Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2006
Publication Date: 6/30/2006
Citation: Tamez-Guerra, P., Damas, G., Iracheta, M.M., Oppert, B.S., Gomez-Flores, R., Rodriguez-Padilla, C. Diferences in susceptibility and physiological fitness of Mexican field Trichoplusia ni (Hubner) strains exposed to Bacillus thuringiensis. Journal of Economic Entomology 99: 937-945. Interpretive Summary: In Mexico, resistance to spray formulations that contain the insecticidal bacterium Bacillus thuringiensis (Bt) has been reported for a major pest of crucifers, the diamondback moth, Plutella xylostella. Because Bt sprays are being used extensively to control crucifer pests, there is concern that other pests will develop resistance, such as the cabbage looper, Trichoplusia ni. Therefore, T. ni insects were collected in various field locations in Mexico and were studied in the laboratory for Bt susceptibility. Differences in susceptibility were detected for field strains compared to laboratory colonies of T. ni with both Bt formulations as well as purified Bt proteins. In addition, physical parameters of field and laboratory strains were compared. These data provide baseline measurements for the response of T. ni to various Bt formulations and toxins, which can be used in monitoring Bt resistance and provide information to develop resistance management plans.
Technical Abstract: The use of different commercial Bacillus thuringiensis (Bt) products in the Bajio guanajuatense area in Mexico began 12 years ago, and resistance to Bt in this area has been reported for Plutella xylostella. The present study provides a baseline response and resistance potential to Bt in field and laboratory strains of Bajio Trichoplusia ni. Differences in susceptibility to Bt among T. ni populations were observed. T. ni neonates collected in Romita, Guanajuato were more susceptible to Bt than those collected in Salvatierra or San Luis de la Paz, Guanajuato. After five generations of XenTari in the laboratory, decreased susceptibility was found only in the Salvatierra insects, with an LC50 that was 2.1-fold greater than that of a Mexican laboratory strain. The XenTari-selected San Luis de la Paz strain was from 16 to 87-fold more resistant to Cry1A protoxins than US and Mexican laboratory strains. Although Cry1Ab is not a component of XenTari, this strain also was significantly less susceptible to Cry1Ab toxin when compared with a US strain, with a resistance ratio of 404. The larval weights and lengths, pupal lengths and pupation percentage were significantly lower for the Salvatierra strain than for all other strains. The relationship of T. ni susceptibilities to Bt Cry toxins and protoxins after several generations of exposure to XenTari and the similarity to studies in P. xylostella are discussed.