Nesting and foraging behavior of Andrena (Callandrena) rudbeckiae Robertson (Hymenoptera: Apoidea: Andrenidae) in Texas

Field: Agricultural and Biological Sciences (Life Sciences)

The foraging and nesting biology of Andrena rudbeckiae was studied over a four year period in central Texas. The few-celled, single-occupant nests are gregarious and structurally simple with the fully lined cells occurring at soil depths of 94 to 118 cm. Females commonly make 6-8 pollen trips per day with pollen trips averaging 33.5 min. Male cells are estimated to require one day to provision while females require two. Pollen collection is restricted to the coneflower genera Rudbeckia and Ratibida (Asteraceae) with time spent per head increasing with the number of staminate phase florets per head. Males patrol the emer gence site, nonflowering shrubs near the nest site, as well as pollen hosts. Bees overwinter both as adults and defecated larvae in their natal cells, an unusual pattern in Andrena. N? mada (Phelonomada) belfragei was regularly present at the nest site and was recovered from nest cells of A. rudbeckiae. This is the first host report for any Phelonomada. Callandrena is easily the largest North American subgenus of Andrena, yet it is biologically among the least well known. Reports of nesting biology are available for only three of the 80 described species (Rozen, 1973; Parker and Bohart, 1982; Parker and Griswold, 1982) and data on foraging behavior are sparse and fragmen tary. Collection records suggest the group is of evolutionary interest since virtually all its members restrict their pollen collection to the Asteraceae, an unusual host choice for Andrena (LaBerge, 1967, 1986). Some species appear to be narrow spe cialists on particular genera (such as Andrena krigiana Robertson on Krigia, A. mel liventris Cresson on Gaillardia) while others (such as A. senticulosa LaBerge) have much broader ranges involving multiple host genera, tribes, or even subfamilies of the Asteraceae (LaBerge, 1967; J.L.N., pers. obs.). Callandrena thus potentially rep resents an informative group for comparative biological studies investigating the ori gin and relative advantages and disadvantages of oligolecty. A prerequisite for such studies is the use of monophyletic groups. Callandrena appears to be monophyletic but this has yet to be rigorously established. Several characters, such as the short maxillary palps and branched scopal hairs, are potential synapomorphies of the group but these characters are not unique to Callandrena (LaBerge, 1967). We have re cently initiated molecular phylogenetic studies to resolve this problem. A further pre requisite for evolutionary studies is an understanding of the ecology and behavior of the members of the taxa involved. The initial portions of our comparative studies include this four year (1993-1996) investigation of the biology of Andrena rudbeckiae in central Texas. A. rudbeckiae is a robust (female body length 13-15 mm), infrequently collected bee, known to be broadly distributed in the central United States (LaBerge, 1967). Central Texas is near the southern limit of its range. Our report is the first on the nesting biology of 1 Current address and to whom correspondence should be sent: Central Melittological Institute, 7307 Running Rope, Austin, Texas 78731. 2 Current address: Department of Botany, The University of Texas, Austin, Texas 78713. Accepted for publication 12 March 1997. This content downloaded from 157.55.39.198 on Thu, 21 Jul 2016 05:34:09 UTC All use subject to http://about.jstor.org/terms VOLUME 70, ISSUE 2 101 Fig. 1. Andrena rudbeckiae nest site at Pendernales Falls State Park, Texas. Nests occurred in the vegetation on both sides of the path. A. rudbeckiae and the first in-depth examination of the foraging and provisioning behavior of any Callandrena. Materials and Methods We studied the nesting biology of A. rudbeckiae at Pedernales Falls State Park (PF), Blanco Co., Texas (30?20,N, 98?15'W). Observations of foraging behavior are from PF and the Brackenridge Field Laboratory (BFL) of the University of Texas, Austin, Travis Co. (30?17'N, 97?47/W). Over the course of several years, we located several small aggregations of 2-6 nests found in open sites dominated by little blue stem (Schizachyrium scoparium (Michx.) Nash) at PF but these areas were not reg ularly monitored. A large nest aggregation at PF was discovered on 17 June 1993 by Dr. Al Hook. All nest aggregations were in deep alluvial sand deposits that form a series of narrow terraces between the rocky, cypress-lined (Taxodium distichum (L.) Rich.) banks of the Pedernales River and the surrounding limestone hills and bluffs which are clad with plateau live oak (Quercus fusiformis Small) and Ashe cedar (Ju niperus ashei Buch.). All nest biology observations are from the large aggregation (Hook's nest site), a site with scattered eastern gama grass (Tripsacum dactyloides (L.) L.) partially shaded by large pecan trees (Carya illinoiensis (Wang.) K. Koch) (Fig. 1). This site is approximately 5 m above the normal river bank but was briefly covered by a meter or more of water in 1994 and 1995 during spring flash floods which occur commonly on the Edwards Plateau. We monitored activity on five days from 18 to 28 June 1993 and visited the nest site at least weekly from 26 April through 24 June in 1994, 26 April through 10 June 1995, and 18 April through 17 June in 1996. In 1993 we marked individual A. rud This content downloaded from 157.55.39.198 on Thu, 21 Jul 2016 05:34:09 UTC All use subject to http://about.jstor.org/terms 102 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY beckiae nests in the center of the PF aggregation with painted nails. The nails were replaced with numbered popsicle sticks when it was found that enlarged or displaced nest tumuli frequently hid the nails. All day monitoring of to provisioning behavior at 26 nests was conducted on 21, 22, and 24 June 1994 and at 28 nests on 8, 16, and 24 June 1994. Most monitored entrances were covered with clear plastic cups to fa cilitate timing. The cups were kept over the entrances throughout the observations periods and were removed only when the bees were actually entering or exiting their nests. This decreased the accuracy of timing but increased the number of nests that could be monitored per observer. A few monitored nests were left uncovered to per mit timings of nest parasite activity. Activities at the nest site and while foraging were timed to the nearest second with hand-held stopwatches. We found no signifi cant differences for provisioning activities between 1993 and 1994 so they are com bined for analysis. Nests were excavated on 28 June 1993 and 2 November 1994 to determine nest structure and recover parasites and overwintering bees. Burrows excavated in 1994 were filled with plaster of paris on 29 June 1993. Larvae were deposited at the Amer ican Museum of Natural History, New York. Bee body weights are from individuals collected at the nest site. Females return ing to the nest without pollen loads were assumed to have been on nectar-only trips. This seems plausible since such trips were usually the only trip of the day. Nonethe less it is possible these females carried significant loads of pollen internally in their crops. This was not determined for A. rudbeckiae but in the case of another Callan drena, Andrena sitililae Viereck, crops of females caught returning to their nests without scopal pollen loads contained large amounts of nectar but relatively small amounts of pollen, suggesting internal pollen transport may not be a major mode for this group (J.L.N., unpubl. data). Pollen loads are from females captured while en tering their nests. The weight of a pollen load was estimated as the difference be tween the fresh weight of a female with her pollen load intact and her fresh weight after nearly all pollen had been removed by careful brushing. The female was then dried to constant weight to obtain a body dry weight. Bees were weighed to 0.1 mg with a Cahn 25 Microbalance. Statistical analyses were performed with the Statview 45 package on a Macintosh 7200/90. Data are presented as the mean ?1 SD with number of observations (n = x) and, if appropriate, the range. Tables 1 and 2 give most of the statistical data summarized in the text.