MAHWAH, N.J. -- Inserra Supermarkets, an 18-unit ShopRite operator here, has acquired the five ShopRites owned by Singer Supermarkets, North Bergen, N.J. Terms of the transaction were not disclosed. Inserra operates 13 stores in northern New Jersey and five stores in New York state. Its volume is estimated at $450 million. Singer operates five stores in northern New Jersey. Its annual volume is about $180 million. Both companies are members of Wakefern Food Corp., Elizabeth, N.J., the retailer-owned cooperative for ShopRite supermarkets.