Herbarium specimens faithfully capture changing plant-insect interactions over years

Biological collections are increasingly mined for historical data on ecological and organismal changes over time. Pressed plant (herbarium) specimens have been harnessed in recent years to investigate how plant-insect interactions have responded to anthropogenic change. In particular, a growing body of research demonstrates that herbaria are promising repositories for long-term data on insect herbivore damage to plants. To determine whether herbarium specimens accurately represent insect herbivory in the field, we compared herbivory estimates derived from whole plants and herbarium specimens. Our study focused on oak trees in the UC Davis Arboretum, where both whole-plant herbivory estimates and herbarium specimens were collected in 2009. In 2021 and 2023, we replicated methods for assessing herbivory on trees from the 2009 study and trained volunteers to collect herbarium specimens from these same trees. We then compared whole-plant and herbarium-based estimates of insect herbivory and how they changed over years. Herbarium specimens faithfully captured herbivory, though, as expected, herbivory was less prevalent on herbarium specimens than on randomly selected branches. Herbivory across all sampling types decreased over years. Though we interpret these results with caution because our data includes just three time points, they are consistent with insect declines documented in recent years. Overall our study indicates that herbarium specimens can be used to estimate changes in herbivory over time, and that it might additionally be possible to apply a simple conversion factor to estimate average levels of herbivory on whole plants from historical specimens.