division of Pest Management (unpublished rec- ords, Sacramento, Calif.), respectively. We as- sumed that results of these 5-year surveys would cover. Each of these holes was filled with a glass approximate the relative overall levels of historical test-tube (12 by 75 mm) containing streaks of hon- pesticide use not only for 1980-84, but for an ear- ey. By illuminating from above, we then drew the lier period of time as well, during which A. meli- parasitoids, which are positively phototropic and nus was present in California; the groves sampled negatively geotropic, into the tubes, where they were mature and had been under single ownership were harvested. Cages were emptied and cleaned for more than the 5-year period surveyed. Also, every 48 h to provide parasitoids that were 0-48 patterns of pesticide use were consistent over time hold. in those groves for which more extensive histories Concentration/mortality data were generated by were available. confining between 10 and 20 adult parasitoids in The influences of in-grove and county-wide pes- a treated disposable plastic cup (30 ml) capped ticide use were evaluated both independently and with untreated polyester gauze upon which undi- in combination using bivariate (one independent luted honey was provided. The cups were treated variable) and multiple (two independent vari- by dipping them for 5 s into insecticide solutions ables) regression analysis from the SPSS' computer formulated in distilled water with a spreader (0.1% statistical package (SPSS 1986, 662-686). For Triton AG-98). Cups were drained onto paper regressions involving tolerances to organophospho- toweling and air dried. Three materials widely used rus (OP) insecticides, the local and regional histo- for California red scale control (carbaryl [Sevin 80 ries of total OP use were the independent vari- sprayable], malathion [Malathion 25 spray able], and ables. For regressions involving tolerances to methidathion [Supracide 2 emulsifiable concen- carbaryl, the corresponding histories of carbamate trate]), one material used for thrips control (di- insecticide use were used. Insecticides of the same methoate [Cygon 400]), and one material used for class (OP or carbamate) were combined in this control of both California red scale and a complex manner in an attempt to include the possible ef- of lepidopterous pests (chlorpyrifos [Lorsban 4 fects of cross-resistance. emulsifiable concentrate]) were tested. An index of overall resistance of each colony At least five concentrations and a water / spread- was calculated relative to that of colony 1. Each er control were tested for each chemical. Vials were colony's LC50 for an insecticide was divided by held at 26 ± 1°C, 87% RH, and a 16:8 photoperiod that of colony 1. A similar ratio was calculated for for 24 h before the tests were scored. Individuals each of the insecticides for which the colony was were considered dead if they were unable to main- tested, and the ratios were totaled and averaged tain a normal posture or walk normally, covering for each colony. The resulting overall average rel- at least 1 mm/s. With a given chemical, all pop- ative resistance values were then regressed against ulations were tested simultaneously. Each test was the total number of insecticide treatments applied 1164 FORUM: JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 79, no. 5
to the grove from which the colony came, against Table 1. Variation in concentration/mortality regres- the county-wide pesticide use, and against the sions of A. melinu. colonies collected from California ex- posed to residues of five insecticides combined level of in-grove and county-wide pes-
ticide use. Insec- Col- nO SI SEM LCso (95% CL) Finally, for each chemical tested, we deter- !icide ony ope ± in mg (AI)/liter
