Predation and pigments: examining alydid seed predation on Lupinus perennis

Background/Question/Methods

Sundial lupine, Lupinus perennis (L.), is an oak savannah, perennial flowering legume that is integral to maintaining reproductive populations of the federally endangered Karner Blue butterfly, Lycaeides melissa samuelis . The seeds of L. perennis can vary from light to dark with intermediate polymorphisms that are speckled to varying degrees. Physical and chemical properties vary by seed coat color in other legume species. In these legume species, darker seeds contain secondary metabolites like tannins and anthocyanins, which are commonly associated with deterring seed predation. These secondary metabolites have also been found to increase seed coat strength. Furthermore, lighter seeds have been found to have weaker seed coats, be more digestible, and have higher nutritional value than darker seeds. The chemical and physical traits of lighter seed coats could make this polymorphism more desirable for seed predators as well as agriculturalists. The seeds of L. perennis are a known food source of ant-mimicking alydid insects, Megalotomus quinquespinosus and Alydus spp. This study determined whether alydid seed predators have a preference for seed coat coloration in L. perennis and simulataneously determined the relative abundance of this seed predator and its impact on lupine populations. Ten populations of L. perennis were surveyed from June 4-June 20, 2012 for seed color frequencies, alydid frequencies, and environmental variables. Within each population, thirty, 4 meter diameter circles were sampled every five meters along transects. Seed color, pod number, alydid sightings, soil color, litter depth, canopy cover, and population area were recorded for each sampling location.

Results/Conclusions

Sites were found to differ significantly in litter depth, canopy cover, total alydids, total pods, and percent dark speckling (p<0.0001). Mary’s Savanna and Central had the darkest seeds while Julia’s Savanna had the lightest seeds and most alydids. These three sites were similar in environmental conditions such as canopy cover, litter depth, and pod production suggesting seed color may not be environmentally controlled. The area of dark speckling on seeds increased with pod abundance (ρ = 0.2841, r = 0.163, p< 0.0001). Alydid presence was more likely with higher pod density (r² = 0.031 p = 0.0028). However, no significant relationship between seed color and alydid abundance was detected.  Though no direct relationship between alydid abundance and seed color was observed, sites with higher alydid abundance tended to have lighter seeds when compared to sites with similar environmental conditions.  Further research will explore the relationship between seed color and insect predation.