locust (n.1) Look up locust at Dictionary.com
"grasshopper," early 14c., borrowed earlier in Old French form languste (c.1200), from Latin locusta "locust, lobster" (see lobster).
In the Hebrew Bible there are nine different names for the insect or for particular species or varieties; in the English Bible they are rendered sometimes 'locust,' sometimes 'beetle,' 'grasshopper,' 'caterpillar,' 'palmerworm,' etc. The precise application of several names is unknown. [OED]
locust (n.2) Look up locust at Dictionary.com
North American tree, 1630s, originally "carob tree" (1610s), whose fruit supposedly resembled the insect (see locust (n.1)). Greek akris "locust" often was applied in the Levant to carob pods. Soon applied in English to other trees as well.