STIMULATION OF PLANT-GROWTH AND APHID POPULATIONS BY A FORMULATION INGREDIENT OF CYMBUSH (CYPERMETHRIN)

Similar increases in the height and aerial fresh weight of cowpeas com pared with control plants were obtained when plants were sprayed with either 50 ppm a.i. (active ingredient) of the pyrethroid insecticide ' Cymbush'(R) containing cypermethrin, or, the equivalent dilution of a blank formulation without the a.i. With broad beans, similar growth st imulation was obtained with the blank at the same or half dose, but no t at a quarter dose. Testing the four formulation components separatel y showed that only the non-ionic emulsifier caused stimulation A compa rison of the stimulation of plant growth by the blank and the non-ioni c emulsifier showed that plant height, shoot fresh weight, leaf fresh weight, leaf dry weight and leaf area were each equally increased by b oth treatments. The non-ionic emulsifier thus accounted for the entire stimulation caused by blank solution. Populations of Aphis fabae Scop oli (Homoptera: Aphididae) were twice as high on broad bean plants spr ayed with the non-ionic emulsifier as on the controls. 'Cymbush' (the complete formulation) sprayed at 25 ppm a.i. was shown to increase the intrinsic rate of natural increase of the aphid through a 22% increas e in fecundity; aphid development time was not affected. The results d o, however, suggest that residual effects of cypermethrin may have par tly counteracted the stimulatory effects of the non-ionic emulsifier. Nevertheless, it is possible that outbreaks of aphids and mites follow ing pyrethroid application, and often assumed without evidence to have been due to natural enemy mortality, may at least in part reflect the plant stimulation caused by the non-ionic emulsifier in the formulati on.