Thermoregulation and Activity Patterns of the Sand Wasp Steniolia longirostra (Say) (Hymenoptera: Sphecidae) in Costa Rica

Abstract

In the neotropical sand wasp Steniolia longirostr-a (Say), female activity levels in the nesting area were negatively correlated to ambient temperature and positively correlated to relative air humidity. The wasps showed high activity below 350C and above 40 percent relative humidity. Females with a low wing load had a lower body temperature during flight compared to females with a high wing load. This may offer females with favorable body proportions benefits, as they can enjoy longer daily provisioning periods. MICROCLIMATE HAS A SUBSTANTIAL EFFECT on the behavior of most insect species. Maintenance of an adequate body temperature and water balance is difficult for small animals, whether using ectothermic (behavioral) or endothermic (physiological) mechanisms for its regulation (May 1985). Numerous studies of thermoregulation in Hymenoptera have been carried out in recent years. However, as in other insect groups, most research has been conducted on temperate zone species (but see May & Casey 1983, Vogt & Heinrich 1987). Sand or digger wasps occupy dry and sunny habitats in temperate and tropical areas. Many species dig their burrows in the ground, exposing themselves to extreme shifts in ambient temperature during surface activities (Nielsen 1945). The few studies published on their thermoregulation suggest that they are ectothermic, that is, regulate their body temperature by behavioral means (Willmer 1985a, O'Neill & O'Neill 1988). One should suspect intraspecific variation in body size of ground-nesting tropical wasps to be related to their thermoand hygroregulatory capacity (as for temperate species, see Wilmer 1985 a). As a consequence, I hypothesized differences in activity patterns between differently sized nesting females. I present here some data on the activity and thermoregulation of a gregarious neotropical sand wasp, Steniolia longirostra (Say) (Hymenoptera: Sphecidae: Nyssoninae), during the hot and dry season in Costa Rica. This genus embraces about fifteen species, ranging from southwestern Canada to Central America. S. longirostra has been recorded from late September through May, mainly from southern Mexico. The females of this medium-sized wasp are characterized by their fast, hovering flight. They dig burrows in dry, sandy ground, where they provision their larvae with flies (Evans & Gillaspy 1964, Gillaspy 1964).