The population dynamics of several herbivorous beetles in a tropical agroecosystem: the effect of intercropping corn, beans and squash in Costa Rica.
Abstract
The population dynamics of six species of chrysomelid beetle pests (Diabrotica viridula (F.), D. balteata Lec., D. adelpha Har., Acalymma theimei (Baly), Cerotoma ruficornis rogersi Jac. and Paranapiacaba waterhousei (Jac.)) were studied in monocrops, dicrops and tricrops of maize, beans (Phaseolus vulgaris) and squash (Cucurbita maxima) for three seasons in Costa Rica. The beetles included both generalist and specialist species and they damaged 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 approximately 6 sampling times each season (for each crop type in each treatment). Whenever an intercrop contained at least one non-host 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 generally 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 tenfold. In the cases when a given beetle species fed on both crop types in a dicrop and when there was no non-host plant present, the general intercrop effect was usually reversed: the numbers of beetles were significantly greater on one or both of the host plants in the dicrop relative to the numbers on the respective monocrops. The differences in beetle abundance appear to have been 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 rather 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 on larger plants so factors besides size of plant are important in determining beetle abundance within one cropping system. The effect of beetle damage could not be separated from the effects of root and shoot interactions of the crops, yet 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 traditional practice of intercropping may therefore be a cultural method of pest control.