Does the size of the seeds and seed pods of the invasive tree Leucaena leucocephala (Fabaceae) affect their utilization by the biological control agent Acanthoscelides macrophthalmus (Chrysomelidae: Bruchinae)?

The Neotropical tree Leucaena leucocephala (Lam.) de Wit (Fabaceae), which was introduced worldwide for agroforestry, has become invasive in several countries. In an attempt to reduce its invasiveness in South Africa, the seed-feeding beetle Acanthoscelides macrophthalmus (Schaeffer) (Chrysomelidae: Bruchinae) was released in 1999 as a biological control agent. The beetle oviposits on seed pods and on loose seeds of its host. Hatching larvae burrow into the seeds to consume the contents, emerging as adults. Since L. leucocephala seeds and seed pods display considerable variation in size (i.e. resource quality) in the field, a laboratory study was conducted to determine whether size affects utilization by A. macrophthalmus. A seed-switching trial was also carried out to determine whether seed pod or seed size was the most consistent oviposition cue. The beetles displayed strong oviposition preferences for larger resources, with significantly higher egg loads on the largest seeds and seed pods. The seed-switching trials suggested that seed pod size was a more important oviposition cue than seed size. These results were verified by field surveys which revealed significant positive relationships between egg loads and seed pod size at two of the three sites surveyed. Since seed size is positively correlated with germination success and seedling fitness in leguminous plants, these oviposition patterns suggest that, in situations where dehisced seeds are utilized, A. macrophthalmus may be targeting the fittest seeds in L. leucocephala populations. This may have positive, albeit subtle, implications for biological control.