Nests of leaf-cutting ants (Hymenoptera: Formicidae: Attini) are abundant disturbances in Neotropical rain forests, and could affect the plant community both while the nests are active and after they are abandoned. We measured the diversity and abundance of understorey plants (>1 m in height) in the area around active and abandoned nests of leaf-cutting ants (Atta cephalotes) at the La Selva Biological Station in Costa Rica. Sampel quadrats on active nests had reduced diversity (number of morphospecies) and abundance of both small (height >10 cm) and large (10 cm–1 m) understorey plants, when compared to the nearby forest floor (3 and 13 m from the nest edge). Abandoned nests had greater diversity and marginally greater abundance of small understorey plants relative to nearby forest; there was no difference in diversity or abundance of large understorey plants. Leaf-cutting ant nests create gaps in the plant understorey when active, but serve as centres of recruitment for small plants after they are abandoned. Thus, like canopy gaps, ant nests could play an important role in recruitment of new individuals and maintenance of plant species diversity in tropical forests.

This paper reviews and analyses literature on the effect of plant species diversity on crop pests. It is observed that populations of several crop insect pest species are depressed under conditions of plant species diversity. The factors responsible for this depression are: parasites, predators, masking, camouflage, repellency, less colonization, resource concentration, unfavourable microclimate, physical obstraction and trap cropping. Natural enemies account for more than half of the cases where the pest population was claimed to be regulated in the weed-diversity systems. However, under conditions of crop–crop diversity, natural enemy action accounted for less than a third of the cases reviewed. It is further observed that most of the reviewed crop–crop diversity systems were of annual crops. It is therefore concluded that under conditions of annual cropping natural enemies are probably of lesser importance than other regulating factors.