Bt crops – some background

November 27, 2013 by Rebecca Nesbit 4 Comments

One of the most widespread genetic modifications of crops is the addition of a Bt gene, so here’s some background on what it is and what it does.

A bacteria found in soil, Bacillus thuringiensis, produces proteins that are toxic to certain insects, and so has been used as a ‘microbial insecticide’ for well over 50 years.

More recently, crops including cotton, potato and corn have been modified to contain the gene for an inactive form of the Bt toxin. The plant itself is therefore producing the toxin which naturally occurs in bacteria. The Bt toxin becomes active in the gut of insects, which is an acidic environment with specific enzymes.

The intention of these Bt crops is to increase yield (by preventing loss to insects) and reduce the need for pesticides.

Over 100 different variants of Bt toxin have been identified, and they don’t all affect the same insects. This specificity is a good thing – a major problem with pesticides in general is their effect on non-target insects. Caterpillars (Lepidoptera) are susceptible to a particular group, Cry1a, and beetles (Coleoptera) are susceptible to a different group, Cry3.

I have found it very hard to find figures about how much yield has been increased by Bt crops. This is partly because there are so many different sorts grown in so many different situations, and nobody has put all this information together. It’s also because I haven’t got all night and I rely on open access publications.

I read claims that Bt corn likely provides a marginal yield increase of 3 – 4% over conventional practices (this is an overall figure – it is much higher when pest populations are large).  In contrast, a 2012 paper in the peer-reviewed journal PNAS writes that “Bt has caused a 24% increase in cotton yield per acre through reduced pest damage and a 50% gain in cotton profit among smallholders”.

Bt was first inserted into tobacco plants in 1985, but these were never commercialised. Bt potato plants were approved for use in the USA in 1995, and Bt maize followed in 1996. Bt crops are now extremely widely cultivated; in 2012 95% of land cultivated with cotton in India was Bt cotton.

In 2009, Monsanto found that the pink bollworm (a moth caterpillar) had become resistant to the Bt gene, Cry1Ac. As a result they brought out a variety which has different Bt genes to control the caterpillars.

Research is also taking place into which other naturally-occurring compounds which act as insecticides could be used in this way.