Locusts are a major threat to crops throughout the world. Usually they live a fairly solitary existence, but under the correct conditions they can come together in huge swarms of millions of individuals, eating any vegetation in their path.
The process that creates swarms is thought to be based on olfaction; how the insects respond to chemicals and pheromones in the environment. Many aspects of insect behaviour are governed by their sense of smell, including which foot to eat, finding a mate or hunting.
If we could target any of these pathways and stop them from working, they could be used as a form of insect control; making them lose their taste for certain crops or stopping them forming such devastating plague-like swarms in the first place.
A new paper from Yan Li et al  has taken steps towards this goal by targeting a gene in the locust involved the response to odours; the odorant receptor co-receptor (Orco) gene.
Insects have many different forms of odour receptor (OR) genes that respond to different chemicals, causing a neuron to fire and signalling to the brain. The migratory locust, for example, has 142 ORs, 134 of each are expressed in the antennae .
Orco forms a complex with the OR and is vital for proper function. Without Orco, the olfaction system cannot work properly,
To knock Orco gene, the group used the CRISPR/Cas9 system to target two sites within the gene, and then bred the insects several generations to establish and characterise the mutations. They then tested the knock-out locusts on a variety of odourants (including plant food and chemicals released during swarming behaviour), looking to see if exposure caused the nerves in the antennae to fire and indicating that the insects were responding to the chemicals.
The Orco homozygous mutants (ie they had two copies of the broken gene) did not respond to any of the odours except hexanoic acid, and did not aggregate together in response to the swarming odour compared to wild type and heterozygotes animals. They also showed other effects linked to behaviour such as mating an oviposition (egg laying).
If the system can be refined in the future to just affect the swarming behaviour, it could have the advantage of removing the 'pest' element of the locust while keeping its place in the ecology of the environment. It would certainly be preferable to broad-spectrum insecticides.
For this to work, however, genetically modified organisms would have to be released into the wild, which is highly controversial and could have its own unintended consequences.
Getting man-made mutations to proliferate into the gene pool of wild populations uses a mechanism that overrides normal genetic inheritance patterns - a technique called gene drive. This fascinating system will form the basis of a future blog post!
 Li, Yan, Jie Zhang, Dafeng Chen, Pengcheng Yang, Feng Jiang, Xianhui Wang, and Le Kang. 2016. “CRISPR/Cas9 in Locusts: Successful Establishment of an Olfactory Deficiency Line by Targeting the Mutagenesis of an Odorant Receptor Co-Receptor (Orco).” Insect Biochemistry and Molecular Biology 79 (October): 27–35. doi:10.1016/j.ibmb.2016.10.003.
 Wang, Zhifeng, Pengcheng Yang, Dafeng Chen, Feng Jiang, Yan Li, Xianhui Wang, and Le Kang. 2015. “Identification and Functional Analysis of Olfactory Receptor Family Reveal Unusual Characteristics of the Olfactory System in the Migratory Locust.” Cellular and Molecular Life Sciences 72 (22). Springer Basel: 4429–43. doi:10.1007/s00018-015-2009-9.