INSECTICIDE RESISTANCE AND RESISTANCE MANAGEMENT Susceptibility of Dipel-Resistant and -Susceptible Ostrinia nubilalis (Lepidoptera: Crambidae) to Individual Bacillus thuringiensis Protoxins HUARONG LI, 1 BRENDA OPPERT, 2 RANDALL A. HIGGINS, 1 FANGNENG HUANG, 3 LAWRENT L. BUSCHMAN, 1 AND KUN YAN ZHU 1 J. Econ. Entomol. 98(4): 1333Ð1340 (2005) ABSTRACT Dipel-resistant and -susceptible strains of Ostrinia nubilalis (Hu ¨ bner) were evaluated for larval mortality and growth inhibition when fed diets containing individual Bacillus thuringiensis protoxins. Resistance ratios for four of the protoxins in Dipel (Cry1Aa, Cry1Ab, Cry1Ac, and Cry2Aa) were 170-, 205-, 524-, and 640-fold, respectively, considerably higher than the 47-fold resistance to Dipel. The Dipel-resistant strain was 36-fold resistant to Cry1Ba, a protoxin not present in Dipel. Another non-Dipel protoxin, Cry1Ca, did not cause signiÞcant mortality for either resistant or susceptible larvae with doses as high as 1.0 mg/ml. In an evaluation of larval growth inhibition, resistance to Cry1Aa, Cry1Ab, Cry1Ac, and Cry1Ba was signiÞcant at concentrations of 0.054 and 0.162 g/ml. However, growth inhibition with Cry2Aa was not signiÞcant at either dose. These data provide information on the spectrum of resistance and cross-resistance to individual Cry protoxins in this strain. KEY WORDS Cry protoxins, Bt resistance management, cross-resistance, European corn borer, Bt toxicity CRYSTALLINE (CRY) PROTEINS PRODUCED by Bacillus thu- ringiensis Berliner (Bt) are toxic to several economi- cally important insect pests, particularly those in the order Lepidoptera (Ho ¨ fte and Whitely 1989, Widner and Whiteley 1989, Schnepf et al. 1998). Several com- mercial insecticidal formulations, such as Dipel ES (Valent BioSciences Co. Libertyville, IL) and Javelin (Thermo Trilogy, Columbia, MD), have been devel- oped from different Bt strains to be used as biological insecticides against insect pests. More recently, the genes controlling expression of these toxins also have been inserted into the genome of certain crop plants to render them resistant to insect pests (Perlak et al. 1990, Adang et al. 1993, Koziel et al. 1993). The European corn borer, Ostrinia nubilalis (Hu ¨ b- ner), is one of the most damaging pests of corn, Zea mays L., in North America and has been one of the targets for commercial applications of Bt insecticidal formulations (McWhorter et al. 1972, Mason et al. 1996, Sloderbeck et al. 2004). This insect also has been the primary target for transgenic Bt corn hybrids. The long-term durability of Bt toxins is threatened for both applications if insects develop resistance to Cry pro- teins (McGaughey 1985, Gould et al. 1992, Tabashnik 1994). It is therefore important to understand the mechanisms of resistance to Cry toxins in laboratory- developed resistant strains so that sound resistance management strategies can be developed that are based on an understanding of the physiological mech- anisms likely to develop in the Þeld. The KS-SC strain of O. nubilalis is 70-fold resistant to Dipel ES, one of the highest levels of resistance to a Bt formulation that has been documented for this insect (Huang et al. 1997, 1999b; Bolin et al. 1999, Chaufaux et al. 2001, Siqueira et al. 2004). This strain was obtained by exposing larvae to low concentrations of Dipel ES (B. thuringiensis subsp. kurstaki HD-1) (Huang et al. 1997). Resistance in this strain is inher- ited as an incompletely dominant trait (Huang et al. 1999a) with moderate levels of heritability (Huang et al. 1999b). Resistance has been associated with reduced digestive proteinase activity resulting in re- duced protoxin activation (Huang et al. 1999c; Li et al. 2004a, 2005). Although resistant larvae had a signiÞ- cant level of resistance to Dipel ES, they were not able to survive on high-expression Bt corn plants (ex- pressing Cry1Ab) in greenhouse trials (Huang et al. 2002). The Dipel ES formulation contains a complex mix- ture of spores, protoxins, and other formulation products (Masson et al. 1989). Cry protoxins found in Dipel ES include Cry1Aa, Cry1Ab, Cry1Ac, Cry2A, and Cry2B. In general, Cry1A protoxins are toxic to various lepidopterans, whereas Cry2 proteins are toxic to lepidopterans and dipterans. Resistance that devel- Mention of trade names or commercial products in this publication is solely for the purpose of providing speciÞc information and does not imply recommendation or endorsement by the USDA. 1 Department of Entomology, Kansas State University, Manhattan, KS 66506. 2 Grain Marketing and Production Research Center, USDAÐARS, 1515 College Ave., Manhattan, KS 66502. 3 Department of Entomology, Louisiana State University AgCenter, Baton Rouge, LA 70803.