Ant-hemipteran mutualism and the persistence and spatial distribution of a predatory ladybeetle in a coffee agroecosystem

Background/Question/Methods The spatial distribution of a predator population is often closely linked to that of its prey. Predators spend more time foraging in areas of high prey density; thus, if their prey exhibit a patchy spatial distribution, it is expected that the predators will exhibit a similar pattern. However, if predation in areas of high prey density is risky for predators, as is the case for insect predators of ant-tended hemipteran, the situation gets complicated. We studied the interaction between a voracious predator, the ladybeetle Azya orbigera (Coleoptera: Coccinellidae) and the mutualism between an important coffee pest, the green scale (Coccus viridis, Hemiptera: Coccidae) and an aggressive tree-nesting ant, Azteca instabilis (Hymenoptera: Formicidae) in a coffee agroecosystem in Chiapas, Mexico. While A. orbigera adult ladybeetles feeding on scales suffer from constant attack by the ants, the larvae have waxy filaments that render them immune to ant attacks and thus are able to forage freely in ant-tended areas, where scale densities are higher and more persistent. Furthermore, in previous studies we have shown that the ants indirectly benefit the ladybeetle larvae by providing them enemy-free space. Consequently, we hypothesized that the distribution of the ladybeetles is closely linked to that of the ants. To test this hypothesis we established a 45-ha plot, marked all A. instabilis nests, and conducted a two-year long ladybeetle survey in sites with and without ants. Results/Conclusions In the dry season, when scale abundance was low, ladybeetles were found almost exclusively in sites with ants. In contrast, in the rainy season ladybeetles were found all over the plot, though in significantly higher densities in sites with ants. Furthermore, we found a significant relationship between the occupancy and persistence of the ladybeetle populations and the clustering of ant nests. The more ant nests within 20 m of the sampled site, the higher the probability the site was occupied by ladybeetles and the higher the probability that it remained occupied for longer. Our results show that the spatial distribution and persistence of A. orbigera populations are greatly influenced by the clustered distribution of A. instabilis. In turn, the persistence of this important predator might be key to the natural population control of green scales in the coffee farm.