Competitive Interactions between Two Bruchid Species (Algarobius spp) Introduced into South Africa for Biological Control of Mesquite Weeds (Prosopis spp)

The introduction into South Africa of two North American seed-feeding bruchids, Algarobius prosopis (LeConte) and Algarabius bottimeri Kingsolver, for biological control of mesquite weeds has met with disparate results. Algarobius prosopis is prolific and damaging while A. bottimeri has established, but is scarce and confined to one locality. Survival of immatures, adult mass, sex ratios, and fecundity were measured for individuals in insectary-reared colonies of A. prosopis and A. bottimeri on five different mesquite taxa that are naturalized in South Africa, including a South American species, Prosopis chilensis (Molina) Stuntz, and four North American taxa, Prosopis velutina Wooton, P. glandulosa var. torreyana (L. Benson) M. C. Johnston, Prosopis glandulosa var. glandulosa J. Torrey, and hybrids, possibly of all four of these species. On all five mesquite taxa, survival from egg to adult was lower in A. bottimeri than in A. prosopis. Though potentially more fecund than A. prosopis, A. bottimeri females that only mated once produced few viable eggs. When neonates competed for the same seed, A. prosopis was dominant and A. bottimeri was usually excluded. Mass rearing and releases of A. bottimeri should be discontinued because all indications are that this species will not have a noticeable impact on the weed.