Components of Nest Provisioning Behavior in Solitary Bees (Hymenoptera: Apoidea)*
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Apidologie 39 (2008) 30–45 Available online at: c INRA/DIB-AGIB/ EDP Sciences, 2008 www.apidologie.org DOI: 10.1051/apido:2007055 Review article Components of nest provisioning behavior in solitary bees (Hymenoptera: Apoidea)* John L. Neff Central Texas Melittological Institute, 7307 Running Rope, Austin, Texas 78731, USA Received 13 June 2007 – Revised 28 September 2007 – Accepted 1 October 2007 Abstract – The components of nest provisioning behavior (resources per cell, transport capacity, trip dura- tion, trips per cell) are examined for a data set derived from the literature and various unpublished studies. While there is a trade-off between transport capacity and trips required per cell, the highest provisioning rates are achieved by bees carrying very large pollen loads at intermediate trip durations. Most solitary bees appear to be either egg or resource limited, so sustained provisioning rates over one cell per day are unusual. Provisioning rate / body size / transport capacity / solitary bees / trip duration 1. INTRODUCTION tradeoffs between provisioning one large cell or two small cells direct, indirect, or nonex- There is a vast literature on the foraging tac- istent? Are provisioning rates related to body tics of bees: what kinds of flowers to visit, how size? long to spend on a flower, how many flowers Provisioning rate is a general term and can in an inflorescence to visit, when to turn, and simply refer to the rate (mass per unit time) so forth. Much of this work is done within a at which foragers bring various food items framework of whether a forager should max- (pollen, nectar, oils, carrion, or whatever) to imize harvesting rate or foraging efficiency their nests. Several factors are involved in nest (harvesting rate adjusted for foraging costs). provisioning rates: resources needed per cell, Virtually none of this research is done with transport capacity, trips needed to complete a any consideration of the possible constraints cell, and cells completed per day. Such a con- on the actual provisioning patterns of solitary cept applies to all bees, but for eusocial taxa it bees. Much of the theory on bee foraging be- is only indirectly related to offspring produc- havior is based on the activities of social bees, tion because of the high costs of colony main- principally Apis and Bombus, but their life his- tenance, the prevalence of food storage, and in tories may not be appropriate models for the Apis, the unknown costs of the production of 14 000 plus species of solitary bees. Questions glandular food. Here I focus on solitary bees that need to be addressed but that have re- and use provisioning rate in a more restricted ceived little attention include the following. sense: the amount of resources collected per What are the implications of egg limitation? hour as a proportion of forager body weight. In What are the implications if provisioning is a few cases, I have included data from the so- limited to one cell per day (or fewer) regard- cial halictines since, unlike the social apids, all less of resource availability? Are the possible are mass provisioners and their nest provision- ing behavior is typically very similar to that Corresponding author: J.L. Neff, of solitary bees. Additionally, some solitary [email protected] bees do include significant amounts of glandu- * Manuscript editor: Bryan Danforth lar materials in their nest provisions (Norden Article published by EDP Sciences and available at http://www.apidologie.org or http://dx.doi.org/10.1051/apido:2007055 Components of nest provisioning behavior in solitary bees (Hymenoptera: Apoidea) 31 et al., 1980; Batra and Norden, 1996) but as Similarly, pollen loads and provision masses we currently have no estimates of the costs of have been reported sometimes as fresh weights, these materials, they are not considered here. sometimes as dry weights, sometimes both, and The amount of time to provision a cell is sometimes as numbers of pollen grains. Studies in determined by a number of parameters. These which it could not be determined if wet or dry include the amount of resources necessary to weights were reported were excluded. Because of produce an offspring of a given size and the the obvious problems of variable moisture levels, amount of resources that can be transported comparisons are made only on a fresh weight: fresh weight or dry weight: dry weight basis. Pollen per foraging trip (the transport capacity). Ide- counts may, with proper precautions, allow an ac- ally, the amount of required resources divided curate estimation of the number of trips required by the transport capacity will yield the re- per cell or the number of flowers a bee may have to quired number of foraging trips to provision visit to