Fire Ant Population Model

Introduction

Infestations of the red imported fire ant, Solenopsis invicta Buren, occur in eleven states within the U.S. and in Puerto Rico, covering over 240 million acres. Colonies may contain one or more queens with the single-queen form occurring in 130 eastern and central counties in Texas. A 1989 survey showed 60% of these counties also had colonies of the multiple-queen form (Porter et al. 1991). That percentage is undoubtedly higher today.

Fire ants are a serious pest in urban environments, because they interfere with gardening and outdoor activities, and occasionally invade human habitations. Fire ants have a very complex colony cycle that begins with a single mated queen or two or more cooperating queens who digs a new nest and lays eggs, and often ends as a large sprawling nest with thousands of workers in different morphological castes plus winged pre-reproductive females and males ready for the mating flight.  The rate of growth of colonies is affected by competition, predation, disease, and parasitism, and the availability of resources (food and nesting sites).

Local climate affects colony dynamics directly by influencing the rate of development of immature ants, and indirectly by affecting food abundance (Porter and Tschinkel 1987). The production of winged reproductives (male and female) is influenced by humidity and rainfall (Bhatkar 1991). Mating flights are sporadic, occurring typically on the first clear, calm day following a period of precipitation. Fire ant dispersal is a result of mating flights and transport of mated queens by wind, water, and human commerce.

Predictions of the population and spatial dynamics of a species with the behavioral complexity of the fire ant will require a systematic integration of relevant individual, colony, population, and community interactions. We feel that this can only be accomplished through the use of simulation methodology, using a clearly planned and defined structure, combined with accurately measured parameters. We propose to build a fire ant simulation model which tracks development of multiple colonies in different stages of development and links their growth to climate and resource data reflecting the range of conditions found in Texas.

Underlying the proposed model would be a spatially-referenced structure which can utilize site specific information on climatic, and biotic and edaphic variables. From these simulations we would be able to predict the population density of fire ants in various habitats and the impact of climatic and management practices on colony success.  A conceptual value of the proposed model would be that by systematically integrating the effect of each of the major factors which impinge upon a colony's net reproductive rate, we would have a means with which to estimate how much additional mortality was needed to suppress the local and regional abundance of the fire ant to acceptable levels.

As such, we would be able to evaluate potential fire ant management strategies, both chemical, microbial, and biological, for regionally managing this pest based on locally prevailing conditions. These results would be generalizable to conditions in other states throughout the southern U.S. A second major value of the proposed model is its ability to capture in a systematic manner what is known about fire ant genetics, biology, and ecology. By comparing simulated and observed estimates of colony behavior and population dynamics, the model would provide a focus for identifying critical missing data and in some cases inappropriate assumptions or hypotheses.

This approach could be of immense value in targeting critical basic and applied research needs. An implicit objective of this proposal is to use the fire ant model as a structure for integrating the knowledge base and expertise of each of the major players in the fire ant project.  Along this line of reasoning, the fire ant model would also be structured to enable future linkage with object-oriented vertebrate population models, which would allow for the joint models to be used to estimate fire ant impact on vertebrate populations for different management scenarios.

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