A genetically modified version of the diamondback moth, which can devastate cruciferous vegetables, significantly reduced pest populations and decreased spread of resistance to the Bt (Bacillus thuringiensis) bacterial proteins engineered into transgenic broccoli to control pests. The transgenic males produce female offspring that do not survive to adulthood, resulting in a population decline. The technology used to create these moths offer a promising sustainable pest management tool, suggest the authors of the study published in BMC Biology. The study was conducted by the British firm Oxitec, which has also developed a GM mosquito designed to control the spread of malaria.
Dr. Max J. Scott, Professor of Entomology, North Carolina State University (webpage):
Expertise: genetic pest management, developing transgenic “male-only” strains of insect pests for genetic control programs, regulation of gene expression in the model insect Drosophila melanogaster
“On the whole this is a robust study. In some of the experimental conditions they only had two replicates. While three would have been better, the results are still fairly clear to me. The authors show that genetic control can be used in combination with or enhance other methods of suppression of pest population. It is appealing because compared to a broad spectrum insecticide genetic control is species specific.
“The use of these male-only transgenic moths has two effects. The first is that it reduces the population expansion overall in the broccoli experiments when in large excess (40X). However, when combined with Bt-broccoli only a modest excess of transgenic males (5x) was needed to reduce the population compared to Bt-crop alone. Secondly it also results in the spread of susceptibility to the Bt toxin produced by transgenic plants.
“Studies combining genetic control of the pest insect (sterile insects) with Bt crops to enhance control have been published before. But the particularly novel aspect of this paper is that they have used fertile males so when females mate with the transgenic male not only do the females die but the susceptibility to Bt is also passed on to the males. This reduces the spread of resistance to Bt in the population. One thing to note is that the susceptibility to Bt was monitored by exposing the caterpillar to the toxin. If they knew at the genetic level what causes Bt resistance it could have been monitored with sequencing.
“Numerous studies have shown that introducing an excess of the male-only strain into a cage with a stable population of insects would result in a decline of the population over a few generations. What is much more challenging to do is to try and suppress a population that is growing. This is what happens in more temperate climates during the growing season. In this study broccoli plants gave the moths the opportunity to expand, which is a tougher test. Despite that they did succeed to keep the population in check even though they were not able to completely suppress the population.
“This is a low risk method on the whole. It is a self-limiting technology as females that inherit the transgene will die. There are some risks discussed in the literature. One is the potential for the gene to move horizontally to a different species. The transgenes are made with so-called ‘jumping genes’ or transposons. In the male-only strains the transgenes can only move if it somehow comes into contact with the specific enzyme used to create the moth. Further to be passed on it will have to transpose to the egg or sperm cells. That is quite a barrier and possibility of this happening is quite low. Beyond that there are also ways to modify these genes so that they don’t move again.
“The other risk that people worry about is that what happens to something that preys on the insects like a bird for example. However the transgenic moths don’t produce a toxin, rather the mechanism of lethality is due to overexpression of a protein that binds to DNA in the nucleus, which is thought to somehow generally interfere with gene expression. There have been feeding studies with predatory mosquito larvae and transgenic mosquitoe larvae that overexpress this protein and no ill effects were seen. Lastly one thing to consider is the female offspring in this case survive to grow as caterpillars and die in later life. the male caterpillars will also eat the crop but develop to adults. The presence of transgenic caterpillars is something the growers might not accept.”
‘Pest control and resistance management through release of insects carrying a male-selecting transgene’ by Tim Harvey-Samuel et al published in BMC Biology on Thursday 16 July 2015
Declared interests (see GENeS register of interests policy):
Dr. Max J. Scott works in the field of transgenic insects. He does not hold or have pending patents but hopes to file a patent later this year.