Induced resistance has been defined as the "qualitative or quantitative enhancement of a plant's defense mechanisms against pests in response to extrinsic physical or chemical stimuli." (Kogan and Paxton, 1983). Physical stimuli can include infection by a pathogenic organism (Jorgensen et al, 1996; Morris et al, 1998) and feeding by insects (Felton and Eichenseer, 1999). Chemical stimuli include such organic compounds as salicylic acid (Hammerschmidt and Smith-Becker, 1999), jasmonic acid (Staswick and Lehman, 1999), and the commercially available Actigard® 50WG (Tally et al., 1999) (Syngenta Crop Protection, P.O. Box 18300, Greensboro, NC 27419). Actigard® 50WG is registered in the U.S. to protect tomatoes, spinach and tobacco from fungal and bacterial pathogens. Srinivas et al. (2001a) showed that Actigard® did induce resistance in soybean to adult feeding of the bean leaf beetle, Cerotoma trifurcata (Forster) (Coleoptera: Chrysomelidae). However, Inbar et al. (2001) concluded that the active ingredient (i.e., BTH) in Actigard® had a negligible effect on whiteflies, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) and the cotton bollworm, Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae), in cotton.
Documentation of induced resistance against insect herbivores has a relatively short history (Karban and Kuc, 1999). The direct effects of induced resistance against insect herbivores have been shown in tomato, Lycopersicon esculentum Miller (Solanaceae) (Thaler, 1999), cotton, Gossypium hirsutum L. (Malvaceae) (Karban and Carey, 1984), soybean, Glycine max (L.) (Leguminosae) (Lin et al., 1990), and a number of other crops (Karban and Baldwin, 1997).
A few studies have been published documenting an induced response in maize, Zea mays L. (Graminae), to fungal pathogens such as Helminthosporium carbonum (Ullstrup) and Puccinia sorghi (Schw.) (Cantone and Dunkle, 1990; Morris et al., 1998). Morse et al. (1991) showed that mechanical crushing damage to maize leaves similar to that of certain insects with chewing mouthparts did result in reduced aphid growth rates and dramatically lower aphid survival when corn leaf aphids fed on the plants subsequent to the initial mechanical damage. Turlings et al. (1993, 1995, 1998) conducted several studies on maize and showed a response to herbivore feeding that attracted parasitoids to the plant in its defense.
The studies reported here were conducted to determine if an induction response can be shown in maize to fall armyworm larvae during the seedling to early vegetative growth stages as a response to initial feeding by the fall armyworm and/or treatment by the chemical inducer Actigard® 50WG.