THE BIOLOGY OF THYSANOPTERA WITH REFERENCE TO THE COTTON PLANT: 1. THE RELATION BETWEEN DEGREE OF INFESTATION AND WATER SUPPLIED

Abstract

Summary. Experiments concerning the degree of infestation of Thrips tabaci upon cotton plants under glasshouse conditions suggest that the infestation factor, or number of thrip stages per 100 sq. cm. of foliage surface, varies inversely with the weight of water supplied. Plants receiving excessive water supply have a lower infestation factor and a lower number of thrip stages per leaf as compared with plants receiving sufficiency of water supply; plants receiving insufficient water supply have a greater infestation factor than plants receiving sufficiency of water supply but have not necessarily a greater number of thrip stages per leaf; owing to their smaller foliage area however they suffer more from thrip attack. Irrigation is more effective in influencing the degree of thrip infestation than is rainfall. Irrigation on a flat soil surface is more effective than irrigation on a furrowed soil surface. The effect of excessive water supply does not appear to be due to mechanical removal of thrips stages from the foliage, nor an injurious effect of soil moisture or atmospheric moisture upon thrips stages, nor alteration of the osmotic concentration of cell sap, although all three factors may operate slightly. It is suggested that one factor concerned is the influence of heavy water supply upon the texture of certain soils, in promoting surface caking which will act inimically to soil pupating species of Thysanoptera.