Preliminary observations on the control of aquatic insects in nursery ponds

Abstract

THE majority of the aquatic insects are known to prey upon and destroy large numbers of young carp fry in nursery ponds, z, 3 The Notonectids, ' Anisops spp. (A. barbata Brooks, A. Kempi Brooks) generally occur in thousands in carp nurseries in Orissa and Bengal during the monsoon season and do immense harm to the young carp fry which are then transplanted in these ponds for culture. These insects are almost of the same size as the weekold carp fry and are, therefore, difficult to segregate from the latter by sieving or other similar methods. When carp fry are netted from nursery ponds these insects are also caught in appreciable numbers and when kept together in the limited space of the hapa or hundi, they become extremely destructive to the fry. Though aquatic in habitat these Notonectids can fly out of water and go to neighbouring ponds as well. They very often come up to the surface of water for taking air for breathing. Spraying oil on water surface to control insects which come up to take atmospheric air, is a wellknown principle, and a routine practice in malaria control. It is also reported that some fish farmers in Bengal use mustard oil to segregate Notonectids and carp fry when they are kept in hapas. As vegetable oils are reported to be safer than mineral oils, 2 a series of experiments was carried out with a few selected common vegetable oils to ascertain whether these Notonectids could be effectively controlled in ponds without adversely affecting the other living animals including fish. The results obtained are briefly detailed in this note.