University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Faculty Publications: Department of Entomology Entomology, Department of August 2000 Baseline Susceptibility of the Corn Earworm (Lepidoptera: Noctuidae) to the Cry1Ab Toxin from Bacillus thuringiensis Blair D. Siegfried University of Nebraska-Lincoln, [email protected] Terrence A. Spencer University of Nebraska-Lincoln, [email protected] Jessica Nearman University of Nebraska-Lincoln Follow this and additional works at: https://digitalcommons.unl.edu/entomologyfacpub Part of the Entomology Commons Siegfried, Blair D.; Spencer, Terrence A.; and Nearman, Jessica, "Baseline Susceptibility of the Corn Earworm (Lepidoptera: Noctuidae) to the Cry1Ab Toxin from Bacillus thuringiensis" (2000). Faculty Publications: Department of Entomology. 34. https://digitalcommons.unl.edu/entomologyfacpub/34 This Article is brought to you for free and open access by the Entomology, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications: Department of Entomology by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Baseline Susceptibility of the Corn Earworm (Lepidoptera: Noctuidae) to the Cry1Ab Toxin from Bacillus thuringiensis Blair D. Siegfried, Terrence Spencer, and Jessica Nearman Journal of Economic Entomology Volume 93, Issue 4 (August 2000) pp. 1265–1268 DOI: 10.1603/0022-0493(2000)093[1265:BSOTCE]2.0.CO;2 Authors may post electronic reprints of their own journal articles after an embargo period of two years has passed from the date of publication. Also, authors must include on the electronic reprint the following statement: This article is the copyright property of the Entomological Society of America and may not be used for any commercial or other private purpose without specific written permission of the Entomological Society of America. INSECTICIDE RESISTANCE AND RESISTANCE MANAGEMENT Baseline Susceptibility of the Corn Earworm (Lepidoptera: Noctuidae) to the Cry1Ab Toxin from Bacillus thuringiensis BLAIR D. SIEGFRIED, TERRENCE SPENCER, AND JESSICA NEARMAN Department of Entomology, 202 Plant Industry Building, University of Nebraska, Lincoln, NE 68583Ð0816 J. Econ. Entomol. 93(4): 1265Ð1268 (2000) ABSTRACT Susceptibility to Cry1Ab toxin from Bacillus thuringiensis (Bt) was determined for 12 Þeld populations of neonate corn earworm, Helicoverpa zea (Boddie), from the United States. Earworm larvae were exposed to artiÞcial diet treated with increasing Bt concentrations, and mortality and growth inhibition were evaluated after 7 d. The range of variation in Bt susceptibility indicated by growth inhibition was very similar to that indicated by mortality. Although interpop- ulation variation in susceptibility to both proteins was observed, the magnitude of the differences was small (less than or equal to Þvefold). These results suggest that the observed susceptibility differences reßect natural variation in Bt susceptibility among corn earworm populations rather than variation caused by prior exposure to selection pressures. Therefore, corn earworms apparently are susceptible to Bt toxins across most of their geographic range. KEY WORDS Baccilus thuringiensis susceptibility, geographic variability, bioassay INCREASING PUBLIC CONCERNS about environmental haz- The insect is a generalist feeder on both weed and crop ards and widespread resistance in pest populations are plants, including cotton, tobacco, and tomato (Metcalf threatening the continued effectiveness of conven- and Metcalf 1993). tional insecticides and should increase the use of in- Corn earworms are unable to overwinter in the secticidal products derived from Bacillus thuringiensis northern United States. However, in southern states, Berliner in the next decade (Tabashnik 1994). This is where populations are potentially exposed to both particularly true for transgenic Bt crops, which rep- transgenic cotton and corn in alternate generations, resent an important new tool for pest management there is a high level of concern regarding the increased (Fischhoff 1996). However, increased use of B. thu- potential for selection pressure and resistance devel- ringiensis in agriculture via transgenic plants and other opment (International Life Sciences Institute 1998). modes of application likely will increase the intensity This is especially true because of the shared binding of selection for Bt resistance in pest populations. site between the Cry1Ab and Cry1Ac toxins that are The possibility of resistance highlights the need to expressed in Bt corn and cotton (Tabashnik 1994, develop and implement resistance management strat- Gould 1998), respectively. Furthermore, the expres- egies to prevent or delay the evolution of resistance to sion level in either crop