mined whether the slope values of the concentra- Carbaryl (suggested field rateb: 960 mg [All/liter) tion/mortality regressions were linearly related to 7 1,182 3.22 ± 0.20 15.4 (13.3-17.5)a 1) the associated LCoo's, 2) the selective pressure 8 1,061 3.42 ± 0.22 17.9 (l5.8-20.1)b experienced by the population (as estimated by 5 1,101 3.91 ± 0.24 19.9 (l8.5-21.4)c the sum total of the in-grove and county-wide pes- 1I 1,153 3.74 ± 0.21 19.9 (18.1-21.8)cd 4 983 4.27 ± 0.25 23.5 (21.5-25.9)e ticide use histories), or 3) the degree to which the 3 498 3.10 ± 0.30 24.0 (20.3-28.2)f level of in-grove pesticide use diverged from that 1 648 4.28 ± 0.42 27.9 (24.4-31.2)g of the county (as estimated by subtracting the in- Chlorpyrifos (suggested field rate: 450 mg [All/liter) grove pesticide use value from the county-wide 7 427 2.70 ± 0.26 0.78 (0.58-0.97)a pesticide use value). 1I 1,337 3.31 ± 0.16 0.84 (0.76-0. 92)ab 3 1,443 2.55 ± 0.14 0.93 (0.81-1.06)c 4 381 3.40 ± 0.32 1.02 (0.87-1.20)d 5 1,035 3.59 ± 0.27 1.11 (0.98-1.23)de Results 8 614 5.69 ± 0.78 1.24 (1.01-1.40)f 1 1,172 4.60 ± 0.51 1.37 (1.l8-1.50)g For each of the five insecticides tested, the con- 2 255 2.42 ± 0.33 1.43 (1.06-2.07)h centration/mortality regressions varied signifi- Dlmelhoale (suggested field rate: 1,200 mg [All/liter) cantly among the A. melinus colonies (Table 1). 9 325 3.06 ± 0.47 2.18 (1.26-2.85)a The hypothesis of equality was rejected for most 7 1,444 2.69 ± 0.15 2.58 (2.19-2.96)b population comparisons for each chemical. 13 410 3.09 ± 0.29 2.73 (2.25-3.20)bc The collection site characteristics and the in- 10 1,168 2.28 ± 0.13 2.73 (2.33-3. 15)d 3 1,143 2.63 ± 0.15 3.55 (3.02-4. 13)e grove and county-wide pesticide use histories for 2 438 2.65 ± 0.29 3.62 (2.64-4.62)ef the 13 colonies tested are shown in Table 2. All 8 1,464 2.55 ± 0.13 3.91 (3.40-4.46)ef but two of the collection sites were commercial 12 196 2.70 ± 0.37 3.95 (2.82-5.79)efg groves. The species of Citrus chosen was generally 6 910 2.71 ± 0.17 4.16 (3.57-4.79)fg 4 1,244 2.72 ± 0.17 4.46 (3.75-5.25)g the one that was predominantly grown in the re- 1 435 2.67 ± 0.49 6.36 (3.49-8.71)h gion being sampled. The overall amounts of insec- Malathion (suggested field rate: 720 mg [All/liter) ticides used varied widely, both regionally among 9 929 2.65 ± 0.15 0.54 (0.45-0.63)a counties (0.099-4.67 kg [AI]/ha per year), and lo- 13 460 3.15 ± 0.24 0.78 (0.57-0.99)b cally among groves (0-15 applications from 1980 7 1,436 2.78 ± 0.13 1.23 (1.09-1.37)c to 1984). No significant relationship between the 10 1,521 3.20 ± 0.19 1.27 (1.l0-1.42)cd 6 1,369 2.50 ± 0.14 1.73 (1.50-1.98)e total (carbamate plus OP) in-grove and county- 12 1,226 4.52 ± 0.35 1.94 (1.72-2.15)f wide insecticide use was found (r2 = 0.0012; P > 3 1,644 3.09 ± 0.20 2.07 (1.84-2.28)g 0.25). Since A. melinus colonies were collected 8 1,459 3.56 ± 0.25 2.29 (1.99-2.57)hc from groves that were chosen for their varied his- 1I 869 4.43 ± 0.31 2.36 (2.07-2.67)h 2 1,294 2.98 ± 0.24 2.37 (2.02-2.70)h tories of pesticide use, a lack of correlation was 4 1,272 3.04 ± 0.15 3.1I (2.69-3.58)i expected. 1 553 2.53 ± 0.31 4.22 (3.33-5. 16)j The results of the regression analyses of the LCso's Methidathion (suggested field rate: 300 mg [All/liter) (Table 1) on the levels of past pesticide use (Table 9 1,102 2.38 ± 0.14 0.44 (0.37-0.51)a 2) are shown in Table 3. The slopes of the signif- 6 1,093 2.43 ± 0.16 2.07 (1.79-2.37)b icant regressions were all positive, indicating a 8 1,121 2.45 ± 0.15 2.30 (2.03-2.60)bc 1 957 2.90 ± 0.22 2.46 (2.13-2.81)cd positive correlation between LCso's and levels of 5 776 2.80 ± 0.24 2.54 (2.19-2.97)cd past pesticide use. Due in part to the small sample 2 1,231 2.08 ± 0.17 2.68 (2.16-3.23)& size (7-11), the levels of significance associated with 3 1,055 2.57 ± 0.17 2.71 (2.32-3.15)de 4 654 2.30 ± 0.20 3.00 (2.54-3.58)e the single independent variable correlations were 12 286 1.99 ± 0.36 3.36 (1.76-7.90)e often inadequate to draw strong conclusions. (For the eight nonsignificant regressions, 0.14 ~ P ~ Concentration/mortality regressions followed by the same let- 0.44.) However, when the influences of the two ter are not significantly different (a = 0.05; likelihood-ratio test factors, the in-grove and county-wide pesticide use [Savin et al. 1977]). o Natural mortality ranged from 0 to 8.8 ± 1.9% and averaged histories, were combined, the resulting regressions 2.2%. were strong enough to be significant for three of b Recommendations of the University of California Cooperative the five chemicals tested (P ~ 0.10). (Due to the Extension citrus treatment guide (Morse & Bailey 1984). c The regression for colony 8 is not significantly different (a = small sample sizes employed, a = 0.10 was used. 0.05) from the regressions of colonies 1I or 