Wednesday, August 8, 2007 - 8:40 AM

OOS 23-3: Persistence of a predator population: High-order effects of an ant-hemipteran mutualism in a coffee plantation

Heidi Liere, University of Michigan

I am studying 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. My research focuses on the interaction between this mutualism and a voracious green scale predator, Azya orbigera (Coleoptera: Coccinellidae), whose larvae have waxy filaments that render them immune to ant attacks and thus are able to prey upon ant-tended scales. I wanted to determine if A. instabilis’ patchy distribution creates high- and low-quality habitats for A. orbigera, and if this spatial heterogeneity influences A. orbigera’s distribution and population stability. I did systematic samples of A. orbigera in an established 45-ha plot. I sampled coffee bushes around 53 trees with and 63 trees without A. instabilis nests during the wet season (June) 2006 and the dry season (February) 2007.  During the dry season, I found statistically more larvae, pupae, and adults around trees with ants. The larvae were more often parasitized in sites without ants and the larvae pupae ratio was statistically greater around trees without ants. These results suggest that, besides having access to high scale-density areas, A. orbigera larvae gain enemy-free space by living in ant-tended plants. Furthermore, during the dry season, A. orbigera’s populations were drastically reduced and were almost exclusively found around trees with ants. My study suggests that the A. instabilis-C. viridis mutualism is key to the persistence of A. orbigera’s populations; this might have great practical implications to the natural control of green scales.

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