Variation in Weight and Postdiapause Development Among Groups of Alfalfa Leafcutting Bees, Megachile rotundata (F.), Reared in Different Locations

Bee Science 1:230-236

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steve@pollinationsystems.ca

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Abstract

Postdiapause developmental rates of three groups of Alfalfa Leafcutting Bee, Megachile rotundata (F.), from a common source were compared. In 1990, bees from Manitoba, Canada, were raised in Idaho, California and Manitoba. Diapausing prepupae from these locations were incubated during 1991 in observation cells and development time and weight of adults were recorded. Maturity of the bee pupae was rated from one to seven, defining six distinct development periods. Overall, bees from Manitoba emerged 2.8 d earlier and weighed 3.8 mg more than the bees from Idaho and California. Most of the difference among groups and between the sexes occurred between the prepupal and teneral pupal stages (Period 1). After Period 1, an average of 147 degree-days were required for adult emergence.

Key words: Alfalfa Leafcutting Bee, Megachile rotundata, development, incubation, weight, latitude

Introduction

The Alfalfa Leafcutting Bee, Megachile rotundata (F.), is the primary pollinator for alfalfa seed in the northwestern United States and western Canada. The bee is a Eurasian species thought to have been introduced to North America several times (Stephen and Osgood 1965). In the Northwest, yields of 2200 kg/ha of seed (2000 lb./A) are possible when the leafcutting bee is used as the pollinator (Johansen et al. 1979). A major industry has developed in Canada to provide disease-free bee larvae to seed producers in the United States.

In the management of leafcutting bees, an understanding of bee development during incubation is important for two reasons. First, the timing of insecticide use for parasitoid control is only recommended during the eighth through thirteenth day (Murrell and Gayton 1987). Second, the timing of adult bee release in the field to coincide with alfalfa bloom is critical to obtain maximum yields. Incubation of leafcutting bee prepupae usually begins 3 weeks prior to alfalfa blossoming. Adult emergence can be delayed by cooling the incubator to 15 or 20°C during the final days of incubation (Rank and Goerzen 1982), if blossoming is later than normal because of cool temperatures or if stormy conditions occur when bees are to be released.

Delaying bee emergence occasionally leads to confusion among growers as to when adult emergence will actually occur. Also, there is concern among bee producers that bees from some locations develop at different rates than bees from other locations. The purpose of this study was to determine whether rearing bees from a common source in different locations could modify bee developmental rates and to determine the thermal requirements for progression of bee pupae through the development stages.

Materials and Methods

During 1990, groups of bees from Fisher Branch, Manitoba, Canada, were reared in Fresno, California, U.S.A. (36°45’N, 119°75’W), Midvale, Idaho, U.S.A. (44°30’N, 116°45’W), and Fisher Branch, Manitoba (51°0’N, 97°30’W). Diapausing offspring from these bees were used in this study. The bees from Idaho and Manitoba had one complete and a partial second generation in 1990 while the California bees had two complete and a partial third generation in 1990.

The diapausing prepupal bees from the three locations were held at 4°C for 9 months. Bees from Manitoba and Cailfornia were subsequently shipped to Idaho. Cocoons were then removed from the wood laminate nest material. Cocoons were opened with a razor blade and the prepupae removed with forceps. The prepupae were then placed in compartmented trays, similar to those used by Hobbs and Richards (1977). Trays consisted of a plastic divider of 100 cells (10 by 10) with plate glass taped to the top and bottom. Each cell was 14 by 14 by 9 mm. One-hundred-fifty prepupae from each location were placed in individual cells. The trays were then placed in a common incubator in darkness at a constant 29.0°C and relative humidity between 50 and 60%.

Each insect was checked daily and rated with respect to maturity. A scale was developed to rate morphologically distinct stages in the development sequence. Bees were rated as follows: stage 1 = prepupa (white, legless prepupa)(Figure 1), stage 2 = teneral pupa (white, recently molted pupa)(Figure 2), stage 3 = light-eye pupa (color of compound eye ranges from light pink to tan)(Figure 3), stage 4 = dark-eye pupa (compound eyes are dark red or brown, as are ocelli)(Figure 4), stage 5 = blackening pupa (body with some pigmentation, beginning with dorsal thorax and mandibles)(Figure 5), stage 6 = black pupa (entire body pigmented)(Figure 6), stage 7 = adult (pupal integument shed, wings expanded, light stripes visible on abdomen)(Figure 7).