2. However, the Additional studies with larger sample sizes will be regression of colony 1I is significantly different from that of col- necessary to confirm these results and reduce the on12. overall risk of type I error.) This suggests that both The regression for colony 1 is not significantly different (a = the in-grove and county-wide pesticide use are im- 0.05) from the regressions of colonies 5 or 3; likewise, the regres- sion for colony 3 is not significantly different from that of colony portant in accurately describing a population's his- 2. However, the regression for colony 2 is significantly different tory of selection pressures. from those of colonies 1 and 5. October 1986 ROSENHEIM & Hoy: PESTICIDE RESISTANCE VARIATION IN A. melinus 1165
t- tr.> tr.> tr.> QO t- t- t- o o o o o o o o t- o ..•. o o"" o"" o"" o"" o o 8 QO o-< o o"" o "C -a.III -< 8 8 8 8 o o"" ~ o o o o o o o :s.• .S t- t- t- ~ co co co co E o o o o o'" ...~ o o 00 QO QO Regressions were also performed using the his- tory of use of the specific chemical for which the :s.. concentration/mortality regression was generated. .S ." ~ 0 A. melinus is a recently introduced species. The 0 genetic variability of species introduced to a new County-wide pesticide use history area as part of a biological control program may be reduced by the limited numbers of individuals collected from the indigenous area, genetic drift and inadvertent selection during the processes of 1.0 C • quarantine and mass-rearing, and additional bot- .• tlenecks occurring during the release and coloni- .:"· zation of the new habitat (Messenger & van den •• 0.8 u Bosch 1971, Hoy 1985b). It is unknown to what o • extent, if any, the process of importing A. melinus ;;" ';; • has reduced the evolutionary flexibility of its Cal- ~ 0.6 • ifornia populations . .~•• :; Previous ipvestigations of the evolution of resis- ~ tance to insecticides among field populations of ,2 • 0.62 ~ 0.4 parasitoids have yielded mixed results. Adams & ~";; • Cross (1967) reported no significant difference be- " ~ 0.2 <5 -Fig. 3. Regression analyses of each colony's overall o o 0.2 0.4 0.6 0.6 1.0 average relative tolerance on (A) its in-grove pesticide Combined in-grove and county-wide pesticide use index use history, (B) its county-wide pesticide use history, and (C) both A and B simultaneously (multiple regression). ( = 0.372 + 0.024(;n-o,ove use) + 0.066(counly-wide useU See Table 2 for values plotted. 1168 FORUM: JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 79, no. 5 tween the tolerances of three field and one labo- vival after pesticide applications may effectively ratory colony of Bracan mellitor Say to five insec- select against resistant individuals (Croft & Morse ticides used on cotton. Similarly, Krukierek et al. 1979, Hoy 1979, Tabashnik & Croft 1985). The (1975) failed to detect differences between the re- last hypothesis, the food limitation hypothesis, has sponses to oxydemeton-methyl of two populations received increased attention recently (Morse & of Trichagramma evanescens Westwood or two Croft 1981, Tabashnik & Croft 1982, 1985, Ta- populations of T. minutum Riley. Strawn (1978) bashnik 1986). was unable to detect significant variation in the A corollary of the food limitation hypothesis responses of either adults of four field populations (Georghiou 1972, Morse & Croft 1981, Croft & of Comperiella bifasciata Howard or pupae of six Strickler 1983, Tabashnik & Croft 1985) is that populations of A. melinus to four OP compounds. resistance is unlikely to develop in a natural enemy Likewise, no significant differences in levels of re- species until after its host or prey (henceforth called sistance to malathion were exhibited by seven field host) has become resistant. Consideration of the populations of Aphytis holoxanthus DeBach patterns of pesticide use in citrus and the resulting (Havron 1983). impact on A. melinus and its host scale popula- In contrast, Schoonees & Giliomee (1982) found tions leads to the conclusion, however, that this a 5.7- and a 65.6-fold difference in levels of resis- corollary may have been described in an overly tance to methidathion in two field populations of restrictive form. Generally, chemicals being dis- Aphytis african us Quednau and two field popu- cussed in this regard