Butterfly, moth, and sawfly diversity and host-use patterns change based on tree planting diversity: the first eight years of forest growth

Background/Question/Methods

Animals make decisions about the environments that their offspring will encounter. The stakes are particularly high for insect herbivores like caterpillars where parental oviposition choices often determine a caterpillar’s sole plant food source until they pupate and become an adult. Collective host-use decisions create patterns of caterpillar diversity and community structure across landscapes. While the diversity and arrangement of hosts has been demonstrated to influence host use decisions in agricultural systems, this has been more difficult to test and understand within the forested ecosystems common to the Eastern United States. Further, it is unclear whether and how the relationship between tree diversity and host use changes as a forest grows and canopy closure occurs. To answer this question, we tracked the colonization and host use of moth, butterfly and sawfly caterpillars in early and late summer during four of six summers between 2016- 2021. We repeatedly sampled ~540 trees of 15 native tree species growing in plots either surrounded by conspecifics (monoculture) or a mixture of 4 or 12 native species (polyculture). The time period sampled represents years 3 – 8 in the establishment of the BiodiversiTREE tree diversity experiment at the Smithsonian Environmental Research Center (SERC) in Edgewater, MD.

Results/Conclusions

In the period of initial forest establishment (years 3 and 4) we find that host trees surrounded by diverse mixtures of trees support a 34-79% higher abundance and a 44-55% higher richness of caterpillars with strengthening positive diversity effects from year 3 to year 4. In year 6 we continued to find positive effects of tree diversity on caterpillar richness and abundance, but the effect size weakened. We will present hot off the press data from the 2021 field season (year 8) documenting whether this weakening trend continues and how host specialization and beta diversity patterns respond as canopy closure increases in the experiment. By characterizing patterns in caterpillar diversity, degree of host specialization, and abundance within a controlled manipulation of host tree diversity, we provide important basic insights into plant-insect interactions and host-use patterns and applied insights into how forest management and restoration decisions impact the biodiversity of insect herbivores within human-managed landscapes.