Effects of Host Plant Density on Herbivores and Their Parasitoids: A Field Experiment with a Native Perennial Legume.

Senna mexicana chapmanii (Fabaceae: Caesalpinoideae), an attractive and threatened species native to pine rocklands of southern Florida, is consumed by folivorous caterpillars of Sulphur butterflies (Lepidoptera: Pieridae). Caterpillars may be deterred or eaten by predators (such as ants, vespid wasps, spiders, and cocinellid beetles), but also very important are parasitoids, both flies (Diptera) and wasps (Hymenoptera). An experimental study is currently underway, investigating the effects of plant density on sulphur caterpillar numbers and rates of parasitization.

Senna mexicana chapmanii plantations have been established at two sites, one in an agricultural region, the other in an urban area; both sites are adjacent to protected pine rockland areas. In three areas at each site, and three replicate plots per area, seedlings were planted in a semicircular array with 16 plants each at distances of 1 m, 5 m, and 10 m from other individuals, representing high, medium, and low hostplant densities, respectively. Starting in June 2015, monitoring of the number of sulphur butterfly eggs, larvae, and pupae, as well as parasitism (percentage of larvae attacked by parasitoids) were recorded for each plant. Caterpillars are collected and reared to determine if they are parasitized, if it is not obvious at the time of observation.  Preliminary data reveals greater caterpillar numbers on low density plants; some parasitoids have also been discovered from caterpillars on those plants.  Continuing this weekly monitoring protocol over the course of a year will provide data to determine if higher plant density leads to greater herbivore numbers and greater levels of parisitization (the expectation), or if hostplant density has a different, or no effect on pierid herbivores and their biocontrol agents.  Elucidating the patterns will be important in understanding the factors that regulate the numbers of folivores and their parasitoids.