Interactions of fire ants and Xylophilous hymenoptera
Jenkins, David Alan
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A review of the evolutionary significance of ants to nesting behaviors expressed in social and solitary wasps and bees is presented. In response to ant predation many Hymenoptera have adopted a variety of nesting behaviors thought to reduce such predation. These include architectural defenses (e.g., pedicels or other structures that restrict access to the nest), glandular defenses, wherein a glandular product of the wasp or bee is used to repel or thwart ants, and active defenses (e.g., method of colony formation in social vespids and prey-carriage mechanisms in sphecids). Trap-nests were used to survey the abundance, seasonal occurrence, and nest architecture of xylophilous Hymenoptera in early successional old field habitats of Georgia and South Carolina from March to September of 2001. Occupants of trap-nests included three Vespidae (Euodynerus megaera (Lepeletier), Ancistrocerus campestris (Saussure), and Monobia quadridens (L.)), four Sphecidae (Isodontia mexicana (Saussure), Solierella plenoculoides (Fox), Trypoxylon collinum (Smith), T. clavatum (Say), and T. striatum Provancher), two Megachilidae (Megachile frigida Smith and Osmia albiventris Cresson), and one Anthophoridae (Xylocopa virginica(L.). This study records the first biological data for S. plenoculoides and M. frigida. The bees (O. albiventris and M. frigida nested early in the season (April-May), whereas the vespid and sphecid wasps nested predominantly in the summer (May-August). No correlation was found for either the number of species nesting per site or the number of nests per site and abundance of red imported fire ants, Solenopsis invicta Buren (RIFA), or plant diversity. However, the four sites in Georgia were more floristically diverse than the four sites in South Carolina and had significantly higher numbers of occupied nests. Only E. megaera nested at all sites and it accounted for 35% of all completed nests. Comparisons made with trap-nest data collected 40 years earlier by Krombein (1967) in various southeastern U.S. localities revealed nest architecture and species differences that are interpreted in light of nest placement and the arrival of the RIFA. Comparing these data with similar data from trap-nesting studies in Europe suggest that placing more stations with fewer trap-nests per station increases nesting rate (i.e., placing 100 trap-nests at 50 locations will trap more Hymenoptera than placing 100 trap-nests at two locations). These data form a baseline for cavity-nesting Hymenoptera diversity of the early successional stages of an old field habitat that may prove important as habitat use changes and as non-native species, such as RIFA, become intergrated and more abundant in the region. In another study, two groups of alfalfa leaf-cutting bees, Megachile rotundata Fabricius, were manipulated to nest in trap-nests at a site in Oconee Co., Georgia which was also monitored for RIFA density. When RIFA were present at lower densities there were no significant differences in nest architecture (closure plug thickness and number of cells) between nests that were close to the ground (<30 cm) and nests that were 150 cm above the ground. Nor were there significant differences in nest architecture between nests in locations where ants were excluded and nests in locations that were accessible to ants. However, when RIFA foragers were more abundant closure plugs were significantly thinner and there were significantly fewer cells in ant-excluded nests than in ant-accessible nests. Also, closure plugs were significantly thicker in nests that were close to the ground than in nests that were further from the ground. These findings suggest that these cavity-nesting bees adapt their nesting behavior in response to exposure to a novel Formicidae species.