Resistance of Field Strains of Beet Armyworm (Lepidoptera: Noctuidae) from Arizona and California to Carbamate Insecticides

Transcription

1 INSECTICIDE RESISTANCE AND RESISTANCE MANAGEMENT Resistance of Field Strains of Beet Armyworm (Lepidoptera: Noctuidae) from Arizona and California to Carbamate Insecticides D. L. KERNS, J. C. PALUMBO, AND T. TELLEZ Department of Entomology, Yuma Valley Agricultural Center, University of Arizona, 6425 West 8th Street, Yuma, AZ J. Econ. Entomol. 91(5): 1038Ð1043 (1998) ABSTRACT Beet armyworms, Spodoptera exigua (Hübner), were collected from spinach, Spinacia oleracea L., lettuce, Lactuca sativa L., cantaloupe, Cucumis melo L., and alfalfa, Medicago sativa L., and their F 1 progeny were evaluated for resistance to methomyl and thiodicarb. No resistance or high levels of resistance were detected to methomyl by using a topical bioassay. Agricultural areas containing a large hectarage of produce crops where methomyl is extensively used throughout most of the year primarily yielded the highest levels of resistance. The highest level of resistance detected came from a strain collected in Imperial County, CA, having a resistance ratio of 552-fold. Low levels of resistance were primarily associated with areas with a small hectarage of produce crops. Detection of high levels of methomyl resistance represents a dramatic shift in susceptibility since Beet armyworms collected from alfalfa before and after an insecticide application containing methomyl exhibited 101-fold and 450-fold resistance, respectively, suggesting selection for resistance. Beet armyworm strains resistant to methomyl were found cross-resistant to thiodicarb when using a diet incorporation bioassay. However, resistance ratios for thiodicarb never exceeded 5-fold. Similarly, when methomyl was bioassayed using a diet incorporation technique resistance ratios never exceeded 5-fold suggesting that the diet incorporation bioassay may underestimate the severity of resistance or that methomyl resistance may involve cuticular penetration. KEY WORDS Spodoptera exigua, insecticide resistance, cross-resistance, methomyl, thiodicarb THE BEET ARMYWORM, Spodoptera exigua (Hübner), is a serious pest of many crops grown in the low-desert regions of Arizona and California (Metcalf and Metcalf 1993). These areas are characterized as being multicropping systems where various crops are grown throughout the year. Most insecticide applications targeting beet armyworms in vegetables, cotton, Gossypium spp., or alfalfa, Medicago sativa L., contain either methomyl or thiodicarb. Because methomyl and thiodicarb are chemically similar, selection for beet armyworms resistant to these products is high. Although beet armyworms have not been reported to be resistant to thiodicarb, they have exhibited variable responses to methomyl. Meinke and Ware (1978) detected a 3-fold level of variability in susceptibility to methomyl in beet armyworms in Arizona. Yoshida and Parrella (1987) reported methomyl resistance in beet armyworms collected from ßoricultural crops in Florida. Methomyl resistance also has been documented in beet armyworms in Mexico and California (Brewer et al. 1990). During the past 5 yr many pest control advisors in Arizona have reported the use of increasing amounts of methomyl tank-mixed with other insecticides to achieve commercially acceptable beet armyworm control. During 1995Ð1997, growers experienced control failures with methomyl in lettuce, Lactuca sativa L., cole crops, and alfalfa. These control failures have been attributed both to poor application and insecticide resistance. However, no supportive data have been published. If resistance is at least partially responsible for control failures, a resistance management plan should be implemented. The objective of this study was to assess the occurrence of methomyl and thiodicarb resistance in beet armyworms from the low desert agricultural regions of Arizona and California, and to determine if cross-resistance occurred between methomyl and thiodicarb. Materials and Methods Insects. From late February 1996 to June 1997, beet armyworm larvae, egg masses, or both were collected from spinach, melons, lettuce or alfalfa growing in the low desert agricultural regions of Arizona and California (Table 1). Two laboratory-susceptible strains were established as comparative references. One came from the USDAÐARS Western Cotton Research Laboratory in Phoenix, AZ (PHO96), and the other from the USDAÐARS laboratory in Stoneville, MS (MIS96). The 4th generation of the Yuma, AZ, strain tested in 1976 (Meinke and Ware 1978) (H-76), and the Yuma, AZ strain tested in 1989 (Aldosari et al. 1996) (H-89) were used as regional historical references for susceptibility to methomyl /98/1038Ð1043$02.00/ Entomological Society of America

