The Function of Burrow Turrets in a Gregariously Nesting Bee

Abstract

Diadasia bituberculata, a gregariously nesting bee, builds earthen entrance tubes (turrets) onto its subterranean nest burrows. Turrets are here shown to function in preventing infiling of burrows by dirt or debris. Both turret design and orientation are important in this context. Selection pressures promoting the construction of turrets by bees probably are related to gregarious behavior, which concentrates excavation and nesting activities in a limited area. The behavior of modifying burrow entrances into projecting tubes has evolved in several groups of ground-dwelling arthropods, including ants, spiders, and bees. Entrance tubes (often called turrets) are especially notable in bees that are gregarious but solitary (in the sense that each female makes her own nest and there is no worker caste). These structures typically are formed as thin-walled earthen extensions of a burrow by the cementing together of small earthen pellets (Fig. 1). Although turrets are constructed by numerous species of bees which have received intensive study, little is known about the manner of turret construction and almost nothing is known about turret function. Among suggestions given a priori to explain the usefulness of bee turrets are protection of the burrow entrance from rainwater, parasites, and debris, and possibly social functions (e.g., Malyshev 1935, Linsley et al. 1952, Linsley and McSwain 1957, Linsley 1958, Eickwort et al. 1977). This study assesses the significance of turrets in the biology of the anthophorid bee, Diadasia bituberculata (Cresson). Diadasia is a genus of bees which is common in arid regions of the Western Hemisphere and which usually nest in aggregations. The range of D. bituberculata is listed by Michener (1951) as California. ANIMALS AND METHODS. Populations of D. bituberculata were studied in the Laguna Mountains of southern California (San Diego Co., elev. 1500 m), where females nested in aggregations of up to several hundred individuals on hard-packed surfaces of dirt roads in recently burned chaparral (2-4 years postfire). The behavior and tunneling activities of several such populations were observed intensively for varying periods during June and July of 1972-74. Casts of burrows and adjoining nest cells were made by pouring liquid latex (Rub-R-Mold) into burrow entrances and allowing it to harden overnight. The di-