Bt inspired pests

September 2013

Cotton field in China. CC photo by tian yake on Flickr

A long running-debate about the sustainability of 'Bt' insecticide-generating GM crops is that because they kill their single most troublesome pest, no broad-spectrum chemicals are applied and other pests will “go crazy”.

The story of Bt cotton in China supports this concern. More than 90% of Chinese cotton is now genetically transformed to control corn boll-worm (CBW). “Spraying specifically for CBW from 1997 when Bt cotton was introduced until 2010 decreased by a spectacular two-thirds. Total sprayings for all insect pests during this period was reduced by a less spectacular 14%. Spraying for non-CBW pests increased by 60%.” (APHID-FRIENDLY COTTON - July 2012)

The secondary pests necessitating all this increased spraying included sap-suckers, such as aphids, and herbivores, such as mirid bugs. The actual secondary problem pest (or pests) emerging is a very local phenomenon, and some have never before been considered 'pests'. Reduced broad-spectrum spraying will allow existing pests to thrive, but why should they vary so widely in type and why should novel pests emerge? Something much more complex is going on.

A team of scientists from Switzerland, Britain and America has been investigating an aspect of pest control which simplistic biotech and agri-chemical approaches ignore: the plant's own defense mechanisms to avoid being eaten.

When plants are damaged, they produce a range of substances, referred to collectively as 'terpenoids' which are 'anti-feedants' and can be insecticidal. This means that healthy plants with CBW damage will generate terpenoids to protect themselves. The defenses triggered by this damage will keep other herbivors at bay, and also sap-suckers which do not, by themselves, inflict enough physical damage to elicit a plant response.

Removing both the CBW damage and the chemical sprays at the same time leaves the plant defenseless, especially against aphids.

The experiment was carried out under laboratory conditions, and didn't translate directly into the field situation, where many more interacting factors come into play.

OUR COMMENT

The indications are that healthy plants interacting with pests will defend themselves. Plants raised in a chemically sterilised environment, or GM plants which flood themselves with a toxin to eliminate a single pest, lack defense mechanisms which in turn disrupts the balance of pests around them: the result is novel pest problems.

Are we causing more problems in our food supply than we're solving with our high-tech approach to plant life?

SOURCES:

• Steffen Hagenbucher et al., 2013, Pest trade-offs in technology: reduced damage by caterpillars in Bt cotton benefits aphids, Proceedings of the Royal Society
• Use of GM cotton linked to rise in aphid numbers, www.scidev.net, 8.04.13