2 October 1998 KERNS ET AL.: INSECTICIDE RESISTANCE IN S. exigua 1039 Table 1. Locations, hosts and collection dates of beet armyworm strains during 1996 and 1997 Strain County Town, State Host Collection date Historically susceptible strains H-75 a Yuma Yuma, AZ Ñ c July or Aug H-89 b Yuma Yuma, AZ Ñ c Oct. or Nov Laboratory susceptible strains MIS96 Ñ c USDAÐARS, Stoneville, MS ArtiÞcial diet Ñ c PHO96 Ñ c USDAÐARS, Phoenix, AZ ArtiÞcial diet Ñ c Field collected strains, 1996 YAC96A Yuma Yuma Valley, AZ Spinach 26 Feb. SAL96 La Paz Salome, AZ Watermelons 19 Aug. DOM96 Yuma Dome Valley, AZ Lettuce 7 Sept. SOM96 Yuma Somerton, AZ Lettuce 11 Sept. TEX96 Yuma Texas Hill, AZ Lettuce 10 Oct. Field collected strains, 1997 GAD97 Yuma Gadsden, AZ Lettuce 5 Feb. YMS97A Yuma Yuma Mesa, AZ Alfalfa 6 June YMS97B Yuma Yuma Mesa, AZ Alfalfa 9 June ICA97 Imperial Holtville, CA Alfalfa 11 June PAR97 La Paz Parker, AZ Alfalfa 14 June BLY97 Riverside Blythe, CA Alfalfa 14 June Laboratory selection for increased methomyl resistance YAC96B Yuma Yuma Valley, AZ Spinach 26 Feb a Taken from Meinke and Ware (1978). b Taken from Aldosari et al. (1996). c Data not available or not applicable. d YAC96B is a population created from YAC96A larvae surviving topical applications of methomyl, 4,000 g (AI)/g. Larvae of all strains were reared on instant diet (Stoneßy Industries, Bryan, TX). Adults were maintained in 3.79-liter clear plastic jars with a mesh top. Adults were fed a 10% sucrose-water solution and allowed to oviposit on strips of paper towels. Environmental conditions were maintained in the rearing area at 25 5 C, 15Ð30% RH, and a photoperiod of 12:12 (L:D) h. The F 1 generations obtained from these adults were tested for resistance to methomyl and thiodicarb. Methomyl Topical Bioassays. Resistance to methomyl was evaluated using the topical procedures described by Meinke and Ware (1978). Insecticide dilutions were made by dissolving technical grade methomyl (98.7%; DuPont, Wilmington, DE) in acetone. Four replicates of 7Ð13 dilutions were used in this study depending on the population tested. Dilutions were calculated based on average larval weights in micrograms of methomyl per gram of larvae. Twenty to Þfty 3rd instars which averaged 9Ð11 mg, were used for each dosage. Topical applications were made with a 0.25-ml syringe and a 27-gauge needle by using a hand-driven micro-applicator (Burkard, Rickmanworth Herts, UK). Each larva was assayed by placing a 0.5- l droplet of insecticide dilution on the dorsal thorax. Control larvae were treated only with acetone. Treated larvae were then transferred to individual 29-ml plastic cups with lids containing 5.0 g of artiþcial diet and held at 25 5 C, 15Ð30% RH, and a photoperiod of 12:12 (L:D) h. After 24 h, larvae that could not move about freely after prodding with a pencil were considered dead. Methomyl and Thiodicarb Diet Incorporation Bioassays. Insecticide dilutions were prepared by dissolving commercially formulated methomyl (Lannate 90S, DuPont) or thiodicarb (Larvin 3.2 E, Rhone-Poulenc, Research Triangle Park, NC) in distilled water. These insecticide solutions were used to hydrate instant mix diet by using 3.0 ml of insecticide solution/1.0 g of diet. Dilutions were calculated based on insecticide concentrations in the artiþcial diet (milligrams of active ingredient per kg of diet). Four replicates of 6 to 8 dilutions of each insecticide were used depending on the population tested. Insecticide-diet mixtures were transferred to 29-ml plastic cups with lids at 4.0 g per cup. Thirty to seventy-þve 2nd instars were used for each dosage; 1 larva was used per cup. Larvae were held at 25 5 C, 15Ð30% RH, and a photoperiod of 12:12 (L:D) h. Larvae not able