A solitary male desert locust (left) facing a gregarious male (right) of the same species. Image: Tom Fayle, University of Cambridge

A team of scientists has shown how the environment shapes learning and memory by training locusts to associate different smells with reward or punishment. 

The new research from the universities of Sussex,  Leicester and Cambridge, published today (21 November) in the journal Current Biology,  examines how locusts associate odours with nutritious or toxic food.  

Desert locusts are notorious for their devastating swarms. However, they do not always live in swarms — they switch between a lone-living ‘solitary phase' and a swarming ‘gregarious’ phase. 

Solitary locusts rely on camouflage to evade predators and they avoid eating toxic plants; but gregarious locusts eat these plants to ‘impregnate’ themselves with toxins to deter predators. The transformation to gregarious behaviour, which happens when solitary locusts are forced together into a crowd, is complete within a few hours. 

Locusts should consider toxic food ‘bad’ while they live alone but ‘good’ when they are in a swarm, which made the researchers ask how swarming locusts learn that ‘bad is the new good’. 

Dr Jeremy Niven, who heads the Laboratory of Evolutionary Computational Neuroscience in the School of Life Sciences at Sussex, says: “Our research shows how animals that undergo a profound transformation in their lifestyle also adapt their learning and memory capabilities to cope with the new environment in which they find themselves.” 

For the study, when solitary locusts were presented with an unfamiliar odour together with toxic food, they assigned it an aversive (‘bad’) value. But if the locust is in a crowd and starting to change to the gregarious state, it assigns an appetitive (‘good’) value to the same odour.  

But if a solitary locust has already learned about an odour and then it finds itself in a crowd, what would happen to its memories? Can it switch the value that it has assigned to the odour, or more precisely, does crowding change the value of a previous memory from aversive to appetitive? Solitary locusts, the researchers found, cannot do this: they are stuck with the value of their already acquired memories. Also, locusts in the transitional period cannot form any new aversive memories, but can still form new appetitive memories.  

The researchers then simulated the context in which the switch to swarming behaviour takes place in the field and found that the gregarious locusts blocked aversion learning,  enabling  those locusts to effectively re-train themselves to learn that the same odour that indicated ‘bad’ now indicates ‘good’. 

Dr Niven adds: “Because newly crowded locusts don’t form memories about toxins they ingest, all they remember is the pleasant side of what they ate, and they ignore the toxin. In this way, a smell previously associated with a toxin can become associated with a pleasant experience. 

“The changes in learning and memory we're proposing don't require the locusts to understand what's happening to them — they just have to feed and form associations.”