Studies on the effect of onion plants on the oviposition behaviour of Psila rosae on caged carrot plants

Abstract

A reduction of 22.5%,31.7%,45.7% and 53.9% in the number of eggs laid on carrot plants by Psila rosae Fab. was obtained when each plant was, respectively, surrounded by either 1, 2, 4 or 8 onion plants in cages. Similarly, the numbers of eggs laid on carrot plants were reduced by 12.3%, 22.2%, 25.6% and 32.1% when the onion plants were substituted with green models, and by 16.0% and 28.7% when the carrot plants were surrounded by 4 and 8 black onion plant models, respectively. Furthermore, reductions of 11.6%, 42.3% and 42.7% in the numbers of P. rosae eggs laid were obtained when a carrot and an onion plant were separated by a distance of either 15.0, 7.5 or 3.75cm, respectively. Onion bulb extract sprayed once ad initium and daily reduced P. rosae oviposition by 55% and 89%, respectively, compared with the unsprayed carrots. These results suggest that disturbance of the oviposition behaviour of P. rosae by onion plants was caused by (i) their diversionary effects, and (ii) their repellent effects, both of which hindered efficient acquisition of an oviposition stimulus by the flies.