A comparison, by sweep sampling, of the insect fauna from corn and sweet potato monocultures and dicultures in Costa...

The insect fauna of 80 day-old plots of corn and sweet potato monocultures and dicultures in Costa Rica were compared using sweep sampling. Six hundred sweeps were taken in each of the three habitats. There were 15% more total species in the diculture than either monoculture but approximately the same total number of individuals. There were 75% more species and approximately 100% more individuals of parasitic Hymenoptera in the diculture than the monocultures. The ratio of numbers of phytophagous individuals to predaceous/parasitic individuals was lowest in the diculture (2.5) and highest in the sweet potato monoculture (14.3). It is suggested that these patterns may be explained if phytophagous insects are limited basically by abundance and diversity of food so that a diculture is at best the sum of two monocultures. However if abundance and diversity of parasitic insects depends more on structural complexity, the result of putting together two monocultures would be synergistic in terms of numbers and species of insects.Two chrysomelid beetles and two leaf-hopper species that were extremely common in the monocultures were significantly less common or absent in the polyculture, and only one leaf-hopper that was rare in the monocultures was relatively more common in the diculture. Comparison of species similarity showed that the corn monoculture and the diculture were much more similar than the sweet potato monoculture and the diculture, and the two monocultures showed the least similarity.Statistically smoothed out species-subsample curves were constructed for each of the three habitats, the curves were fitted to a mathematical model, and they were then extended in order to predict the theoretical total number of species in all three communities sampled. Extrapolation of the curves suggests that approximately 33% of the total sweepable insect community was sampled in the three habitats.One year after the initial sweep samples were taken, the populations of two sweet potato pests, Diabrotica balteata and Diabrotica adelpha, were sampled ten times over a 120 day period in plots of corn and sweet potato monocultures and dicultures. Approximately 50 days after planting, the numbers of both beetles on sweet potato in monocultures were much higher than in dicultures. This trend continued the rest of the season, the difference reaching a maximum approximately 90 days after planting.These data suggest that indigenous agriculturalists are correct: increasing resource diversity in a cropping system may act as a form of biological control, by increasing the relative abundance and diversity of the predaceous parasitic fauna and decreasing the abundance of the major herbivores.