Understanding how herbivore and parasitoid populations are spatially, temporally and genetically structured has broad implications for the theory and application of biological control.  We are studying a tritrophic system that includes wild bean plants in the genus Phaseolus (P.vulgaris and P. coccineus), bruchid beetles that attack these beans and three species of parasitoids that attack the larvae of the beetles inside the beans. The objectives of our study are: 1) to examine the temporal and spatial patterns of this tritrophic system across several populations in Central Mexico and 2) to determine how these patterns relate to the levels of genetic variability and the population genetic structure of the dominant parasitoid species.  Mexico is a likely center of origin for the genus Phaseolus.  Individuals of the parasitoid species H. depressus, H. missouriensis, and H. butcheri, were collected from infested beans in 10 populations over a period of two years.  These wasps have established ranges that vary in either the bean species on which they specialize or the altitude at which hosts are attacked.  While each wasp has its specific range they coexist in the common bean, P. vulgaris.

Results/Conclusions

We found that H. butcheri has a more restricted distribution relative to the other two species.  While it was collected from seeds of P. vulgaris and P. coccineus infested by the same species of bruchid at median altitudes (1230m-2240m) it was absent outside of this range, despite the availability of the same beetle host/plant complex. Additionally, sequence analyses of 28SD2 and ITS2 nuclear regions show H. butcheri to be more polymorphic than either H. depressus or H. missouriensis.  Individuals morphologically identified as H. butcheri cluster into four genotype groups, while each of the other species possesses their own common genotype.  This greater genetic diversity argues for increased genetic drift, either due to decreased dispersal ability between populations (perhaps due to a more limited altitudinal range), and/or an earlier establishment.  It also explains previous difficulty in assessing the population structure of this species.   Investigations into the extent of this molecular divergence and its underlying causes are ongoing.