Field survey and greenhouse evaluation of non-rice host plants of the striped stem borer, Chilo suppressalis (Lepidoptera: Pyralidae), as refuges for resistance management of rice transformed with Bacillus thuringiensis toxin genes.

The striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Pyralidae), is the most abundant stem borer of rice Oryza sativa L. (Poaceae) in temperate Asia. Chilo suppressalis also occurs throughout the tropical rice-growing areas of Asia, where it is generally second in abundance to the yellow stem borer, Scirpophaga incertulas (Walker) (Lepidoptera: Pyralidae) (Pathak & Khan, 1994). Stem borers are the primary target pest for control by rice varieties transformed with toxin genes from Bacillus thuringiensis Berliner (Bacillaceae) (Bt). Bt rice is under development in several countries but has not yet been released to farmers (Cohen et al., 2000; Ye et al., 2001). The need to implement resistance management strategies to delay the development of pest resistance to Bt toxins in transgenic crops is widely recognized (Gould, 1998; EPAUSDA, 1999; Cohen et al., 2000). The most promising strategy entails the use of plants with a high dose of toxin in combination with the maintenance of crop ‘refuges’, i.e. nonBt plants that serve to maintain Bt-susceptible insects in the pest population (Gould, 1998). Refuges can consist of non-Bt cultivars of the same species as the Bt crop as well as, for polyphagous pests, alternative host species. Refuges consisting of alternative hosts are an attractive option for Bt rice because it will be difficult to enforce the maintenance of refuges of non-Bt rice by small, low-income farmers. In contrast to S. incertulas, which is restricted to Oryza spp., C. suppressalis is considered to be polyphagous. The bibliography of Khan et al. (1991) reports 41 species of host plants of C. suppressalis in six families, primarily wild and cultivated Poaceae. This information suggests that non-rice hosts might serve as refuges for C. suppressalis. However, a critical review of the literature reveals that this host list is based on a very limited amount of experimental or field survey data (see Discussion). The objective of this study was to survey the abundance of C. suppressalis in five grass species that are common in and around rice fields in tropical and subtropical Asia (Soerjani et al., 1987) and are listed as alternative hosts of C. suppressalis by Khan et al. (1991), and to evaluate the suitability of these grasses as hosts of C. suppressalis under greenhouse conditions.