Coevolution of Cassia (Leguminosae) and Its Seed Beetle Predators (Bruchidae)

Abstract

Most of the bruchid species feeding on Cassia are in the genus Sennius . The beetles are divided into 2 groups, the specialists that feed on 1–3 species of host plants and the generalists that feed on seeds of 5–10 species of host plants. The generalist species are: all continuous breeders; most feed on many of the same host plant species; all have a broad geographic distribution; all attack a greater percentage of seeds than do the specialist species; all feed in a broader range of seed volumes than the specialist species; all tend to range in length from 2.0 to 2.3 mm. Four of the generalist species demonstrate competition: each species usually destroys a greater percentage of seeds when other bruchid species are not attacking the same seeds at the same time. The hypothesis is forwarded that species of Cassia that harbor 5–7 bruchid species in their seeds have few or no toxins in their seeds while the other species of Cassia that have 1–3 bruchid species in their seeds are very toxic. We suggest that a limited-resource competition strategy is used by some species of plants with non-toxic seeds to reduce predator levels. In this scheme plants that mature fruits at different times provide a limited resource (seeds) for which bruchid species compete. Subsequent generations of bruchids are reduced due to production of fewer offspring resulting from competition with other bruchid species. Each seed produced as a result of cross pollination is potentially different from every other seed in the same fruit. Thus every new seed, as a new individual, potentially has different protective mechanisms against seed predators than its parents.