are not directed against the lations of C. bifasciata, respectively. Strawn (1978) host population. Rather, they are being used to found significant interpopulation variation in re- control other key pests in the agroecosystem. sponse to parathion by C. bifasciata pupae and in Therefore, resistance in the host population is not the tolerances of A. melinus adults to dimethoate, always necessary; any means by which this popu- methidathion, and parathion. Unfortunately, be- lation can survive the pesticide application and cause Strawn's tests were performed using single- thereby continue to serve as hosts to the natural dose bioassays and because probit regressions were enemy species will circumvent the problem of food not generated, further comparison of these results limitation. with ours is difficult. Finally, Hsieh (1984) found There are two general means by which a non- a ca. 2-fold difference between the tolerances ex- resistant host population can survive an insecticide hibited by two field populations of Diaeretiella application (Fig. 1). First, a non-resistant host pop- rapae (M'Intosh) to methomyl. Thus the increase ulation will be relatively unaffected by a chemical in resistance levels exhibited by parasitoids pro- application that it does not physically contact. For duced by natural selection in the field has varied example, dimethoate applied to control citrus thrips from undetectable to substantial. In none of these is applied only to the periphery of the tree (Table studies, in which explicit comparisons of different 4). The dimethoate will, therefore, contact only a populations were made, was the increased level of fraction of the total California red scale popula- resistance adequate to enable adult parasitoids to tion, which is distributed throughout the tree survive field application rates of insecticides. The (Ebeling 1959). The same is true for chlorpyrifos, pattern of evolution of resistance among parasit- which is applied to the outside of the tree for con- oids clearly does not resemble the explosive emer- trol of various orangeworms (Table 4). The appli- gence of resistance in pest species (Georghiou & cation of pesticides to a restricted portion of the Mellon 1983). tree may be described as a form of ecological se- Several possible explanations for this apparent lectivity towards the host population. Second, a relative inability of parasitoids to develop resis- natural tolerance relative to the tolerance of the tance have been proposed. The evolution of resis- target pest species will also enable the host popu- tance in field populations of parasitoids may often lation to survive treatment. Thus, California red go unnoticed (Croft & Brown 1975). The differ- scale, while potentially controllable with dimeth- ential exposures of parasitoids relative to their hosts oate (which is registered for control of A. auran- due to morphological, behavioral, or ecological dif- tii), largely appears to be tolerant of the lower ferences may lead to a greater effective toxicity concentrations applied for control of citrus thrips. (Georghiou 1972). Because parasitoids are often The results of our study may be considered with initially less tolerant of pesticides than their hosts, regard to this broadened concept of the food lim- a larger increase in tolerance may be required to itation hypothesis and its corollary. The absence achieve effective levels of resistance (J.A.R., un- of a substantial host population should, according published data). The lack of genetic flexibility to the hypothesis, retard the evolution of resistance among the highly ecologically specialized parasit- in A. melinus to carbaryl, malathion, and methi- oids may restrict the evolution of resistance (Huf- da'thion (used to control scales) but not to dimeth- faker 1971, Georghiou 1972). The relatively low oate (applied for thrips control). The evolution of activity of preadapting detoxifying enzyme sys- resistance to chlorpyrifos (applied for control of tems may limit the potential for resistance (Croft either scales or orangeworms) should fall some- & Strickler 1983). Finally, the reliance of parasit- where between these two extremes. oid populations upon the host population for sur- Are these predictions reflected in the ranges of October 1986 ROSENHEIM& Hoy: PESTICIDERESISTANCEVARIATIONIN A. melinus 1169 Table 4. Recommendations" of the University of California Cooperative Extension for the treatment of citrus with carbaryl, chlorpyrifos, dimethoate, malathion, and methidathion Earliest recommendation 1984-86 recommendation Pesticide and Target species Cover- Cover- formulation Year Concn age Concn age typei' type Carbaryl A. auranlii 1966-67 1.20-1.44 g/liter TC Same as 1966-67 80WP (1-1.2 Ib/loo gal) Chlorpyrifos Several Lepidopteran 1984-86 1.24-12.4 ml/liter/ha OC Same 0.48 kg/liter E pests (1-2 qt/l00-5oo gal/acre) (4lb/gal) A. auranll/ 1984-86 0.94 ml/liter TOC Same (0.75 pt/loo gal) Dimethoate S. citri 1966-67 3.75 ml/liter MS 4.64-18.5 mlfliter OC 0.32 kg/liter E (3 pts/loo gal) 3-6 pts/l00-2oo gal/acre" (2.67 Ib/gal) Malathion A. aurantii 1957d 3.00-4.20 g/liter TC 2.88-4.07 g/liter TC 25WP (2.5-3.5 Ib/loo gal) (2.4-3.4 Ib/loo gal) Methidathion A. auranlii 1976-78 1.25 ml/liter TOC Same as 1976-78 0.24 kg/liter E (1 pt/loo gal) (2Ib/gal) U Recommendations published in the irregularly issued citrus treatment guides (e.g., Morse & Bailey 1984). /,Abbreviations used are taken verbatim from treatment guides (e.g., Morse & Bailey 1984). MS, mist spray; low Iiterage, 936-2,807 liters per hectare (100-300 gallons per acre), applications without droplet size restrictions achieving limited droplet depositions on tree surfaces; OC, outside coverage; median Iiterage, not more than 4,676 liters per hectare (500 gallons per acre), applications achieving thorough distribution to outside or peripheral parts of the tree only; TC, thorough coverage: high Iiterage, 114-132 liters per tree (30-35 gallons per tree), applications achieving thorough film wetting of all interior and exterior parts of the tree; TOC, thorough distribution coverage: median Iiterage, 76-114 liters per tree (20-30 gallons per tree), applications achieving thorough distribution to all interior and exterior parts of tree without necessity of obtaining film wetting. " For the first time, in the 1984-86 Treatment Guide, this recommendation was accompanied by a footnote reading "Non-resistant thrips only." of Because A. melinus was introduced in 1957, this is the earliest relevant recommendation. resistance values observed in the populations tested existence of cross-resistance patterns, will affect the with each of these insecticides? Because not all outcome. We conclude that, at least in this system, colonies were tested with all of the chemicals, the food limitation does not appear to be the key fac- ranges of resistance values must be considered with tor in determining the extent to which resistance caution. To compare the ranges of resistance val- has developed. ues for two chemicals, only those colonies tested The Role of Past Selection Pressures. One of with both chemicals should be considered (e.g., to our two initial goals was to evaluate thp role of compare the ranges of LCso's of carbaryl with di- past selection pressures in any observed interpop- methoate, only colonies 1, 3, 4, 7, and 8 should be ulation patterns of variation. To measure selection included). The LC50's for these colonies (Table 1) pressures, spatial delimitation of a population was indicate that the maximum ranges of resistance required. Two important considerations were the are 27.9/15.4 = 1.8-fold for carbaryl and 6.36/ geographic distribution of potential hosts and the 2.58 = 2.5-fold for dimethoate. The ranges of re- dispersal ability of the parasitoid. sistance values observed for malathion and di- The distribution of hosts of A. melinus proved methoate were 7.8- and 2.9-fold, and methidathi- to be difficult to describe. Although the distribu- on and dimethoate were 7.7- and 2.9-fold, respec- tion of commercially cultured citrus in California tively. Analogous figures for the three scalicides, is known to consist of several fairly discrete regions carbaryl, malathion, and methidathion, compared in widely separated valleys bounded by substantial with chlorpyrifos are 1.8- and 1.7-fold, 3.4- and mountain ranges (Fig. 1), we concluded that com- 1.8-fold, and 1.3- and 1.5-fold, respectively. The mercial citrus did not represent the entire host food limitation hypothesis predicts that the second plant pool. Substantial scale populations also exist