Host age choice for oviposition in Pauesia juniperorum (Hymenoptera : Braconidae : Aphidiinae) and its effect on the parasitoid's biology and host population growth

Pauesia juniperorum has been selected as a potential agent for the biologic al control of Cinara cupressivora, an important introduced pest of conifers in Africa. As part of the pre-introductory assessment studies, selection o f different host age categories for oviposition was studied in choice and n o choice experiments. The duration of development and adult size of parasit oid progeny, developing in different host age catagories were compared. The effects of parasitism on survival and reproduction of five categories of t he apterous morph of the host ranging in age from 3-15 days was also studie d The age of hosts had a significant influence on the degree with which dif ferent categories were parasitized Host defensive behaviour; which ina ease d with age, influenced the outcome of attempts by the parasitoid to oviposi t. The effect was greater in older hosts, but in young hosts their small si ze was more important in enabling aphids to escape parasitism. The duration of development decreased while adult size increased with age of hosts. Thi s demonstrated that fitness attributes of P. juniperorum were affected by l arval ontogeny with older/larger hosts being more optimal. The ability of t he parasitoid to exploit hosts of varying age was suggestive of a high degr ee of phenotypic plasticity which is potentially of adaptive value. Hosts p arasitized before they were 9-days-old became mummies without reproducing. Those that were 9 days or older achieved part of their reproductive capacit y before dying. Thus, the impact of P. juniperorum is markedly reduced when attacking older hosts.