Several terms are used to describe Bt corn hybrids. It is important to understand what these terms mean and how they relate to resistance management.
When genes are inserted into a crop plant, each successful transformation is referred to as an event. Even if the same genes are inserted into a plant, two different events may differ in how the genes are expressed in the plants, resulting in different effects on insects.
Bt corn hybrids have had genes inserted from a naturally occurring soil bacterium, Bacillus thuringiensis (Bt), which produce proteins with insecticidal properties. A variety of protein toxins have been isolated from Bacillus thuringiensis. Different Bt toxins have activity against different types of insects. Some toxins have activity against lepidopteran insects such as European corn borer, and some have activity against beetles, like corn rootworms.
The initial Bt proteins that were incorporated in corn and other crop plants were crystalline proteins, and are designated by a number and one or more letters (e.g., Cry1Ab, Cry1F). More recently other Bt proteins have been incorporated into crop plants, e.g., the VIP protein (Vip 3A) commercialized by Syngenta in Viptera hybrids.
Bt proteins bind with receptor cells in the insect gut. Different insect species are more or less susceptible to different Bt proteins, depending on how well the protein fits the gut receptor cells of different species.
One resistance management strategy is to use crop hybrids with different Bt proteins over time to avoid resistance development to a particular protein. More recently seed companies have produced hybrids with two or more proteins active against a particular insect. These are referred to as pyramids. Insects are less likely to develop resistance to two toxins at once rather than to a single toxin. Because of this the EPA has approved reduced refuge requirements for pyramid hybrids.
Initially Bt corn hybrids had one Bt protein active against European corn borer, or one Bt protein active against corn rootworms, and an external refuge of 20% non-Bt corn in the northern Corn Belt. Now there are multiple insect control traits available in Bt corn hybrids and multiple refuge requirements for different hybrids.
As you select hybrids, be sure the hybrid you select has the appropriate insect control traits (and other traits) you desire, and that you understand the refuge requirements for that hybrid. Even if you use hybrids from different companies over time, you still may be planting hybrids expressing the same Bt toxins. To avoid development of resistance, it is important to rotate different single protein hybrids or use pyramided hybrids with multiple Bt proteins against the insects you need protection from.
A good source of information on Bt proteins expressed by different hybrids and their refuge requirements is the Handy Bt Trait Table from Michigan State University.