SELECTION AND GENETIC-ANALYSIS OF A HELIOTHIS-VIRESCENS (LEPIDOPTERA, NOCTUIDAE) STRAIN WITH HIGH-LEVELS OF RESISTANCE TO BACILLUS-THURINGIENSIS TOXINS

Heliothis virescens (F.), tobacco budworms eggs were collected from 3 adjacent counties in North Carolina. A laboratory strain (YDK) was established from these eggs using precautions to avoid loss of genetic diversity. A subset of this laboratory strain (YHDS) was selected on artificial diet containing the Bacillus thuringiensis (Berliner) toxin, CryIA(c). In the first 12 episodes of selection only moderate resistance (approximate to 7- to 8-fold) was found. However, after 19 episodes of selection the strain had developed >500-fold resistance to the CryIA(C) toxin. Further selection lead to higher levels of resistance with the greatest resistance ratio recorded being approximate to 10,000-fold. The YHD2 strain was cross-resistant to CryIA(a), CryIA(b) and CryIF. We also found some resistance to CryIB, CryIC and CryIIA, but the level of resistance to these toxins was more moderate. Reciprocal genetic crosses between the resistant and control strains indicated that resistance to CryIA(c) and to CryIA(b) was partially recessive, but that the low level of resistance to CryIIA was more dominant. Progeny from backcrosses of Fl larvae to the resistant parent were placed on artificial diet containing a concentration of CryIA(b) that had previously been found to slow the growth of F-1 larvae. When these larvae were weighed after 10 d, two clearly demarcated size classes were found in approximate to 1:1 ratios as is expected in backcrosses when a single locus (or a set of tightly linked loci) is coding for a major component of a recessive trait. Adults that developed from the larger size class of larvae were mated and their offspring inherited the ability to grow well on CryIA(b). As expected from the single locus model, one quarter of the offspring from matings of the small backcross larvae grew well on CryIA(b). Results of this selection experiment indicate that the initial frequency of this resistance trait could be approximately 10(-3), but field tests will be needed to confirm this rough estimate.