Mustard aphids, Lipaphis erysimi (Homoptera: Aphididae) is a widely distributed serious pest of oil seed crops and vegetables in India, which is parasitized by Diaeretiella rapae in good number at Meerut. Aphids are the tiny global insects, yet these are capable of destroy the crop due to their fast multiplication rate, virus transmission capability and draining the plant sap rapidly. These easily migrate form one crop to another and established colony soon. Its effect is more serious in Northwestern U.P. Among its natural enemies, Diaeretiella rapae, checks its fast multiplication up to some extent, though, it appears slightly late.Effect of parasitism on host aphid has been observed in terms of morphology, flight ability, feeding, locomotion, moulting behaviour and reproductive capability. Drastic changes occur in the host after infestation by parasitoid and finally host is converted into mummy and dies, causing cent percent mortality. On this potential biocontrol agent has been made by many, late. which are being presented in this book. This book is an essential resource for students of entomology and parasitology, academic researchers and advisory service personnel.