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 dens ities that are much higher than normally occur in their native habitat s. When Solenopsis fire ants were introduced into the United States, t heir numerous natural enemies were left behind in South America. To co mpare intercontinental fire ant densities, we selected 13 areas in Sou th America and another 12 areas in North America. Sample areas were pa ired with weather stations and distributed across a broad range of cli matic conditions. In each area, we measured fire and densities at 5 pr eselected 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 and densities in lawns and grazing land. Fire and popula tion along roadsides in North America were 4-7 times higher than fire and population in South America. Similar intercontinental differences were found in lawns and on grazing lands. These intercontinental diffe rences in fire and abundance were not associated with sampling conditi ons, seasonal variability, habitat differences, or the frequency of po lygyny. Although several correlations were found with long-term weathe r conditions, careful inspection of the data suggests that these corre lations were probably more coincidental that causal. Cultural differen ces in roadside maintenance may explain some to the intercontinental d ifferences in grazing land and mowed lawns. Bait tests showed that com petition with other ants was much more important in South America; how ever, we were not able to determine whether this was a major cause of intercontinental differences or largely a consequence of other factors study was correlational, we were unable to determine the cause(s) of the large intercontinental difference in fire ant abundance that we ob served. 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 t he unusually high densities of fire ants found in North America.