When each bee reached the adult stage it was placed in a labeled petri dish and frozen. Adult bees were sexed and weighed with a precision of ±0.1 mg after death.

Six postdiapause periods were analyzed statistically, i.e., Period 1 was the number of days from stage 1 to stage 2, Period 2 was the number of days from stage 2 to stage 3, etc. If a period lasted less than 24 hr, the length of the period was scored as 0 d. Analysis of variance was used to test for significant differences (a = 0.05) among locations and between sexes for the postdiapause periods and the weight of the bees. The protected least significant difference method was used to separate the means.

Results

The proportion of pupae surviving to adulthood differed among locations (Chi-square = 13.78; df = 2; P < 0.01). Survival (% ± 95% C.I.) was 90.0 ± 5.0%, 87.3 ± 5.7%, and 75.3 ± 8.0% for the Manitoba, Idaho and California locations, respectively. Among the surviving bees, no interaction between sex and location occurred for any of the periods (F = 0.33 to 2.85; df = 2,361; P > 0.05). The first period ranged from 7 to 20 d in length. For Period 1, males (mean = 10.4 d) developed significantly faster than females (12.0 d)(F = 62.17; df = 1,354; P < 0.0001), and a significant difference among locations occurred (F = 78.16; df = 2,354; P < 0.0001) (Figure 8). Bees reared in Idaho and California required 2.8 d longer than bees from Manitoba in Period 1. Period 2 ranged from 0 to 3 d. No significant difference between males (1.2 d) and females (1.2 d) was observed (F = 0.24; df = 1,354; P > 0.50)(Figure 8), but a significant difference among locations occurred (F = 4.69; df = 2,354; P < 0.01). Bees reared in Manitoba required 0.2 d longer than the Idaho and California bees to progress through Period 2.

The third period ranged from 0 to 4 d. Males (1.8 d) developed significantly faster than females (2.0 d) in Period 3 (F = 6.97; df = 1,354; P < 0.01), and a significant difference among locations was observed (F = 7.95; df = 2,354; P < 0.001)(Figure 8). The bees reared in Idaho and California required 0.3 d longer than the bees from Manitoba in this period. Period 4 ranged from 1 to 5 d in length. In Period 4, no significant difference between males (3.5 d) and females (3.4 d) occurred (F = 1.55; df = 1,354; P > 0.20)(Figure 8). A significant difference among treatments for location was observed (F = 15.10; df = 2,354; P < 0.0001), with bees from Manitoba requiring 0.3 d longer than the bees reared in Idaho and California. The fifth period ranged from 0 to 4 d. No difference between males (1.5 d) and females (1.5 d) occurred in Period 5 (F = 1.32; df = 1,354; P > 0.20)(Figure 8). A significant difference among locations was observed (F = 6.21; df = 2,354; P < 0.01), with the bees reared in California requiring 0.3 d longer than the other locations.
Period 6 ranged from 1 to 5 d in length. Males (2.7 d) developed significantly faster than females (3.2 d) (F = 51.54; df = 1,354; P < 0.0001)(Figure 8). A significant difference among locations occurred (F = 4.06; df = 1,354; P < 0.05), with Idaho bees requiring 0.3 d longer than bees from Manitoba, while California bees were intermediate. For complete postdiapause development, i.e. prepupal to adult stage, no interaction between sex and location occurred (F = 1.39; df = 2,354; P > 0.20). Females took 2.2 d longer than males (F = 105.65; df = 1,354; P < 0.0001). Overall, bees reared in California and Idaho developed significantly slower than the bees from Manitoba (F = 73.32; df = 2,354; P < 0.0001). Manitoba bees emerged 2.8 d earlier than the other locations. It is clear that Period 1 had the most influence on the overall postdiapause development of the bees. No significant interaction between sex and location for bee weight was observed (F = 1.39; df = 2,353; P > 0.05). As expected, female bees weighed significantly more than males (F = 145.34; df = 1,353; P < 0.0001). The females (36.5 ± 0.4 mg, mean ± sem) weighed 7.6 mg more than the males (28.9 ± 0.4 mg). A significant difference among locations occurred for weight (F = 18.81; df = 2,353; P < 0.0001), with bees from Manitoba (36.0 ± 0.7 mg) weighing 3.8 mg more than bees reared in Idaho (32.1 ± 0.5 mg) and California (32.2 ± 0.7 mg). Sex ratios did not differ significantly among locations (Chi-square = 0.49; df = 2; P > 0.50). Overall, females accounted for 60.0 ± 6.5% (95% C.I.) of the adult bees.

