A simulation model of the role of parasitoids in the population development of Sitobion avenae (Hemiptera: Aphididae) on cereals.
A simulation model is described which demonstrates the role of parasitic Hymenoptera in influencing the numbers of the aphid Sitobion avenae on cereals. It is based on dissection of aphids collected in southern UK for immature parasites and the use of physiological time to predict mummification and therefore death. The physiological time unit (quarter-instar period) was also used to calculate simulated aphid population development in the field in the absence of all mortalities, and the difference between the simulated and actual populations was attributed to 'total mortality'. The predicted number of aphids mummifying in each instar-period was then subtracted from 'total mortality', leaving 'residual mortality', the components of which were not identified. On average, simulated populations in the absence of parasitism increased at flowering to a level 7 times higher than in simulations with this mortality included. In most fields, parasitism was initially the main mortality factor, but later its influence was much less relative to residual mortality. The potential of cereal aphid parasites to exhibit an economically useful suppression of their host numbers is considered in the context of the required accuracy of pest forecasting schemes and their consequent value over no control or prophylaxis.