Body Size As an Estimator of Production Costs in a Solitary Bee

Ecological Entomology (2002) 27, 129±137 Body size as an estimator of production costs in a solitary bee 1,2 2 JORDI BOSCH and N A R C IÂ SVICENS 1Biology Department, UtahState University, U.S.A. and 2Departament de Biologia Animal, Facultat de Biologia, Universitat de Barcelona, Spain Abstract. 1. Body weight is often used as an estimator of production costs in aculeate Hymenoptera; however, due to differences between sexes in metabolic rates and water content, conversion of provision weight to body weight may differ between males and females. As a result, the cost of producing female progeny may often have been overestimated. 2. Provision weight and body weight loss throughout development were measured in a solitary bee, Osmia cornuta (Latreille), to detect potential differences between sexes in food weight=body weight conversion. 3. Male O. cornuta invest a larger proportion of larval weight in cocoon spinning, and presumably have higher metabolic rates than females during the larval period; however, this is compensated by a slightly longer larval period in females. 4. Overall, body weight loss throughout the life cycle does not differ significantly between sexes. As a result, cost production ratios calculated from provision weights and from adult body weights are almost identical. 5. The validity of other weight (cocoon, faeces) and linear (head width, intertegular span, wing length, cocoon length, and cell length) measures as estimators of production costs is also discussed. 6. Valid estimators of production costs vary across species due to differences in sex weight ratio, cocoon shape, provision size in reference to cell size, and adult body size. Key words. Body size, Hymenoptera, Megachilidae, Osmia cornuta, parental investment, provision size, sex allocation. Introduction 1982; Vicens, 1997). Other studies have attributed higher foraging efficacy to larger females due to their ability to fly Individual body size is often related to fitness in insects and faster and visit more flowers per unit time (Pyke, 1978), other animals. In Aculeate Hymenoptera (ants, wasps, bees), transport larger pollen±nectar loads (Klostermeyer et al., several studies have documented advantages to larger than 1973), and forage at lower ambient temperatures (Heinrich, average individuals in the establishment of mating territories 1976; Willmer, 1985b; Stone, 1994). On the other hand, (Alcock et al., 1977; Severinghaus et al., 1981; O'Neill, 1983; a number of studies has not found consistent, positive Alcock, 1990; Sugiura, 1991), nest usurpation (Tepedino & correlations between female body size and fecundity Torchio, 1994; Kim, 1997; J. Bosch, unpublished), fecundity (Tepedino & Torchio, 1982; Frohlich & Tepedino, 1986; and=or progeny body size (Alcock, 1979, 1984; Freeman, Johnson, 1990; J. Bosch, unpublished). In fact, in the wasp 1981; Willmer, 1985a; Sugiura & Maeta, 1989; Johnson, Bembix rostrata LinneÂ , female body size was correlated inver- 1990), survival during development (J. Bosch, unpublished), sely to fecundity (Larsson & TengoÈ , 1989). Larger individuals and survival during overwintering (Tepedino & Torchio, may also suffer higher parasitism rates (MuÈ ller et al., 1996). Most aculeate species build nests that females (and=or workers in social species) provision withfood for their Correspondence: J. Bosch, USDA-ARS, Bee Biology and System- progeny. Because of the correlation between the amount of atics Laboratory, UtahState University, Old Main Hill 5310, Logan, food ingested by a larva and adult body size (Klostermeyer UT 84322-5310, U.S.A. E-mail: [email protected] et al., 1973; Freeman, 1981; Johnson, 1988; this study), and # 2002 The Royal Entomological Society 129 130 Jordi Bosch and NarcõÂs Vicens because the heritability of body size appears to be low cells (Bosch, 1994b). Body weight ratio (mean female (Tepedino et al., 1984), bothfreshand dry adult body weight=mean male weight) is 1.6±1.8 (Bosch, 1994b; Vicens, weight have been used in studies of parental investment 1997). In agreement with Fisher's theory (1958), the larger as estimators of progeny production costs (e.g. Trivers & provision weight allocated to female individuals is com- Hare, 1976; Cowan, 1981; Tepedino & Torchio, 1982; pensated by the production of a larger number of male Pamilo & Rosengren, 1983; Frohlich & Tepedino, 1986; progeny, so that parental investment within an O. cornuta Danforth, 1990); however the use of body weight as a cost population is equal in bothsexes (Vicens, 1997). Osmia production estimator has been questioned because of the cornuta eggs hatch in about a week, and the first-instar differences between sexes in metabolic conversion rates. In larva remains within the split egg chorion without feeding. many aculeates, females are larger than males, and as a Four other feeding larval instars follow. The final instar result have a lower energetic cost of respiration per unit body defecates, completes consumption of the provision, and weight than males (Peakin, 1972; Boomsma & Isaaks, 1985; spins a cocoon withsilk strands from