Discussion

Richards and Whitfield (1988) reported that 295 degree-days, with a temperature threshold of 15.7°C, were required for 50% adult emergence of the Alfalfa Leafcutting Bee. They found no differences in postdiapause development among four different locations within Canada. In contrast, we found significant differences among three more widely separated locations. Bees from Manitoba required 20.6 d at 29°C to reach 50% adult emergence, or 274 degree-days. Bees reared one season in Idaho or California took 23.3 and 23.4 d, respectively, for 50% emergence. These bees averaged 311 degree-days for postdiapause development.

Pankiw et al. (1980) reported differences among latitudes of Alfalfa Leafcutting Bees; however, as Richards and Whitfield (1988) point out, storage conditions varied considerably in their study which prevents valid comparisons. According to our study, Alfalfa Leafcutting Bee development can be modified by changes in latitude. We observed differences among locations of up to 12% of the postdiapause developmental period.

Why the leafcutting bees reared in California and Idaho developed slower than bees from Manitoba is not clear. It is noteworthy that these bees performed “best” in their home environment, with regard to weight and development time. In general though, wild bees tend to emerge earlier in northern latitudes to fully exploit the intense bloom in the relatively short season. We speculate that a short postdiapause period might be advantageous for the Alfalfa Leafcutting Bee in northern latitudes. Our study indicates that Alfalfa Leafcutting Bees lengthened their postdiapause development with only one season of exposure to more southern latitudes.

Adult bee weight has been shown to vary with nest hole size (Klostermeyer et al. 1973), emergence day (Rothschild 1979), rearing temperature (Tepedino and Parker 1986), and diapausing versus non-diapausing bees (Tepedino and Parker 1988). We found that bee size can also vary with location. Klostermeyer et al. (1973) demonstrated that provision size affected the size of the adult bee and Bohart and Nye (1971 unpublished data) found indications that the amount of nectar added to the pollen mass affected bee size. Thus, it is possible that smaller provisions or less nectar was provided to the Idaho and California bees. Because all locations were from alfalfa seed fields, lack of adequate pollen would not seem to be a factor. Perhaps optimal foraging conditions were longer lasting or more nectar was available in Manitoba compared to California and Idaho.

Differences between the sexes in developmental time occurred during the first, third and sixth postdiapause periods. Period 1 accounted for 68% of the overall difference between the sexes. Development time in the first period also accounted for virtually all (99%) of the difference among the locations. Thus, the time required to terminate diapause and initiate pupation is subject to the most variation among populations and between the sexes. To aid bee producers who are uncertain of when adult bees will emerge, some guidelines have been developed from this data set to determine when adults will emerge based on pupal morphology (Table 1). Because most variation occurs in the first postdiapause period, the data for periods two through six were combined across locations to obtain means. From the table, for example, if the average stage of a sample of bees is dark-eye (4), then an additional 7.9 d are still required at 29°C for 50% adult emergence (Table 1), which equates to 105 degree-days. Likewise, if most bees are in the dark-pupa stage (6), then the average adult emergence should be in 3 d at 29°C or 40 degree-days.

Acknowledgments

We thank Carolyn Nyberg for technical assistance. Thanks are also due to Daniel Mayer and Keith Dorschner for reviewing the manuscript and offering suggestions. This research was funded by grants from USDA-ARS, USDA-APHIS, FMC Corporation, and the Idaho Alfalfa Seed Commission. Scientific paper no. 91769, University of Idaho Experiment Station.

