With such vast monocultures of GM corn being grown in America, most of which now self-infuse with the same or similar 'Bt' insecticides to kill the same or similar moth infestation, you might expect the pests to be reducing in abundance under the biotech-inspired onslaught.
Indeed, although no investigation has been made into the cause, earworm populations in American fields have been declining. Long-term field monitoring from 1996 when the first Bt crops were entering the US landscape to 2016 found the pest reduced by up to 86%.
This should be good news for farmers, but counter-intuitively, over the same period, tests on sweetcorn sentinel plants  indicated an increase or no change in damage to both GM and non-GM plants.
Laboratory tests confirmed that earworms have developed resistance, not only to the Bt toxin most commonly generated by GM maize, but to multiple Bt toxins pyramided in newer GM varieties.
In India, field-evolved resistance of bollworm to Bt-toxin in GM cotton was noted several years previously. At the time, Monsanto blamed farmer ignorance and regulatory failure:
"failure to plant non-Bt crop refuges, failure to procure non-Bt seed for the refuges, failure to scout the crop for pest emergence, failure to switch to Monsanto's Bollgard 11 which has had two Bt genes inserted, over-use of pesticides, failure to manage the infected crop residues and unopened bolls after harvest, poor tillage and inadequate grazing of cattle on the GM stubble, and previous illegal planting of unsuitable Bt cotton crops." 
None of these 'failures' apply to US maize farmers on any scale, so what's the problem? The authors of the long-term study voiced the concern that the 'refuge-in-a-bag' (RIB) system which uses seed premixed with 5% non-GM seed might have been a 'key misstep'.
RIB is a biotech industry innovation touted to farmers as fast and convenient, enabling GM seed to be planted "fence row to fence row" without the bother of planting and harvesting a non-GM refuge area to maintain the population of Bt-susceptible pests.
Unfortunately, RIB in the real world seems simply to provide a huge area of low-level Bt presence which not only fails to delay resistance, but actively encourages it.
In the words of its authors, the long-term US study has "important implications for sustainable corn production, biotechnology regulatory policies, and sustainability of Bt biotechnology"
Leaving aside the industry claims that it is the user's fault that GM crops aren't working as advertised, not any design fault in their high-tech plants, we may be witnessing an example of what another team of scientists realised a few months after the long-term maize results were published: pests can choose to minimise the Bt toxin in their diet and can choose a healthy diet which lessens the impact of the Bt toxin(s) they're exposed to. The net result is that the moth larvae are not killed outright, but are poisoned just enough to delay their development so they spend a lot longer eating the crop.
This, indeed, has important implications for sustainability and regulation of Bt crops:
- If we want sustainable crop production we have to ditch the simplistic, silver-bullet, gene-obsessed mind-set of biotech scientists in favour of holistic and ecologically-based principles of agriculture.
- Regulators should be promoting No.1 above, not Bt nor any other GM crop
 Sweetcorn 'sentinel' plants were used to study what was going on between pests and the crop in the corn fields. Sweetcorn is planted at a time which is especially susceptible to earworm, so that lack of opportunity for infestation won't skew the readings. Also it is cropped early before the Bt toxin levels begin to drop due to the aging of the plants, so that reduced pest control won't skew the results.
 Bt LOVING INSECTS- February 2011
- Galen P. Dively, et al., 30 December 2016, Field-Evolved Resistance to corn Earworm to Cry Proteins Expressed by Transgenic Sweet Corn, PLOS ONE
- Carey Gillam, New Research shows Failing of GMO Insect resistance, Corn Crop in Jeopardy, Huffington Post, 1.01.17
Photo: Creative Commons