Spatial and Temporal Dynamics of a Fungal Pathogen Promote Pattern Formation in a Tropical Agroecosystem

Abstract

Abstract:Recent studies have shown that the spatial pattern of nests of an arboreal ant, Azteca instabilis (Hymenoptera: Formicidae), in a tropical coffee agroecosystem may emerge through self-organization. The proposed self-organization process involves both local expansion and density-dependent mortality of the ant colonies. We explored a possible mechanism for the density-dependent mortality involving the entomopathogenic fungus Lecanicillium lecanii. L. lecanii attacks a scale insect, Coccus viridis (Coccidae, Hemiptera), which is tended by A. instabilis in a mutualistic association. By attacking C. viridis, L. lecanii may have an indirect, negative effect on ant colony survival. To explore this hypothesis, we conducted investigations into the spatial and temporal distributions of L. lecanii. We measured incidence and severity at 4 spatial scales: (1) throughout a 45 hectare study plot; (2) in two 40 X 50 meter plots; (3) on coffee bushes within 4 m of two ant nests; and (3) on individual branches in a single coffee bush. The plot-level censuses did not reveal a clear spatial pattern, but the finer scale surveys show distinct patterns in the spread of infection over time. We also developed a simple cellular automata model of the coupled ant nest-L. lecanii system which is able to produce spatial patterns qualitatively and quantitatively similar to that found in the field. The accumulated evidence suggests that L. lecanii may very well be responsible for the density-dependent control thought necessary for spatial pattern formation of ant nests in this system.

Population Responses to Environmental Change in a Tropical Ant: The Interaction of Spatial and Temporal Dynamics

“…It could have been the case, as originally suggested [5] that the phorid fly parasitoid was the main agent of control while under the new system either the beetle [9], [18] or the fungal disease [7], [19] may be the culprit. Current evidence is insufficient to favor one or the other, but the change in the nature of the spatial distribution suggests this further mechanistic question.…”

Antagonism between two natural enemies improves biological control of a coffee pest: The importance of dominance hierarchies

“…Otherwise, total number of scales was estimated at 0 or 50. This census protocol was previously established by other investigators using the same study system and found to have a 93% efficiency (R 2 = 0.926) when compared to direct counting methods (Jackson et al, 2012, Jackson et al, 2009Perfecto and Vandermeer, 2006). Each plant was surveyed in sequential, numerical order.…”

“…For each of these bushes, L. lecanii infection was estimated for the entire bush from 0% to 100% in 5% intervals (Jackson et al, 2012, Jackson et al, 2009. To more accurately census A. orbigera populations, surveys were done about a week after C. viridis and L. lecanii surveys to minimize disturbance to the flying insect community.…”

“…Immobility makes C. viridis an easy target for predation by A. orbigera, a voracious consumer of C. viridis in both its adult and larval stages (Liere and Perfecto, 2008;Uno, 2007). In addition, regular epizootics of the fungal disease, L. lecanii (Easwaramoorthy and Jayaraj, 1978;Jackson et al, 2009) can completely decimate large C. viridis populations. In our system, we envision two possible scenarios: predation could facilitate the spread of the fungus as adult beetles travel from large clusters of infected scales unwittingly carrying conidia to new, uninfected patches, or alternatively, consumption of infected scales by the predator can inhibit growth and spread of the fungus.…”

Self‐organization of background habitat determines the nature of population spatial structure

“…This framework is motivated by the concrete case of the spatial patterning of the arboreal ant Azteca instabilis (Hymenoptera: Formicidae) in a coffee farm in southern Mexico (Perfecto and Vandermeer 2008), and the use of that spatial pattern by its mutualist associate, the green coffee scale Coccus viridis (Hemiptera: Coccidae). The ant, in association with one or more natural enemies, generates a scale‐free distribution of clustered nests in a uniform environment (Vandermeer et al 2008, Jackson et al 2009). Several other populations (beetles, spiders, fungi, in addition to the scale insect itself) become associated with these clusters in a complex fashion (Liere and Perfecto 2008, Livingston et al 2008, Vandermeer et al 2009), with each of the other populations using the clusters of ant nests as basic habitat patches.…”