Many plants are eaten by a wide variety of insects, and herbivorous, or plant-eating, insects sometimes consume almost half of the leaves that a plant grows each year. Plants in tropical forests tend to be better defended against being eaten by insects than plants in temperate forests, and the tropical plants are also more diverse. One explanation for both higher diversity and better defense may be that insects eat more leaves in tropical forests than in temperate forests, selecting for plants with more defense and preventing dominance by just a few plant species. However, it is not really known whether herbivory is higher in tropical forests, because no previous studies have measured herbivory in a consistent way across a wide range of latitudes. The goal of this study is to compare the consumption of leaves of plants in the cucumber family by insects in three locations, one tropical (Panama), one subtropical (Florida), and one temperate (Wisconsin). To keep factors such as nutrient content of leaves and local environmental conditions constant, the investigators will grow plants in greenhouses and then transplant them into natural environments at each of the sites. They will then record the number and type of insects visiting the plants, and how much those insects consume of each plant. This will occur over the main growing season of the plants at each site. The study will end when fruits and seeds have been produced, which will reveal how herbivore damage to leaves can influence plant fertility.
Understanding how herbivory changes from tropical to temperate climates will help predict how change in climate might change amounts of herbivory in the future. For example, rise in temperature might cause tropical insects to expand into higher latitudes and increase the amount of plants that are eaten in the temperate zone. In the case of insects that eat crops, this could impact agriculture. Knowing more about current patterns of herbivory may help people adapt to future change through measures such as changes in pest control management. This project will also provide research training for undergraduates and conduct outreach to public schools.
As observed with many other biotic interactions, herbivory is thought to increase at lower latitude (Coley and Aide 1989, Coley and Barone 1996, Hillebrand 2004, Novotny et al. 2006, Dyer et al. 2007, Schemske et al. 2009). Supporting evidence, however, is surprisingly sparse (Moles et al. 2011). We planted varieties of cucumber (C. sativus) and pumpkin (C. pepo) across a latitudinal gradient and monitored the abundance and type of insect herbivores that visited the plants as well as how much leaf area was consumed. We found that herbivory rates were much higher at higher latitueds. Therefore, the common assertion that herbivory rates are higher at lower latitudes needs to be reevaluated, since we found contradictory evidence in two species. Additionally, while it is thought that herbivory rates and diversity may correlate with increased plant defense at lower latitudes, we found that these patterns were still present when controlling for chemical defense traits (i.e. using genetically similar plants, from the same seed stock) and controlling for structural defenses (providing all plants with the same concentration of silica fertilizer). We suggest that the pattern of higher herbivory rates at higher latitudes observed for one of our study species (C. sativus) is due to an increase in generalist herbivore diversity and abundance and increased day length allowing a longer feeding period. During the course of this study, we have trained five undergraduate students in both field and laboratory practices, several of whom are going on to pursue careers in science and attend graduate studies in the sciences. Additionally, we made presentations to middle and high school classes each year about the study in order to expose these students to "real science." These results have been presented at a major scientific conference and are currently being submitted as part of Kristine Callis' dissertation.