Biases in estimation of insect herbivory from herbarium specimens

Information regarding plant damage by insects in the past is essential to explore impacts of climate change on herbivory. We asked whether insect herbivory measured from herbarium specimens reflects the levels of herbivory occurring in nature at the time of herbarium sampling. We compared herbivory measurements between herbarium specimens collected by botany students and ecological samples collected simultaneously by the authors by a method that minimized unconscious biases, and asked herbarium curators to select one of two plant specimens, which differed in leaf damage, for their collections. Both collectors and curators generally preferred specimens with lesser leaf damage, but the strength of this preference varied among persons. In addition, the differences in measured leaf damage between ecological samples and herbarium specimens varied among plant species and increased with the increase in field herbivory. Consequently, leaf damage in herbarium specimens did not correlate with the actual level of herbivory. We conclude that studies of herbarium specimens produce biased information on past levels of herbivory, because leaf damage measured from herbarium specimens not only underestimates field herbivory, but it is not proportional to the level of damage occurring in nature due to multiple factors that cannot be controlled in data analysis.

area is unacceptable 1 .
Inform the students about the number of species of leaf-bearing woody plants that can be found in the study area, and remind them that dwarf shrubs (bilberry, blueberry etc.) are also classified as woody plants. Also indicate the most common species that do not belong to the local flora and therefore should not be collected.
Students should be ordered to work individually; that is, each student should collect plants on their own (not as a group). A desirable number of specimens is 12-15 of each of 7-8 woody plant species (totalling approximately 100 specimens). Thus, if the group includes 15 students, each student should be asked to collect 7 plant specimens; if the group includes 20 students, each student should be asked to collect 5 plant specimens, and so on.
Upon completion of sampling, the teacher will go over the collected samples to check for (1) the presence of introduced species; if any specimen of a non-native plant is found, it is trashed, and the student is ordered to collect a specimen of another (native) species that is not already present in their collection; (2) the size of the collected specimens; if any specimen is too small relative to the size of herbarium sheet (A3), the student is ordered to collect a larger specimen of the same species.
The teacher should reject only non-native species; there are no other grounds for rejection of specimens collected by students. The specimens which are "not typical", dirty, damaged, without flowers and fruits, etc. will remain in the herbarium on the same rights as "ideal" specimens. The teacher may explain to a student the shortcomings of the collected material but should not require re-sampling.
The collected specimens should be press-dried. Mounting of samples on herbarium sheets is not required (but can be done). The label of each specimen should include the name (or other identifier) of the collector. When the sampling is completed, the teacher is expected to immediately supply M. Kozlov with (1) the list of all species of woody plants collected by students; and (2) a map showing the exact boundaries of the site from which the plant specimens were collected. The samples should be transferred to M. Kozlov after the drying process is complete.

Supplementary Methods S2
Protocol used to collect samples for measurements of leaf losses to defoliating insects in ecological research The choice of plant individuals and branches should be random. Therefore, choose an individual of the target plant species which is growing at least 5 m apart from you, and choose a branch that you will sample. It is critically important that you point at the branch from at least 5 m distance, and that you do not change your choice later on -even if you found this branch dirty or damaged when you approach the plant.
Cut the selected branch in such a way, that it bears about 50 leaves. If the selected branch has less than 50 leaves, then cut the additional branch which is next to the branch you have selected. Repeat these procedures until you collect samples from five individuals of each target species.
Collect all leaves from each branch into individual plastic bag. Do not trash any leaf, even if it is heavily injured or dirty. If some pest had completely eaten a leaf -collect leaf remnants (for example, a petiole).

Introduction
• This presentation contains 21 pairs of images of plant specimens. The specimens within each pair belong to the same species and were collected in the same locality on the same date. • Please assume that you are selecting plant specimens from extensive field samples for your herbarium. You are limited by both storage space and workforce; this means that you can add only one specimen from each pair to your collection. • Please compare the images within each pair (please use a large, at least 11 inch, monitor and magnify the image to see the details if necessary) and delete the image of the specimen that you would discard. (To delete the image, return to Normal view, click on the image with the left mouse button and press "Delete" on the keyboard.) After you do that, each slide will contain only one image, showing the plant specimen that you have selected for your herbarium. • We would find it most helpful if, on the final slide, you could list the criteria you used to select between the two specimens within each pair, and if you could answer a couple of questions about your experience in plant sampling and in curatorial work. When processing and storing the data, we will not link your responses to your name. • Please save the edited version of the presentation and return it to Mikhail Kozlov (mikoz@utu.fi). If you do that, we will assume that you will allow us to include your name and affiliation in the list of experts who contributed to our project (to be published as electronic supplementary material to a forthcoming paper).
Please list the criteria you used to select between the two specimens within each pair : Please provide some information about you (by underlining or boldfacing the correct answers): • Your experience with plant sampling for herbarium collections: less than 10 years, 10-20 years, more than 20 years • Your experience with curation of herbarium collections: less than 10 years, 10-20 years, more than 20 years • The number of scientific papers authored/co-authored by you: less than 10; 10-50; more than 50. • Your age: less than 45 years; more than 45 years.
Please save this file and return it to Mikhail Kozlov (mikoz@utu.fi).

Supplementary Data S1
Losses of leaf area to insects in field collected samples (columns are space-separated).