A tree in the eyes of a moth - temporal variation in oak leaf quality and leaf-miner performance

When examined at any moment in time, different parts of an individual oak tree exhibit almost as large differences in quality as different trees. But how consistent are such patterns in time? In this paper, we use intraclass correlations to assess the temporal consistency of host plant quality at several spatial scales. As measures of quality, we use both individual chemical attributes (phenolic contents) and the overall performance (larval survival) of the host-specific leaf-miner Tischeria ekebladella. Concentrations of 24 phenolic compounds were monitored on seven trees throughout a season. Variation in mine initiation and larval survival rates was assessed for individuals transplanted to another set of trees early versus later in the season, while year-to-year variation in larval survival was studied through stratified surveys of wild individuals during three years. At all time scales considered, measures of host quality were moderately consistent: a tree favourable in quality at one point in time often remained so, but there was abundant variation around this relationship (rho=0.4-0.6). One hierarchical level deviated from this general pattern: on individual branches, larval survival rates varied randomly among years (rho=0). Our study suggests that the quality of trees, and in particular of smaller units within trees, may be difficult to predict both in space and in time. To account for this, insects might benefit from adopting a bet-hedging strategy when selecting resources.