value of each of these paired numbers, represent- in common, small, dooryard citrus plantings ing the extent of evolved resistance to chemicals (DeBach 1965). A. aurantii also infests many other not impacting the host population, should be host plants. McKenzie (1956) considered the species greater than the first value, representing the extent to be one of California's 22 omnivorous armored of evolved resistance to chemicals that reduce the scales, and the total number of plant hosts may be host population. However, our observations sug- large (Quayle 1911, McKenzie 1946). Compound- gest the reverse. This is a limited test of the hy- ing this situation in California, A. melinus devel- pothesis; many other factors, including the length ops not only on A. aurantii but also on nine other of time and the degree to which the chemicals species of armored scales (Rosen & De Bach 1979; have been used (Tables 2 and 4) and the possible S. C. Warner, personal communication). Most of 1170 FORUM: JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 79, no. 5 these additional hosts are also omnivorous; Mc- tance observed. Although suggestive data are pre- Kenzie (1956) listed over 80 plant genera, includ- sented in these studies, we are not aware of any ing many commercially cultivated species, as "only instance in which these hypotheses have been tested a few of the more preferred hosts." statistically. Hopefully, future studies will include The significance of these alternate hosts is dif- analyses of 1) past selection pressures, and 2) the ficult to assess, and their collective contribution to roles of migration and host distribution in affecting the total host pool may be less than that of A. the relative importance of local and regional pat- aurantii (R. F. Luck, personal communication). A. terns of pesticide use. Progress in the nascent field melinus is not the dominant parasite of any of the of resistance management, which attempts to pro- alternate hosts (R. F. Luck & S. C. Warner, per- long the useful life of pesticides by slowing or halt- sonal communications), and some of the scales are ing the evolution of resistance, is dependent upon relatively rare. However, alternate host scale/host the ability to implement specific pesticide use plant combinations may provide refugia and av- strategies. The results of the studies we propose enues for population movement both within and should be relevant to determining the geographic between the major citrus-growing areas. scale over which such resistance management pro- Although the dispersal ability of A. melinus has grams should be implemented for optimal effect. not been investigated specifically, reports of the Implications for IPM, The results of this study parasitoid's spread following introduction into new are encouraging because the observed trend to- areas provides a crude estimate of its mobility. wards increased resistance levels indicate a possi- From studying the spread of A. melinus in Greece ble means of increasing the effectiveness of A. following its introduction there, DeBach & Argy- melinus within an IPM framework. However, for riou (1967) concluded that its effective rate of dis- at least three reasons, these results are not yet cause persal was 75-100 km per year, even across rela- for complacency. First, levels of resistance exhib- tively barren land cultivated only in scattered areas. ited by the populations sampled were not sufficient De Bach & Sundby (1963) noted that A. melinus to enable them to survive field application rates of could cross a 15-km barrier of barren high hills commonly used insecticides. This conclusion is in separating California's San Fernando and Simi accordance with other recent studies performed Valleys. On a smaller scale, the ability of the para- both in the laboratory (Bellows et al. 1985, Morse sitoid to spread throughout an Australian citrus & Bellows 1986) and the field (Griffiths et al. 1985). grove within 10 months of its release was docu- Second, we have no way to predict whether or not mented by Campbell (1976). Thus, A. melinus ap- the observed trend towards increased resistance will pears to have a well-developed ability to disperse. continue in the future. Finally, A. melinus does The diffuse nature of the host pool of A. meli- not exist in