Bt toxins and the diamondback moth: Portrait of a pesticide and a pest

Fighting agricultural pests is an arms race against evolution, and the diamondback moth is one of the most adaptive combatants. But its resistance to pesticides has also made it an important research species.

 
  The diamondback moth is both a bothersome pest and a useful research subject.   David McClenaghan/CSIRO  (CC BY 3.0)

The diamondback moth is both a bothersome pest and a useful research subject. David McClenaghan/CSIRO (CC BY 3.0)

 

The crops we grow are not just valuable to us, but also to the numerous pest species that feed on them. It's an ongoing arms race, with pests evolving resistance to overused pesticides, and farmers using new products and strategies to stay one step ahead.

One species of particular note, not only for the damage it causes but also its ability to adapt to pesticides, is the diamondback moth (Plutella xylostella). It feeds on brassica crops, a group of plants that supports a multi-billion dollar agricultural industry, and includes mustards, canola, and cruciferous vegetables like cabbage, broccoli and kale. Due to its remarkable reproductive qualities, the moth has already caused US$4-5bn of crop damage worldwide. Its development takes as little as 14 days, and adults can lay hundreds of eggs. It is also a talented migrator, quickly colonising areas as soon as weather conditions are favourable.

This combination of short generation time, high reproductive ability and ease of distribution are all hallmarks of both a potent pest and a skilled adapter to new threats. This makes the lives of farmers difficult, as the diamondback moth is often the first species to evolve resistance to new pesticides.

  A cluster of rod-shaped  Bacillus thuringiensis  bacteria under the microscope, removed from their usual habitat in the soil.   Dr. Sahay/Wikimedia Commons  (CC BY-SA 3.0)

A cluster of rod-shaped Bacillus thuringiensis bacteria under the microscope, removed from their usual habitat in the soil. Dr. Sahay/Wikimedia Commons (CC BY-SA 3.0)

One type of pesticide used to control the diamondback moth, and a range of other pests, is derived from a bacterium called Bacillus thuringiensis, or Bt. There are over 800 separate toxins associated with Bt, most of them classed as delta-endotoxins or Cry toxins. These toxins get their name from the large crystalline structures they form in the bacterial cell.

When consumed by an insect, Bt toxins are activated by the insect’s gut environment, binding to proteins on the outside of cells and breaking them open. This damage to the gut typically kills the insect by septicaemia (blood poisoning).

Bt toxins were first used in spray-on pesticides in the 1920s, and have been used in commercially grown transgenic crops since 1996. One type of Bt toxin, Cry1Ac, is particularly potent against moth and butterfly larvae, and by 2012 was expressed in 40% of genetically modified crops worldwide — although only the spray form is currently used on brassica crops.

"Transgenic plants expressing multiple Bt toxins, particularly in cotton, have had positive impacts on crop yields," said Simon Baxter, a molecular biologist at the University of Adelaide, "and organic farmers are able to use Bt sprays, as these products are biological.”

Dr Baxter is especially interested in how Bt toxins kill their target species, and whether they can be better applied in agriculture.

  The larval form of the diamondback moth.   David Cappaert, Michigan State University/Bugwood.org  (CC BY-NC 3.0)

The larval form of the diamondback moth. David Cappaert, Michigan State University/Bugwood.org (CC BY-NC 3.0)

Because the wild Bt bacterium lives on the surface of leaves, its toxins are most effective at killing insects with chewing, rather than sucking, mouthparts. This makes the diamondback moth an ideal target pest, and spray forms of Bt-based pesticides are widely used as control measures for the species.

"Bt spray products are often more expensive than chemical insecticides," said Dr Baxter. "However, they have many benefits, including high specificity to targeted pests and low environmental impact."

Bt toxins only affect a very narrow range of species targets, allowing farmers to target pest species without affecting humans or causing collateral damage to beneficial insects like honeybees and other pollinators.

Specificity does, however, have its downsides. Beyond the need to employ several types of toxin when multiple orders of insects are present, Bt specificity makes the evolution of resistance more likely. This is because, as with other toxins that target a single receptor, the insect population only needs to evolve a single mutation in this protein to disrupt the toxic process. This has already been observed in some diamondback moth populations: new mutations in a single cell membrane-bound protein, ABCC2, disrupted the interaction between the toxin and the protein, making them resistant to Bt toxins.

  Diamondback moth larvae, seen here on a cabbage, have caused billions of dollars of crop damage.   Whitney Cranshaw, Colorado State University/Bugwood.org  (CC BY 3.0) 

Diamondback moth larvae, seen here on a cabbage, have caused billions of dollars of crop damage. Whitney Cranshaw, Colorado State University/Bugwood.org (CC BY 3.0) 

Thankfully, farmers can take steps to decrease the chance of resistance evolving in an insect population. It is possible to use several types of Bt toxin on the same crop, and provided that these toxins target different proteins within the insect, resistance can only evolve if mutations arise in all these genes at the same time — an unlikely prospect.

Another option is to cycle the toxins used in a crop over time. As a mutation in the targeted functional protein is needed for resistance, this can impede the ability of resistant members of the population to compete with non-resistant members for other food sources when the toxin isn't around.

All in all, Bt toxins are both a powerful and precise tool for dealing with pests in agriculture. They are easily applied for use against a wide array of common pests and deal very little in the way of collateral damage. Harmless to mammals and quickly decomposing, Bt toxins are worthwhile candidates for a sustainable solution to pests with little risk of interference with beneficial species.

Even so, the arms race is still to be won. Populations of Bt-resistant diamondback moths, first observed in the 1980s, are now serving as research subjects to further our knowledge of Bt toxins. Dr Baxter hopes that his research will yield ever more efficient forms of pest control.

"There is little understanding of how a toxin interacts with its receptor," he said. "A molecular and cellular understanding of this interaction will hopefully lead to improved and more efficient Bt toxin varieties.”