to move about freely after prodding with a lead pencil after 24 h for methomyl-treated larvae, and 96 h for thiodicarb-treated larvae, were considered dead. Statistical Analysis. Dosage-mortality curves for all bioassays were estimated using ProbitÕs analysis (SAS Institute 1989). Dosages required to kill 50% (LD 50 )of the populations were estimated using the Probit regression, 95% conþdence limits (CL) were used to separate differences. Resistance ratios (RR) for all bioassays were calculated by dividing the LD 50 of the Þeld strain by the LD 50 of PHO96 insecticide-susceptible strain. Results and Discussion Methomyl Topical Assays. The Þeld strains of beet armyworm tested for resistance to methomyl in 1975 and 1989, H-75 and H-89, respectively, represent methomyl susceptibility before reported Þeld control

3 1040 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 5 Table 2. Dose-mortality responses of beet armyworm strains to methomyl applied topically Strain a n Slope SE LD 50 (95% CL) g(ai)/g RR b 2 df Historically susceptible populations H (10.12Ð26.04) 1.0 Ñ c Ñ c H (41.11Ð61.66) 2.41 Ñ c Ñ c Laboratory-susceptible populations MIS (15.49Ð24.18) PHO (14.01Ð30.21) Field collected populations (F 1 generation tested) YAC96A ,504 (2,316Ð186,359) d SAL ,419 (2,114Ð2,745) DOM (702.58Ð1,240) SOM (609.21Ð1,165) TEX ,976 (3,405Ð7,890) GAD ,406 (4,174Ð6,936) YMS97A ,208 (1,363Ð3,704) YMS97B ,793 (3,838Ð64,918) d ICA ,024 (7,098Ð25,694) PAR (159.63Ð850.31) BLY (8.44Ð250.12) Laboratory selection for increased methomyl resistance (F 4 generation tested) YAC96B ,227 (6,362Ð64,089) a Collection information presented in Table 1. b RR, resistance ratio calculated by dividing the LD 50 of the strain tested by the LD 50 of the PHO96 strain. c Data not available. d Chi-square signiþcant (P 0.05). failures (Table 1). Because of overlapping 95% CL between H-75 and the laboratory strains PHO96 and MIS96, the laboratory strains are representative of methomyl-susceptible Þeld strains before growers began reporting Þeld control failures (Table 2). Thus, PHO96 and MIS96 can serve as susceptible reference points for determining methomyl resistance. Methomyl has been used for beet armyworm control in the desert southwest of the United States for 20 yr. However, development of methomyl resistance has been slow. Before 1987, agriculture in Yuma County, AZ, was dominated by long-season cotton, alfalfa, and small grains. This cropping system produced periods during the year where beet armyworms were present with little insecticide use (Hill 1968). Additionally, highly polyphagous pests often develop resistance more slowly than more monophagous species because they have a greater potential to maintain insecticide susceptible reservoirs when inhabiting areas where insecticide-free alternative hosts are present (Georghiou and Taylor 1986). The presence of insecticide-free periods and alternative hosts may explain the lack of methomyl resistance before 1987 (Meinke and Ware 1978, Brewer et al. 1990, Aldosari et al. 1996). Since 1987, agriculture in Yuma has changed from being dominated by cotton to having a large hectarage of lettuce and other produce crops that are extremely sensitive to insect damage. The current multicropping system in Yuma promotes year-round use of insecticides on a variety of crops. Methomyl is by far the most widely used insecticide for insect control in produce in Arizona and California. Presently, the low-insecticide refuges only for beet armyworms in Yuma are weeds and alfalfa. However, during dry years when desert ßora is scarce, and in years when the value of alfalfa justiþes insecticide applications, these refuges can be lost. During 1995Ð1997, Yuma experienced drought conditions and insecticide applications for beet armyworm in alfalfa became economically beneþcial. Consequently, many beet armyworm populations collected during 1996Ð1997 demonstrated high levels (RR 100Ð200) of resistance to topical applications of methomyl relative to known susceptiblelaboratory and historical Þeld strains (Table 2). Resistance levels of beet armyworms collected from Yuma County ranged from moderately resistant (RR 25Ð99) in the DOM96 and SOM96 strains to highly resistant (RR 100Ð199) in the YMS97A strain, and extremely resistant (RR 200) in the YAC96A, TEX96, GAD97, and YMS97B strains. A signiþcant chi-square value was found for the YAC96A and YMS97A strains suggesting a poor Þt of the data to the probit model (Robertson and Preisler 1992). The highest levels of resistance detected in Yuma County occurred in October and February, which occur during the traditionally heavy methomyl use-period in lettuce during September to mid-november and February to May (Table 1). The YMS97A strain was collected 1 d before an application that included methomyl and had an LD 50 of 2,208 g of methomyl per gram of larvae and a probit regression slope of Two days after the insecticide application, YMS97B was collected and had an LD 50 of 9,793 g of methomyl per gram of larvae, a 4.43-fold shift toward higher resistance. The YMS97B strain had a probit regression slope of , much ßatter than that of the YMS97A strain. This suggests a shift toward a greater mixture of homozygous suscepti-

4 October 1998 KERNS ET AL.: INSECTICIDE RESISTANCE IN S. exigua 1041 Table 3. Dose-mortality responses of beet armyworm strains treated with thiodicarb by using a diet incorporation bioassay Strain a n Slope SE LC 50 (95% CL) mg(ai)/kg RR b 2 df Laboratory susceptible populations MIS (346.55Ð638.00) PHO (605.86Ð811.73) Field collected populations (F 1 generations) SAL ,099 (1,876Ð2,393) DOM ,569 (2,283Ð2,921) SOM ,698 (2,415Ð3,042) TEX ,117 (2,965Ð3,276) Laboratory selection for increased methomyl resistance (F 8 generation) YAC96B c ,402 (2,273Ð2,524) a Collection information presented in Table 1. b RR, resistance ratio calculated by dividing the LD 50 of the strain tested by the LD 50 of the PHO96 strain. c YAC96B is a population created from YAC96A larvae surviving 4,000 g (AI)/g. ble, heterozygotes and homozygous resistant phenotypes, indicating the involvement of major gene resistance (Matsumura 1985). Most Þeld strains exhibited low slope values, suggesting the heterogeneity for resistant types within the strain evaluated. The beet armyworm strain obtained from Imperial County, CA (ICA97), came from an area where methomyl had been extensively used in produce in previous months, followed by heavy use in alfalfa. This strain exhibited the highest level of methomyl resistance detected among all of the Þeld strains tested, with a resistance ratio of fold. Data collected during 1996Ð1997 conþrmed methomyl resistance in Þeld strains of beet armyworm, and a rapid shift toward higher resistance since 1989 when the highest resistance ratio was 2.41-fold. A rapid increase in resistance indicates monogenetically controlled resistance (Plapp et al. 1979). The most susceptible strains tested were collected from alfalfa from Riverside County, CA, the BLY96 strain, which had no detectable resistance (RR 1), and from La Paz County, AZ, the PAR96 strain which had a low level of resistance (RR 2Ð25). Both of these counties are located in areas dominated by cotton and alfalfa, with little winter vegetable production. Thus, methomyl is not extensively used year round in these areas allowing insecticide-free periods when susceptible genotypes can mediate resistance expression. However, development of resistance is still possible. The SAL96 strain, like the PAR97 strain, was collected from La Paz County but came from an area of the county dominated by melon crops and few alternative crops. This strain was exposed to 2 applications of methomyl before its collection. Consequently, SAL96 was high in resistance to methomyl with a resistance ratio of fold. The YAC97A strain collected from spinach had a very high level of resistance, with a resistance ratio of fold. Larvae from this strain that survived methomyl dosages 4,000 g of methomyl per gram of larvae were selected to form the new strain YAC96B. When the F 4 generation of YAC97B was tested for resistance to methomyl, it showed a signiþcant shift toward a higher level of resistance, fold, suggesting more homogeneity for resistance. However, the low value of the slope indicates that some individuals heterozygous for resistance survived the 4,000- g dosages. Methomyl and Thiodicarb Diet Incorporation Bioassays. Susceptible laboratory strains, YAC96B, and the Þeld strains having moderate and high levels of resistance to methomyl were evaluated