POPULATION BIOLOGY AND PARASITISM OF HESSIAN FLY (MAYETIOLA-DESTRUCTOR) (DIPTERA, CECIDOMYIIDAE) ON BROMUS-WILLDENOWII IN NEW-ZEALAND

Populations of the Hessian fly (Mayetiola destructor (Say)) were studied during 1987-89 in prairie grass (Bromus willdenowii (Kunth)) fields near Hamilton in the North Island of New Zealand, Populations had three generations per year and overwintered as pupae from June to September. Pupae were present all year and egg-laying commenced in early spring (September). Peak egg, larval, and pupal densities were 302-1236, 1164-3197, and 437-992/m2, respectively. At three sites in 1987/88 and two sites in 1988/89, the number of prairie grass tillers killed were 88, 78, and 86%, and 23 and 34%, respectively. An average (+/- SE) of 802 +/- 56 centigrade degree-days (DD) accumulated between generations. Levels of pupal parasitism were between 26 and 42% and 3 and 34% in 1988 and 1989, respectively. Platygaster hiemalis (Forbes) (Scelionoidea: Platygasteridae) was the most abundant parasitoid (92% of parasitised pupae) and, on average, 3.2 P. hiemalis adults emerged per parasitised pupa. This research has demonstrated that prairie grass is an important host of Hessian fly and has indicated that control techniques need to be developed if the persistence of prairie grass as a forage crop is to be improved.