Intercontinental Differences in the Abundance of Solenopsis Fire Ants (Hymenoptera: Formicidae): Escape from Natural Enemies?

The absence of natural enemies often allows exotic pests to reach densities that are much higher than normally occur in their native habitats. When Solenopsis fire ants were introduced into the United States, their numerous natural enemies were left behind in South America. To compare intercontinental fire ant densities, we selected 13 areas in South America and another 12 areas in North America. Sample areas were paired with weather stations and distributed across a broad range of climatic conditions. In each area, we measured fire ant densities at 5 preselected roadside sites that were at least 5 km apart. At each site, we also measured foraging activity, checked for polygyne colonies, and recorded various kinds of environmental data. In most areas, we also measured fire ant densities in lawns and grazing land. Fire ant populations along roadsides in North America were 4-7 times higher than fire ant populations in South America. Similar intercontinental differences were found in lawns and on grazing lands. These intercontinental differences in fire ant abundance were not associated with sampling conditions, seasonal variability, habitat differences, or the frequency of polygyny. Although several correlations were found with long-term weather conditions, careful inspection of the data suggests that these correlations were probably more coincidental than causal. Cultural differences in roadside maintenance may explain some of the intercontinental differences in fire ant abundance, but they did not account for equivalent intercontinental differences in grazing land and mowed lawns. Bait tests showed that competition with other ants was much more important in South America; however, we were not able to determine whether this was a major cause of intercontinental differences or largely a consequence of other factors such as the numerous pathogens and parasites that are found in South America. Because this study was correlational, we were unable to determine the cause(s) of the large intercontinental difference in fire ant abundance that we observed. However, we were able to largely exclude a number of possible explanations for the differences, including sampling, season, polygyny, climate, and aspects of habitat. By a process of elimination, escape from natural enemies remains among the most likely explanations for the unusually high densities of fire ants found in North America.