PARTIAL APPLICATION OF INSECTICIDE TO BROAD BEAN (VICIA-FABAE) AS A MEANS OF CONTROLLING BEAN APHID (APHIS-FABAE) AND BEAN WEEVIL (SITONA-LINEATUS)

The effects upon the nature of insect pest attack of treating only a c ertain proportion (0%, 25%, 50%, 75% and 100%) of the plants in a fiel d bean (Vicia fabae) stand, with a systemic insecticide (aldicarb as T emik 10G, Rhone Poulenc Drugs Ltd; 10% a.i.) were assessed. Numbers of both the black bean aphid (Aphis fabae) and the bean weevil (Sitona l ineatus) were proportionally reduced on untreated plants in plots with an increasing proportion of insecticide treated plants. A similar res ponse was also observed with bean weevil leaf damage scores. It is pos tulated that the observed effects are due to a reduction in insect num bers per plot brought about by redistribution of insects after initial invasion. Insects initially alighting on untreated plants later migra te to treated plants and either die or leave the plot altogether. This resulted in proportionally fewer insects on the untreated plants than would be expected.