WK Kellogg reported a slump in sales in its first quarter this year because of the “weaker than expected consumption trends.”

Ferrero International SA agreed to acquire WK Kellogg Co. for an enterprise value of $3.1 billion, pushing the Italian family-owned candy business further into the lucrative US market.

U.S. sales of the colorfully packaged morning staple have been in a decades-long decline, a trend back in the spotlight with news that Italian confectioner Ferrero Group plans to purchase the American...