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House Infestation and Outdoor Winter Foraging by the Winter Ant, Prenolepis imparis Say (Hymenoptera: Formicidae), in Saltillo, Mexico

The ants (Formicidae) as a group are very thermophilic. Abundance and diversity of ants usually decrease at higher latitudes, and even boreal ants are normally active only on warmer days of the year (Hölldobler and Wilson 1990). The small North American formicine Prenolepis imparis Say is an exception to the trend. It is a cryophilic ant, active at temperatures near freezing (Wheeler 1930, Tschinkel 1987, Hölldobler and Wilson 1990). In northern Florida, P. imparis foraging and above-ground activity occur only during the coldest months (November through late March or early April); annually, the workers remain in the sealed nest (underground) for 7 to 8 months, through spring and summer (Tschinkel 1987). A number of reports described winter activity of the ant aboveground, but few observations described foraging. A collaborator of Wheeler (1930) reported foraging on honey placed at the nest entrance, between -1.1 and 0.8oC. Talbot (1943) reported that aboveground activity began between 1.7 and 4.4oC in Missouri and Ohio, with numbers of workers increasing rapidly toward the warmer temperature. Lynch et al. (1980) reported a colder foraging limit of 5oC for populations of this ant in hardwood forest in Maryland, USA. Prenolepis imparis occurs from southern Ontario, Canada, to the USAMexico border at New Mexico, and essentially from coast to coast in the USA (Wheeler 1930) but probably avoids the warmer parts of this range (Tschinkel 1987). It occurs in western, northern, and Central Texas but it appears to be absent from South Texas (O’Keefe et al. 2000). There are few reports of the ant from Mexico: from the Baja Peninsula (Johnson and Ward 2002) and Hidalgo State (Quiroz-Robledo and Valenzuela-González 1993). Wheeler (1930) described the Mexican varieties P. imparis colimana Wheeler and P. imparis veracruzensis Wheeler; the first from 2,286 meters above sea level, at Volcan de Colima, on the Sierra Madre Occidental range in Colima State (oak-pine forest). The subspecies veracruzensis was described from a single specimen from Maltrata, State of Veracruz, on the Sierra Madre Oriental, at 2,430 m above sea level; the natural vegetation was temperate cloud forest. The validity of Wheeler’s varieties of P. imparis has been questioned (Mackay and Mackay 2002). An urban household in downtown Saltillo (population 800,000; N 25o 25 ‘55’, W 100o 59 ‘36’; 1,600 m above sea level) was infested with ants. Saltillo is in a Chihuahuan desert plain adjacent to the pine-oak forests of the Sierra Madre Oriental. The homeowner called the author and reported ant workers commonly active in a bathroom (on the floor and sink) and also in the kitchen, where aggregations of >50 workers were frequently observed, attracted to food leftovers