Sauterelle vs. locuste
While both the French words locuste and sauterelle can be associated with the English word locust or grasshopper, they refer to different types of insects and contexts. The main difference lies in whether you are talking about a harmless everyday insect or a destructive, swarming agricultural pest.
Sauterelle
B1The word sauterelle is the standard, everyday French term for a grasshopper. Although colloquially or in older texts it is sometimes used to mean locust, it generally refers to the solitary, harmless jumping insect with long antennae that people safely observe in nature.
Les enfants courent dans l'herbe haute pour attraper une sauterelle.
(The children run in the tall grass to catch a grasshopper.)
Il a trouvé une petite sauterelle verte posée sur la fenêtre de sa chambre.
(He found a small green grasshopper resting on his bedroom window.)
Le chant de la sauterelle est très caractéristique des soirées chaudes en été.
(The song of the grasshopper is very characteristic of warm summer evenings.)
Le chat a passé toute la matinée à chasser une sauterelle dans le jardin.
(The cat spent the whole morning chasing a grasshopper in the garden.)
Une sauterelle possède de longues pattes arrière qui lui permettent de faire des bonds immenses.
(A grasshopper has long back legs that allow it to make huge leaps.)
Locuste
C1The word locuste specifically translates to locust in the true sense of a swarming, gregarious insect that destroys vegetation. It is a more formal, scientific, or historical term used exclusively when discussing plagues, massive agricultural disasters, or entomology.
Une énorme nuée de locustes a complètement caché le soleil pendant l'après-midi.
(A huge swarm of locusts completely hid the sun during the afternoon.)
Les scientifiques tentent de comprendre ce qui pousse cette espèce pacifique à se transformer en locuste.
(Scientists are trying to understand what triggers this peaceful species to transform into a locust.)
Les textes anciens décrivent une invasion de locustes comme une terrible punition divine.
(Ancient texts describe an invasion of locusts as a terrible divine punishment.)
Les agriculteurs de cette région agricole redoutent plus que tout l'arrivée de la locuste.
(The farmers in this agricultural region fear the arrival of the locust more than anything.)
Le vent du désert a poussé les millions de locustes directement vers les récoltes de maïs.
(The desert wind pushed the millions of locusts directly toward the corn crops.)
Summary
In short, use sauterelle when talking about a regular grasshopper hopping around the yard, and reserve locuste for the swarming, crop-destroying locusts often mentioned in biological studies, agricultural crises, or historical plagues.