The Population Dynamics of Several Herbivorous Beetles in a Tropical Agroecosystem: The Effect of Intercropping Corn, Beans and Squash in Costa Rica

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Abstract
The population dynamics of 6 spp. of chrysomelid beetle pests (Diabrotica balteata, D. adelpha, D. viridula, Acalymma thiemei, Cerotoma ruficornis and Paranapiacaba waterhousei) were studied in monocrops, dicrops and tricrops of maize, beans and squash for 3 seasons in Costa Rica. The beetles include both generalist and specialist species and they damage the plants by consuming the flowers and leaves, by transmitting viral and fungal diseases, and by eating the roots of the plants. The numbers of beetles per unit plant biomass were determined at .apprx. 6 sample times each season (for each crop type in each treatment). Whenever an intercrop contained at least 1 nonhost plant for a given beetle species, the numbers of that beetle species per host plant in the intercrop were significantly reduced relative to the numbers in the monocrop. This pattern was observed for each of the species 40 and 60 days after planting and continued until the end of the season when the differences in the numbers of beetles per host plant between monocrops and intercrops were sometimes 10-fold. In the cases when a given beetle species fed on both crop types in a dicrop and when there was no nonhost plant present, the general intercrop effect was usually reversed: the numbers of beetles were significantly greater on 1 or both of the host plants in the dicrop relative to the numbers on the respective monocrops. The differences in beetle abundance appear to be caused by differences in plant species richness of the plots per se rather than by differences in host plant or total plant density between the different treatments. Laboratory studies showed that beetles preferred to eat the more diseased squash leaves growing in the intercrops than the healthier leaves in the monocrops, so the decreased beetle abundance in intercrops does not conform with apparent differences in leaf palatability. Within squash monocrops the number of adult beetles was only weakly correlated with the size of individual plants. There were significantly more of several beetle species per unit squash biomass on smaller than larger plant so factors besides size of plant are important in determining beetle abundance within 1 cropping system. The effect of beetle damage was not separated from the effects of root and shoot interactions of the crops. Considering the mechanisms and time at which the beetles typically cause economic damage to the crops, it is possible that decreased beetle abundance contributed to greater yields of the intercrops. The practice of intercropping may be a cultural method of pest control.
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