What are insect resistant
crops?
Insect resistant plants are created by inserting a gene that produces a
protein toxic to insects. The most common gene of this type is from the
soil bacterium Bacillus thuringiensis. This gene codes for a protein
called the Bt toxin that binds to the gut of the insect and prevents it
digesting the food it eats. Several types of Bt exist each one specific to
a group of insect species. The insect gut is sufficiently different from
the animal digestive tract that Bt has no effect on animals including
humans. It is produced as an inactive protoxin which is activated in the
gut of the insect by the alkaline conditions that are found there. If it
is eaten in human food, it first encounters the acid in the stomach which
not only will not activate the protoxin to active Bt but will make it
permanently inactive and it is then rapidly broken down in the
intestine.
Could insect resistant crops
eliminate beneficial insects?
Insect resistant crops could indeed have a negative impact on
beneficial insect populations. It all comes down to an evaluation of the
risks as opposed to the benefits.
Some crops require constant spraying with insecticides (e.g. potatoes
for Colorado beetle and cotton for the boll weevil). In these crops, use
of insecticide has been dramatically reduced by the introduction of insect
resistant varieties. This has reduced the impact of insecticides on useful
insects and lessened the effect on animals and humans. It is, therefore,
important to determine the impact of spraying against the impact of the
insect resistant plants. Studies so far, have suggested that the impact on
beneficial insects of the insect resistant plants is minimal. In addition,
many insects, e.g. the Colorado beetle are becoming insecticide resistant
and this is leading to use of more toxic or multiple insecticides. The use
of Bt containing plants could overcome this problem.
Insects may become resistant to the Bt toxin and crops will again have
to be sprayed. This is a genuine concern although insect resistance has
not been as much of a problem as was feared. It is fundamentally a
management problem. Farmers are required to leave twenty per cent of their
crops as non-insect resistant varieties as a haven for the insect pest.
Any resistant insect is likely to breed with a non-resistant form and the
resistant trait will be lost. A second strategy is to constantly modify
the Bt toxin to overcome any resistance. Insect resistance to Bt is a
recognized problem for the companies that sell insect resistant crops.
The pollen or other plant parts can kill beneficial insects. This is
correct; monarch butterflies that are forced to eat milk weeds coated with
pollen from Bt corn plants are damaged. This is not surprising since the
monarch and the European corn borer moth are related insects. However,
there is no evidence that this occurs in nature and if it does it is
unlikely to have a major impact on the monarch population. It is something
that should be monitored. However, there are varieties of Bt corn in which
the Bt toxin is not in the pollen and these will not have the problem of
poisoning monarch butterflies. In addition, milkweed is eradicated from
corn fields and is regarded as a noxious weed. Most of the butterflies
feed on milkweed that is found in waste land or other non-cultivated land
free from corn pollen.
It should be noted that there is a significant loss in the yield of the
corn crop each year amounting to a loss of revenue to American farmers of
about $1.2 billion because of the European corn borer. Since the insect is
buried in the stem of the corn, it is difficult to kill with insecticides.
It is however, well controlled by Bt expression in the plant.
If the Bt gene is transferred to wild plants beneficial insects will be
killed. This is correct. Again it is necessary to consider the problem in
relation to the crop and potential wild plants that might be affected. It
should be carefully monitored.
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