Interactions between phytophagous insects and their host plants can produce physiological, morphological and behavioural changes on the population of the parasites. Such changes due to phenotypic plasticity may be relevant in both the specialisation process and host races or ecotypes formation. In this work the occurrence of phenotypic plasticity (physiological and behavioural) on distinct aphids clones of Brevicoryne brassicae (L.) was investigated utilising two different hosts: cabbage (Brassica oleraceae var. acephala) and broccoli (B. oleraceae var. italica). Nimphs, with approximately the same age, were placed individually on the underneath leaf surface on each tested host. Their developmental time and fecundity were observed daily in order to calculate a fitness index (r m) or performance. Experiments to determine preference (behavioural plasticity) were done inside the laboratory, basing on the choice of eighty aphids (20 per clone) for a particular host. Measurements of variability on plastic responses among clones were done following a quantitative genetic model. Clones demonstrated low physiological plasticity (F=3.412; P=0.067) probably due to the great genetic similarity between the two variety of plants used as hosts. Small variability was verified for this kind of plasticity among the performance of the analysed clones (F=1.281; P=0.283). Nevertheless, the clones presented significant preference for a particular host (chi2=4.66; P<0.05), which demonstrates the conditioning behaviour (individuals choosing the hosts were they fed during their development). Aparently the selection pressure was not hard enough to promote biotype or race formation in B. brassicae adapted to the host varieties tested.
Insecta; phenotypic plasticity; Hopkins principle; conditioning behaviour; quantitative genetics