References

HOBBS, G.A., AND K.W. RICHARDS. 1977. An examination of methods used in western Canada to estimate populations of alfalfa leafcutter bees. Bee World 59:67-70.

JOHANSEN, C., C. BAIRD, R. BITNER, G. FISHER, J. UNDERRAGA AND R. LAUDERDALE. 1979. Alfalfa seed insect pest management. Western Regional Extension Publication 0012.

KLOSTERMEYER, E.C., S.J. MECH, JR. AND W.B. RASMUSSEN. 1973. Sex and weight of Megachile rotundata (Hymenoptera: Meagachilidae) progeny associated with provision weights. J. Kansas Entomol. Soc. 46:536-548.

MURRELL, D. AND D. GAYTON. 1987. Alfalfa seed and leafcutter bee production in Saskatchewan. Saskatchewan Agriculture, Soils and Crops Branch. 10 pp.

PANKIW, P., J.A.C. LIEVERSE, AND B. SIEMENS. 1980. The relationship between latitude and the emergence of alfalfa leafcutter bees, Megachile rotundata (Hymenoptera: Megachilidae). Can. Ent. 112:555-558.

RANK, G.H. AND D.W. GOERZEN. 1982. Effect of incubation temperatures on emergence of Megachile rotundata (Hymenoptera: Megachilidae). J. Econ. Entomol. 75:467-471.

RICHARDS, K.W., AND G.H. WHITFIELD. 1988. Emergence and survival of leafcutter bees, Megachile rotundata, held at constant incubation temperatures (Hymenoptera: Megachilidae). J. Apicultural Res. 27:197-204.

ROTHSCHILD, M. 1979. Factors influencing size and sex ratio in Megachile rotundata (Hymenoptera: Megachilidae). J. Kansas Entomol. Soc. 53:392-401.

STEPHEN, W.P. AND C.E. OSGOOD. 1965. The induction of emergence in the leaf-cutter bee Megachile rotundata, an important pollinator of alfalfa. J. Econ. Entomol. 58:284-286.

TEPEDINO, V.J. AND F.D. PARKER. 1986. Effect of rearing temperature on mortality, second-generation emergence, and size of adult in Megachile rotundata (Hymenoptera: Megachilidae). J. Econ. Entomol. 79:974-977.

TEPEDINO, V.J. AND F.D. PARKER. 1988. Alternation of sex ratio in a partially bivoltine bee, Megachile rotundata (Hymenoptera: Megachilidae). Ann. Entomol. Soc. Am. 81:467-476.

Table 1. Requirements, in days and degree-days, for average adult emergence of Alfalfa Leafcutting Bees from pupal stages two through six. Parma, Idaho, 1991.

Pupal Stage	Days to 50% Emergence	Degree-Days to 50% Emergence
2	11.0	147
3	9.8	130
4	7.9	105
5	4.5	60
6	3.0	40

Incubation temperature = 29°C.

Figure Captions 
Figure 1-7. Stages 1-7 of Alfalfa Leafcutting Bee postdiapause development.

Figure 8. Days required for Periods 1-6 of Alfalfa Leafcutting Bee males and females from Manitoba (MAN), Idaho (IDA), and California (CAL), subsequently reared in Idaho. Period 1 = prepupa to teneral pupa, Period 2 = teneral to light-eye pupa, Period 3 = light-eye to dark-eye pupa, Period 4 = dark-eye to blackening pupa, Period 5 = blackening to black pupa and Period 6 = black pupa to adult. Vertical lines represent SEM. Locations with different letters are significantly different by the protected LSD method.

Figure 9. Compartmented trays with immature leafcutting bees. Stages of bees in row 1 (left to right): 4, 4, 4, 4; row 2: 4, 3, 4, 2; row 3: 1, 3, 4, 2.

Figure 10. Stages of bees in row 1 (left to right): 6, 6, 6, 6; row 2: 6, 5, 5, 4; row 3: 4, 5, 6, 4.