[Market-farming] sweet corn - Bt resistance?

Shoemaker, William H wshoemak at illinois.edu
Thu Jan 5 14:29:30 EST 2012

Good point. There is another problem for resistance development with worm-resistant Bt corn; scale of exposure. The Bt corn varieties have refuge requirements. I believe it is 25% of the acreage that must be planted to non-Bt corn to preserve a portion of the pest population. That may seem counter-intuitive but the idea is to keep the non-resistant pest population healthy so that any resistance genes that emerge are diluted and masked by the non-resistant portion of the pest population as they interbreed. The problem is that farmer compliance with refuge requirements isn't as good as it should be, especially over the last couple of years. So when farmers used Bt dust they were limiting exposure to the pest in a number of ways. Other farmers use other pesticides on the same pest population. The pest may not be exposed in each generation. There may be lots of acreage of other crops the pest can thrive on. Or there may be an abundance of native plants that support the pest population (tomatoes and their nightshade relatives for example). In the end, there are 10's of millions acres of Bt corn planted. The scale itself increases risk for resistance development.

Its important to note that use of Bt on one pest does not influence resistance development for another unrelated pest. If they don't interbreed, they don't pass resistance genes.


William H. Shoemaker

University of Illinois, Crop Sciences

St Charles Horticulture Research Center

535 Randall Road, St Charles, IL  60174

630-584-7254, wshoemak at illinois.edu<mailto:wshoemak at illinois.edu>

From: market-farming-bounces at lists.ibiblio.org [market-farming-bounces at lists.ibiblio.org] on behalf of Road's End Farm [organic87 at frontiernet.net]
Sent: Thursday, January 05, 2012 1:14 PM
To: Market Farming
Subject: Re: [Market-farming] sweet corn - Bt resistance?

BT kurstaki etc. applied as a spray or a dust is only present in the field for a short period of time after each application. GMO plants expressing BT toxin do so continuously throughout the season, and with probable residue remaining in the field for some time after harvest. The latter situation is likely to produce resistance much faster.

-- Rivka; Finger Lakes NY, Zone 5 mostly
Fresh-market organic produce, small scale

On Jan 5, 2012, at 2:05 PM, <sunnfarm at netscape.com<mailto:sunnfarm at netscape.com>> wrote:

I have used BT since 1972 and BT kurstaki or Dipel is just as effective today as it was 40 years ago. I can not name a single synthetic that has been resistance free for 10 years yet alone 40. Bt is a natural product and must have evolved along with the insects it controls. That is millions of years of exposure.
Even before gmo's BT was applied to millions of acres annually without resistance. So I don't think there will be any resistance anytime soon with Bt corn. Keep in mind that corn worms have many host plants to preserve there susceptibility to the toxin.... Bob.

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