for cross-resistance to thiodicarb. Thiodicarb is chemically similar to methomyl; it is essentially 2 methomyl molecules linked together (Ware 1994). In the insect, thiodicarb is oxidatively cleaved to produce the primary toxicant methomyl. Because of the chemical similarities between these insecticides, we expected that resistance to methomyl would confer cross-resistance to thiodicarb. Although thiodicarb resistance has not been reported for beet armyworm, it has been reported in other lepidopterous pests (Elzen 1991, Campanhola and Plapp 1989, Karunaratne and Plapp 1993, Leonard and Burris 1991, Elzen et al. 1992, Vinatier et al. 1992, Martin et al. 1997). Cross-resistance between methomyl, pyrethroids, and organophosphates has been reported for beet armyworm (Aldosari et al. 1996) and with thiodicarb for tobacco budworm, Heliothis virescens (F.) (Martin et al. 1997). When thiodicarb was fed in artiþcial diet to beet armyworm larvae of Þeld strains collected in this study, resistance ratios were low, ranging from to 4.45-fold (Table 3). Of the Þeld strains tested, TEX96 had the greatest LC 50 value for thiodicarb, which was signiþcantly greater than the other Þeld strains, except for SOM96. Consequently, among the Þeld strains evaluated for thiodicarb resistance, TEX96 also had the greatest LD 50 for topically applied methomyl (Table 2), suggesting cross-resistance between thiodicarb and methomyl. Although the resistance ratio values for thiodicarb are considerably less than those detected for the topical methomyl bioassays, this is not necessarily an indication of a low level of resistance to thiodicarb, but may indicate differences in the relative sensitivity between topical and oral bioassay techniques. Bioassays where insects are immersed or allowed free contact to the insecticide rather than point-source dermal exposure commonly produce lower LC 50 and resistance ratio values

5 1042 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 5 Table 4. Dose-mortality responses of beet armyworm strains treated with methomyl by using a diet incorporation bioassay Strain a n Slope SE LD 50 (95% CL) mg(ai)/kg RR b 2 df PHO (257.26Ð411.26) YAC96B ,642 (1,459Ð1,814) a Collection information presented in Table 1. The YAC96B strain is 12th generation since its selection with 4,000 g of methomyl. b RR, resistance ratio calculated by dividing the LD 50 of the strain tested by the LD 50 of the PHO96 strain. and have steeper response curves (Robertson and Haverty 1981, ffrench-constant and Roush 1990, Dennehy et al. 1983). Thus, the low resistance ratios detected for thiodicarb may not be indicative of low levels of resistance but a low level of methodological sensitivity. Likewise, when methomyl was bioassayed using the diet incorporation method, the resistance ratio for YAC96B (RR 4.99-fold) was notably reduced relative to the topical assay (RR fold) (Tables 2 and 4). However, if the oral ingestion bioassays are not insensitive relative to a topical bioassay, these data could be interpreted to suggest that a reduction in cuticular penetration might be involved in methomyl resistance. Methomyl is a contact poison, whereas thiodicarb must be ingested to induce its highest level of toxicity. This implies that by-passing the cuticle may alleviate much of the resistance. However, the mechanism for methomyl and thiodicarb resistance in beet armyworms from the low-desert areas of Arizona and California is not clear and will require further study. Acknowledgments We thank T. J. Dennehy (University of Arizona), for his advice concerning the interpretation of the results of this study. We thank E. T. Natwick (University of California), M. Rethwisch, (University of Arizona), and J. Nigh for aiding in collecting samples. 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6 October 1998 KERNS ET AL.: INSECTICIDE RESISTANCE IN S. exigua 1043 larvin to tobacco budworm larvae, pp. 867Ð870. In Proceedings, Beltwide Cotton Production and Research Conference. National Cotton Council of America, Memphis, TN. Ware, G. W The pesticide book, 4th ed. Thomson Publications, Fresno, CA. Yoshida, H. A., and M. P. Parrella The beet armyworm in ßoricultural crops. Calif. Agric. 41: 13Ð15. Received for publication 29 August 1997; accepted 29 May 1998.