Monsanto Co.’s $45 billion bid for Swiss rival Syngenta AG highlights the growing global importance of agricultural pesticides despite efforts in the U.S. and elsewhere to reduce the use of the bug- and weed-killing chemicals.

The proposed combination—which Syngenta’s board rejected—would rank as the biggest agribusiness merger ever, according to Dealogic, and make the merged entity the leading global supplier of seeds and crop sprays to farmers from Indiana to India. Syngenta, the world’s largest pesticide seller, said the...