Evolution of life-history timing in a leafmining moth: Phenotypic selection in patches with manipulated development time

Phyllonorycter mespilella (Hübner) is a leafmining moth with two stages of larval development: the initial sap-feeding (SF) stage followed by the tissue-feeding (TF) stage. Phenotypic selection by parasitoids on the duration of the SF stage (SF duration) was measured in artificial patches of larvae placed in the field during the diapausing generation. Pretreatment of larvae with different photoperiods allowed creation of patches that varied in the time-course of appearance of TF larvae. The shorter the photoperiod pretreatment, the sooner TF larvae tended to appear. Some patches were left exposed and others were caged to exclude parasitoids. Positive directional selection on SF duration was detected in exposed patches, and no selection was detected in caged patches. Directional selection in exposed patches was caused by both parasitoid oviposition and other unidentified sources of mortality. The other sources of mortality may have included host feeding by parasitoids on TF larvae. A larger proportion of parasitoid eggs were oviposited on TF larvae in patches where TF larvae appeared the earliest, but this variation in parasitoid oviposition did not result in significant differences in directional selection intensity among patches with early, intermediate and late appearance of TF larvae. Although the general form of the fitness function was very similar when compared among patch types, no significant directional selection could be detected in patches where TF larvae appeared late, and the causes of directional selection appeared to vary between patches where TF larvae appeared at early and intermediate dates.