plant is probably insufÞcient B. thuringiensis toxins (Hokkanen and Wearing 1994). to achieve a high dose for this pest species because of These strategies are dependent on the development of generally higher tolerance to the Bt toxins (Interna- effective resistance monitoring programs capable of tional Life Sciences Institute 1998). early detection of resistance that will allow implemen- Variation in susceptibility to the Cry1Ac has been tation of appropriate management decisions in a documented previously among geographically dis- timely manner (Dennehey 1987). The initial steps in tinct H. zea populations (Stone and Sims 1993, Sims et implementing such programs include development of al. 1996) and annual evaluations of susceptibility to appropriate bioassay techniques and establishment of Cry1Ac are conducted for insects collected from cot- baseline susceptibility data among populations across ton. The objective of the current study was to establish the geographic range of the target species. With this a baseline of susceptibility to Cry1Ab toxins from information, potential population susceptibility geographically distinct populations of H. zea collected changes in response to selection with B. thuringiensis from corn, emphasizing areas where there is likely to can be identiÞed (Fischhoff 1996). be exposure to both transgenic corn and cotton. The corn earworm, Helicoverpa zea (Boddie), is a serious pest of maize, Zea mays L., in the United States Materials and Methods (Mason et al. 1996) and a major target pest for control with transgenic Bt corn (Fischhoff 1996). This pest Insects. Populations of corn earworm were col- causes primary damage to corn plants by feeding on lected from nine states from both northern and south- ears, although whorl feeding does occur in early gen- ern areas of the corn belt and bioassayed to determine erations (Neunzig 1969, Keith and Witkowski 1996). susceptibility to the Cry1Ab Bt endotoxin (Table 1). 0022-0493/00/1265Ð1268$02.00/0 ᭧ 2000 Entomological Society of America 1266 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 93, no. 4 Table 1. Source description of European corn borer popula- tein used for bioassays consisted of puriÞed Cry1Ab tions used to establish baseline susceptibility to the Cry1Ab toxin obtained from the B. thuringiensis kurstaki strain from B. thuringiensis HD1Ð9, which produces only Cry1Ab protein (pro- Parental Generation Sample vided by the Novartis, Research Triangle Park, NC). Collection site Population Bioassayed size The crystal protein preparation was obtained by den- sity gradient centrifugation and contains Ϸ98% crystal Loxley, AL 200 F1 784 Salisbury, MD 199 F 748 protein as determined by phase contrast microscopy. 2 Ͻ Upper Marlboro, MD 152 F2 1,120 Individual neonate larvae ( 24 h after hatching) were Fayetteville, AK 150 F1ÐF2 896 placed in wells, and mortality and individual larval Aurora, NE 236 F1 784 Ͼ weight recorded 7 d later. Control treatments con- Plymouth, NC 100 F1 672 Leland, MS 117 F1ÐF2 672 sisted of wells treated with 0.1% Triton-X 100. When Stoneville, MS 196 F2 672 mortality was recorded, larvae that had not grown Loxley, AL 200 F1ÐF3 1,120 beyond Þrst instar and weighed Յ10 mg were consid- Washington County, MS (Bt) 175 F1ÐF2 748 Ͼ ered to be dead. As a result, the criterion for mortality Washington County, MS 200 F1ÐF3 896 Ͼ Rosemont, MN 100 F1 896 used in this study accounts for both severe growth a Laboratory Colony ÑF5 672 inhibition and death. Statistical Analyses. Bioassays were conducted in a Colony initiated in 1997 from Stratford, TX, Þeld collection. duplicate on three different dates and included at least Þve Bt concentrations that produced signiÞcant Populations were obtained by Novartis personnel as growth inhibition and mortality as deÞned previously. late instars from both Þeld corn and sweet corn and Because of difÞculties in obtaining enough larvae from sent to the University of Nebraska. Additional collec- a single generation, results from multiple generations tions were obtained as larvae or pupae from corn were combined for some populations. Individual larval plants by the Monsanto Company and provided to weights were recorded after 7-d exposures and trans- Agripest Crop Consulting, Zebulon, NC, for rearing. formed to percentage of growth inhibition relative to Field-collected insects were reared to adults and al- the controls, and these data were analyzed by non- lowed to mate using standardized rearing techniques linear regression (SAS Institute 1988, Marc¸on et al. (Stone and Sims 1991). Eggs from the mated females 1999). Mortality data were analyzed by probit analysis were collected and allowed to hatch. Neonate larvae (Finney 1971) using POLO-PC