thesalivary glands. MacKay, 1985). Females also tend to have higher body The cocoon isolates the then dormant fifth-instar larva water contents and to accumulate larger amounts of non- (prepupa) from the faecal particles. Pupation and adult- metabolising fat reserves (Peakin, 1972; Trivers & Hare, hood occur towards the end of the summer or beginning 1976; Boomsma & Isaaks, 1985; Nielsen et al., 1985). For of the autumn. Adults remain within their cocoon through- these reasons, parental investment in aculeate females may out the autumn and winter, and emerge the following spring often have been overestimated (Trivers & Hare, 1976; in synchrony with the bloom of fruit trees, the main pollen Boomsma, 1989; Danforth, 1990; Helms, 1994). and nectar source for nesting females (MaÂ rquez et al., 1994). In this study, provision weight and body weight throughout development and overwintering were measured in males Methods and females of a megachilid solitary bee, Osmia cornuta (Latreille), to determine whether adult body weight is an Bee population and rearing methods acceptable progeny production cost estimator in this species. The following questions were asked: Are there Bees used in the study were derived from drilled nest- differences between sexes in food weight=body weight blocks placed at several sites in Vall d'en Bas (Girona, NE conversion? If so, during which stage do these differences Spain) at the beginning of March 1994. Nest-blocks were occur? Do estimates of production cost ratio (cost of solid wooden blocks with25 drilled holeswitha paper straw producing a female=cost of producing a male) vary when (15 cm long, 8 mm inside diameter) inserted in eachhole. calculated using male and female adult body weight (both Newly plugged straws (straws withplugs still wet) were freshand dry) versus weightof provisions allocated to each removed each week and taken to the laboratory, where sex by the parental generation? they were dissected using a razor blade. Within each nest, Sometimes, for instance in field studies, bothmeasures cells with unhatched eggs were dated assuming an approxi- (adult body weight and provision weight) are difficult to mate cell production of half a cell per day (Vicens, 1997). obtain. At other times, the two measures are simply not Eggs were sexed, based on cell position and provision size available, for instance in individuals failing to reachtheadult within the nest (Bosch, 1994b). The sex of each individual stage or in pinned specimens. In these cases, other body was confirmed in later developmental stages. measurements (intertegular span, wing length, head width) Eggs from 13 female and 24 male cells were removed using (Cane, 1987; Rust, 1991; Roulston & Cane, 2000) or related a pair of forceps and weighed. The provisions of these cells were measures (cell length or volume, cocoon weight, faeces weight) also weighed immediately after nest dissection (fresh weight) are used as acceptable substitutes of body weight (Trivers & and later dried (110 C for 24 h) and reweighed (dry weight). Hare, 1976; Helms, 1994). A second objective of this study was The contents of another 103 female and 93 male cells to evaluate the validity of these measures as estimators of (provision egg) were placed individually in clay blocks provision weight and adult body weight in O. cornuta. (Torchio & Bosch, 1992), and kept in an incubator at 20:30 C (12:12 hthermoperiod)and 50±70 % RH throughout development. On 31 October, when all individuals had Biology of Osmia cornuta reached the adult stage, bees (in their cocoons) were trans- ferred to a refrigerator for a wintering period of 138 days Osmia cornuta is a spring-flying bee that nests in pre- at 3 C. established cavities, where it builds a linear series of cells provisioned witha pollen±nectar loaf and separated by mud partitions (TaseÂ i, 1973; Vicens et al., 1993). When offered Weight loss throughout the life cycle a choice of cavity dimensions, O. cornuta females prefer to nest in 8-mm diameter holes (Bosch, 1994a). Fertilised The developmental stage of individuals placed in clay (diploid) eggs produce female progeny and are usually laid blocks was checked every other day. After cocoon spinning, on larger provisions in the innermost cells within a nest cocoons were placed in clear gel capsules and x-rayed every cavity, whereas unfertilised (haploid) eggs produce male pro- other day to detect pupation (Stephen & Undurraga, 1976). geny and are allocated smaller provisions in the outermost Because the pupal period of O. cornuta lasts about 30 days # 2002 The Royal Entomological Society, Ecological Entomology, 27, 129±137 Body size in a solitary bee 131 (J. Bosch, unpublished), all individuals were assumed to Simple correlation and simple linear regression have reached adulthood at 45 days after pupation. Adult- (Y A BX) were used to analyse the relationship between hood was confirmed for each individual at the end of weight variables, and ANCOVA's slope homogeneity test was the wintering period. Each individual was weighed at the used to establishparallelism between regression lines.

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