complete a load or provision a cell, but sim- a cell. Finally, foraging trip duration (plus the ple pollen grain numbers are not useful for compar- time required to unload pollen in the nest) isons between species. In a number of cases, data times the number of foraging trips will yield on pollen loads were extracted from graphs when provisioning time per cell. All of these fac- not given explicitly in the text. tors are variables, so in many cases, determin- Conversion ratios were calculated by dividing ing the amount of time to provision a cell may average provision dry mass by average bee dry be considerably more complicated than is sug- mass. Since many species have strong sexual size gested by the simple scheme indicated above. dimorphisms, I made an attempt to associate sexes For bees provisioning more than one cell per with appropriate provision masses when I could de- day, additional factors to consider are empty termine them from position in the nest or bimodal cell availability and egg availability. distributions of provision masses. When the distri- This review examines each of these compo- bution of pollen masses was not bimodal, despite nents to see if there are general patterns across strong adult size dimorphism, I determined an av- solitary bee taxa or even if there are enough erage bee weight from the average of male and fe- males weights adjusted by the sex ratio, if known. data to discern such patterns. Each factor is If the sex ratio was not known, I employed the con- discussed in turn using published literature and servative, but sometimes erroneous, assumption of data I have collected over the last 30 years. the sex ratio being inversely proportional to the in- vestment ratio. 2. MATERIALS AND METHODS 3. RESULTS A database on the various components of provi- 3.1. Resources per cell sioning rates for 92 bee species was compiled from the literature and various unpublished studies (both Comparisons of the amounts of resources my own and that of other workers) and is available required to produce an offspring are compli- from the author. For data gleaned from the pub- cated by many factors, the most obvious of lished literature a variety of methods were used. which is that one is dealing with mixed cur- Methods of cited authors are given when discussing rencies. Provision masses of solitary bees typ- their results. For many species, the data are quite ically consist of mixtures of pollen and sugar fragmentary and heterogeneous. Body weights are variously reported as fresh weight, dry weight, or (from nectar), the proportions or which are omitted entirely. For species where weights were usually unknown. In a few cases, they may also contain floral oils (with or without added not provided, body dry weights (BMdry)werees- timated by using the strong correlation between sugar) (Vinson and Frankie, 1999; Camilo, trans-tegular distance (TTD) and dry body mass 2005; and pers. obs.), or glandular substances 2.94 (BMdry = 2.70 + TTD , Danforth, 1989b). Studies added by the female (Norden et al., 1980). in which female weights were not given and speci- One simple, and perhaps simpleminded, ap- mens were not available for measurement were ex- proach is to ignore these differences and cal- cluded. culate conversion ratios of the provision dry 32 J.L. Neff mass divided by the bee dry mass. While far in many factors (Todd and Bretherick, 1942; from ideal, this approach yields values permit- Baker and Baker, 1979; Roulston and Cane, ting comparisons between species. Among ex- 2000). Pollen protein content is commonly amples I have found, dry weight ratios ranged considered the most important factor for bees from 2.75:1 to 8.33:1 and averaged 4.66:1 (Roulston and Cane, 2000 Roulston et al., while fresh weight ratios ranged from 1.90:1 2000). The protein content of pollens collected to 3.67:1 and averaged 2.87:1 (Tab. I). I have by bees ranges from 12 to 61% (Roulston excluded the values from the pioneering work et al., 2000). One might expect that bees col- of Strickler (1979) since much of the mass lecting high protein pollen might require less of the provisions was apparently lost dur- pollen to produce offspring of a given mass ing washing. Provision masses of Hoplitis an- than those collecting low protein pollen but thocopoides, which were washed to remove there are few data on this point. In a feeding sugars in order to determine the mass of col- experiment using different pollens with differ- lected pollen, had only 22% of the mass of in- ent protein contents collected by Lasioglossum tact provision masses (Strickler, 1979, 1982). zephyrum, a mass provisioning, polylectic, so- This loss is considerably greater than what cial halictine, offspring mass increased with would be expected if only sugars were re- increasing protein content in a linear man- moved. The high conversion ratios seen in ner (Roulston and Cane, 2002).