Some Bt Sweet Corn No Longer Effective

I received a few calls this summer from sweet corn growers who wondered why they had corn earworm larvae in their Bt sweet corn. The short answer is that corn earworm has now developed resistance to the older set of sweet corn toxins and only the newest toxin will provide good control.

I ran a sweet corn sentinel plot trial at Lubbock this summer to determine the ability of Bt sweet corn to control corn earworm (as compared to non-Bt sweet corn).

The bottom line was that I found roughly twice as many healthy corn earworm larvae in the older types of Bt corn than in the non-Bt corn. There are a few reasons for this; a) corn earworm is largely resistant to Cry1Ab and Cry1A.105+Cry2Ab2, toxins which worked well several years ago, and b) it has been shown in the scientific literature that resistant larvae on Bt sweet corn largely lose their cannibalistic drive. On non-Bt corn there is usually one big surviving larva per ear by the time the larvae are older. On Bt sweet corn where the insects are resistant or partially resistant to the toxin(s) they are not as prone to cannibalism and there are often multiple surviving older larvae.

Here are some data from this summer's trial at the AgriLife Research Center at Lubbock.

The two columns on the right give us an idea about the health of the larvae. Corn earworm has six instars, and those reaching third instar will probably not be killed by Bt and have a good chance of completing development. I did notice a slight developmental delay in resistant caterpillars on the older Bt toxins as compared to non-Bt corn, but it was minor, perhaps a setback of a couple of days in development.

Attribute II has the older Cry toxin from Attribute and the newer toxin, Vip3a. At least for the High Plains of Texas, if you are a commercial sweet corn grower and want to use a Bt hybrid, it would be worth sourcing seed with Vip3a as one of the toxins.