How aspen tree height influences aspen leaf miner (Phyllocnistis populiella) oviposition and performance

Abstract

Under the optimal oviposition theory, insects are expected to lay eggs on hosts that maximize the success of their offspring. Tree height is known to be an important factor influencing the distribution of phytophagous insects because some species perform better at a distinct range of heights. This difference in performance could lead to incorrect estimates of population parameters if surveys are only conducted on one host plant height. Aspen leaf miners (Phyllocnistis populiella) have undergone a major outbreak in interior Alaska over the last two decades. We quantified patterns of aspen leaf miner oviposition and juvenile survival over 2 years and found that aspen leaf miners were approximately 1.5 times more likely to survive on tall trees than short trees. Parasitism and both egg and larval predation were lower on tall trees. Aspen leaf miners on tall trees also had larger pupal masses than those on short trees. Although aspen leaf miners performed better on tall trees, the number of eggs laid per leaf did not significantly differ by tree height. There were no significant differences in leaf foliar nitrogen between tall and short trees. We also found little differences in wind speed between tall and short trees that could explain ovipositional patterns. Ovipositional patterns may partially reflect the difference in phenology between tall and short aspen trees. Aspen leaf miners only lay eggs on new leaves. Tall aspen trees leafed out 7 days earlier on average than short aspen trees, and tall trees, unlike short trees, ceased to produce new leaves after budburst. Consequently, there was little overlap in the availability of tall and short aspen trees for oviposition, so even if aspen leaf miners have a preference for laying more eggs on tall than short trees, they can only act on it during the short time period when tall trees are available for oviposition. The results suggest that population projections based on data collected from only short trees may underestimate future aspen leaf miner population growth due to lower juvenile survival rates and pupal masses on short trees. More broadly, the results highlight the importance of examining multiple tree heights when studying the performance and population dynamics of phytophagous insects. They also suggest that phenological differences between plants may constrain insects from using higher quality hosts.