The object of this paper is to discuss the ecology of grass grub, Costelytra zealandica White, and porina, Wiseana cervinata (Walker), New Zealand’s most important pastoral insect pests, and to survey what has been achieved in ecological understanding of these insects. The biology and ecology of either or both insects have previously been reviewed by Dumbleton (1942), Miller (1945), Kelsey (1951a, b) and Hoy (1963). The lack of fundamental biological information on grass grub (and this is equally applicable to porina caterpillar) and the need for longterm team studies were stressed, although little sustained, co-ordinated research has since eventuated. Examination of the literature clearly indicates that the lack of fundamental ecological research on these pasture pests is directly due to the introduction of organochlorine insecticides, particularly DDT, to control both porina and grass grub. These insecticides proved cheap, persistent in soil, relatively safe to handle and easy to apply (Kelsey and Hay., 1950; Kelsey 1951a, b, 1952, 1959). DDT was so effective that it led to complacency, in spite of the warning by Miller (1945) that in his view no single control would suffice. There was failure to anticipate: (1) Development of insect resistance. to insecticides. (2) Possible alteration of tolerance limits for insecticides in foodstuffs, by countries importing New Zealand produce. (3) Failure of DDT to be effective in certain soils. (4) The need for fundamental knowledge, in case the efficacy of DDT and related insecticides diminished, and new methods had to be found.