an ecological vacuum. While popula- nus, coupled with the parasitoid's mobility, indi- tions of this parasitoid may be evolving increased cated that an investigation of both the history of tolerance levels, the pest populations against which pesticide use in the grove where the population the chemicals are being applied may simulta- sample was collected and some measure of pesti- neously be evolving increased resistance. Growers cide use in the surrounding areas would be nec- may be forced, thereby, to increase their insecti- essary to adequately describe the historical selec- cide application rates or shift to the use of new tion pressures experienced by the population. insecticides. These responses represent an intensi- Although not precise and not. explaining all of the fication of old selection pressures and the creation variability in LC~o's of the populations sampled, of new selective pressures upon the parasitoid pop- the combined influences of these two factors ap- ulation. Thus, the parasitoid may be engaged in pear to provide a means of explaining the essential an evolutionary race to resistance with the key features of the observed patterns. Part of the unex- pests of citrus. plained variability may also be attributable to the Since the introduction of A. melinus in 1957, artificial movement of A. melinus populations re- California citrus growers have had relatively few sulting from the activities of commercial insecta- problems with resistant insect pests. For all the ries and biological control workers. insecticides tested, the recommended application Unfortunately, it is difficult to compare these rates have remained essentially unchanged from conclusions with those drawn from other studies 1957 to the present (Table 4). A warning regarding dealing with either beneficial or pest arthropods. resistant populations does, however, accompany the Many authors have hypothesized that either the recommended application rates for dimethoate for local history of pesticide use (e.g., Georghiou 1966, thrips control. The citrus thrips is the only key Herne 1971, Penman et al. 1976, Strawn 1978, insect pest of citrus that has recently developed Hoy & Knop 1979, Schoonees & Giliomee 1982, resistance to an insecticide widely used for its con- Mansour 1984, Quisenberry et al. 1984, Robertson trol (Morse & Brawner 1986). The declining effec- & Stock 1985, Schmidt et al. 1985, Georghiou tiveness of dimethoate is reflected in the increasing 1986), regional history of pesticide use (e.g., Geor- application rates used by some growers. Fig. 4 pro- ghiou 1972, Grafton-Cardwell & Hoy 1985), or vides one such example taken from a grove in Butte some combination of the two (e.g., Follett et al. County. Within the last few years, many growers 1985) have generated patterns of variable resis- have begun using insecticides other than dimeth- October 1986 ROSENHEIM & Hoy: PESTICIDE RESISTANCE VARIATION TN A. melinus 1171 •• 5 References Cited 0; U •• Abdelrahman, I. 1973. A laboratory spraying appa- ~4 ratus. Aust. J. Agric. Res. 24: 135-142 . •• Adams, C. H. & W. H. CroSS. 1967. Insecticide re- i sistance in Bracon mellUor, a parasite of the boll ••~ 3 weevil. J. Econ. Entomol. 60: 1016-1020. c ~ Bartlett, B. R. 1964. Integration of chemical and bi- u• ological control, pp. 489-514. In P. 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Thus, A. melinus may al- nus De Bach (Hymenoptera, Aphelinidae) in Aonidi- ready have missed the opportunity to become re- ella aurantii (Mask.) (Hemiptera, Coccidae) on cit- sistant to dimethoate. The other insecticides tested rus in South Australia. Bull. Entomol. Res. 65: 659- in this study continue to be used, but further in- 668. creases in the resistance levels exhibited by A. mel- Croft, B. A. & A. W. A. Brown. 1975. Responses of arthropod natural enemies to insecticides. Annu. Rev. in us will be necessary before they can be used Entomo1. 20: 285-335. selectively. Croft, B. A. & J. G. Morse. 1979. Recent advances One means to obtain these additional increases on pesticide resistance in natural enemies. Ento- in resistance is through a program of artificial se- mophaga 24: 3-11. lection in the laboratory. Artificial selection has Croft, B. A. & K. Strickler. 1983. 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