Resistant Pests Plague Organic Orchards, Too

Organic apple orchards that had counted on a natural virus to control common worms are finding that the worms are becoming resistant to the virus -- just as other farm pests become resistant to pesticides over time. In some organic orchards in Southwest Germany, the coding moth caterpillar is showing signs of resistance to the granulovirus, and scientists have identified a single gene that can make the moth caterpillar 100,000 times less susceptible to the virus.

The study was published in Science and conducted by a team of insect virologists and geneticists from the Agricultural Service Centre of Rhineland-Palatinate, the German Federal Biological Research Centre, the University of Hohenheim, and the Max Planck Institute for Chemical Ecology. Here's some additional information from the research institutions, about how the study was conducted, and what the results were:

Starting in 2005, codling moths collected from 13 organic orchards in southwest Germany were tested in the laboratory to confirm that the insects could tolerate granulovirus amounts more than a thousand times higher than previously. Genetic studies showed that the resistance could be transmitted from parents to offspring via one of the sex chromosomes - which helps to explain how the resistance increased so quickly. The sex chromosomes in humans are called X and Y, with XX females and XY males. This is reversed in moths, where the sex chromosomes are called Z and W, with ZZ males and ZW females. The researchers found that the gene for granulovirus resistance occurs on the Z chromosome. Female caterpillars need only a single copy of the resistance gene to be nearly 100,000 times less susceptible to granulovirus infection. They stay healthy and survive to reproduce, when most others have been killed. Until now very effective in pest management: the granulovirus. Sons from matings between these highly resistant females and susceptible males carry a virus resistance gene on just one of their two Z chromosomes.

"This means of inheritance offers the quickest possible way for the insects to evolve resistance" says Prof. David Heckel of the MPICE. "If the apple grower increases virus applications to try to control the damage caused by the resistant population, the opposite results. Selection for resistance accelerates and the frequency of the gene on the Z-chromosome increases even faster in the population." The study poses serious questions for the organic farming industry.

If natural pest controls prove ineffective over time, it may be more difficult to develop alternatives. Time will tell, and research like this could help define strategies for retaining the effectiveness of important biological controls.

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