Vip3A, the first vegetative insecticidal protein found in Bacillus thuringiensis (Bt), is uniquely different from Cry proteins and has a broad spectrum against multiple Lepidopteran corn pests. MIR162, a maize event genetically modified to expresses Vip3A, has been very effective in providing comprehensive protection for above ground corn tissues from infestation by major corn pests that include western bean cutworm (Striacosta albicosta, WBCW), corn earworm (Helicoverpa zea, CEW), and Fall armyworm (Spodoptera frugiperda, FAW). However, development of insect resistance to Vip3A (MIR162) is a serious concern because resistance can threaten the durability of this valuable trait. With an aim to preserve the value of Vip3A from being compromised due to resistance development, we have attempted to 1) understand the effectiveness of Vip3A (MIR162) against populations of the above pests that have been recently collected from the fields, and 2) select Vip3A-resistant insect strain for better understanding of the mode of action (MOA) and mechanism of resistance (MOR). Our results, combined with comprehensive data from literature, clearly indicate that Vip3A (and MIR162) is still very effective against these three pests. However, it is also clear that resistance to Vip3A is likely to develop as evidenced by increased Vip3A resistance alleles among field populations, especially in FAW. This prediction is further supported by reports of successful selection of Vip3A-resistant FAW and CEW strains in laboratories. By subjecting a FAW population with field genetics to Vip3A exposure, we have selected a Vip3A-resistant FAW strain, FAW_Vip3AR. In addition to having >9800-fold resistance to Vip3A, FAW_Vip3AR larvae also survive on Vip3A-expressing MIR162 corn plants that kill 100% of susceptible larvae. Feeding assays in the absence of Vip3A using both plants or plant materials and artificial diet strongly suggest that fitness costs are associated with Vip3A resistance manifested as slow larval growth and prolonged development.

Zhimou Wen

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