Improving Alfalfa Leafcutting Bee Incubation and Emergence

Methods

Results

Table 1

Table 2

Table 3

Table 4

Discussion

Introduction and Objectives 

Every year alfalfa seed growers spend $160 per acre or more on cells of Megachile rotundata, the alfalfa leafcutting bee, usually from Canadian suppliers, to pollinate their seed fields (Rimbey and Smathers 1999). Bees from Canada usually have a much greater percentage of live cells (about 75%) than do bees reared in the US (about 45%), based on x-ray analysis during the winter while bee larvae are overwintering (Fairey et al. 1996). However, emergence rates in the field may be significantly lower. Larvae may look healthy in x-rays, yet not successfully emerge. The live larvae that do not successfully emerge in the spring represent a financial loss to the grower. If we knew how large those losses were, and why they were occurring, we might be able to reduce them. The objectives of this study were to compare actual emergence rates of loose cell alfalfa leafcutting bees with expected emergence rates based on x-ray analysis and to determine whether mortality is greater in males or in females (the latter being more important for pollination).

Methods

In April 1999, six growers in Idaho and eastern Oregon provided us with 9 samples of bee cells from their Canadian purchases. We x-rayed all samples, and obtained an initial estimate of the % of live prepupae in the sample. Cells containing live prepupae were placed in two rearing dishes. One dish was incubated immediately to determine % emergence and sex ratio of emerged bees. The second dish was stored in controlled temperature chambers in our laboratory until late May, when it was incubated for % emergence and sex ratio. A second sample of cells was obtained from the same bee lots in May, just as the growers were beginning to incubate their bees. These cells were kept in the grower’s cold storage facility during the last month of winter storage. These cells were x-rayed, and a sample of live prepupae were placed in rearing dishes to incubate for % emergence. Finally, in July after bees had been released in the field, we collected samples of emerged cells from the original bee lots. We counted cells that had emerged, and opened cells without emergence to see what remained.

In 2000, five growers in Idaho provided us with 7 samples of bee cells from their Canadian purchases, one sample right before incubation (May-June), and one sample after emergence (July). The first sample was x-rayed to obtain an initial estimate of the percent live prepupae in the sample. Cells containing live prepupae were placed in rearing dishes to determine percent emergence and the sex ratio of the bees. The second sample of cells was obtained after bees had been released in the field. As in 1999, we counted cells that had emerged, and opened cells without emergence to see what remained.

Results

Among the 1999 samples, there were 81% ± 6% (mean ± sd) live prepupae in the original April samples and 79% ± 6% live prepupae in the grower samples from May (Table 1). This was not a significant difference. The 2000 samples that were acquired before incubation contained 80 ± 4% live prepupae, not significantly different from 1999 samples.

Among the 1999 cells that appeared to have live larvae, there was little change in mortality during the last month of cold storage (Table 2, bottom row). In the initial April sex ratio test, 97% ± 3% of the cells that appeared to contain live prepupae actually hatched. Live cells stored in our facility had 95% ± 3% emergence, and cells stored in grower facilities had 94% ± 3% emergence.

The April 1999 samples averaged 35% ± 5% female (Table 2). This is the percent female bees that is reported to be average for Canadian loose cell (Baird and Bitner 1991). The percentage of females rose to an average of 46% ± 7% in our facilities, and 44 ± 10% in grower facilities in May. The May samples from the lab and grower facilities were not significantly different, but they did differ significantly from the April sample. In May, 2000 the female live count averaged 46 ± 2%, again not significantly different from the May, 1999 samples. The increase in % females in May is likely due to disproportionate male mortality. Among the dead pupae and adults in sex ratio tests, 87 ± 8% were males in May, 2000 samples. However, we do not understand why there was so little mortality overall between the April and May samples, and yet a significant increase in percent females in the last month of winter storage.

Only 68% ± 8% of the cells actually emerged when released in the field in 1999 (Table 1). This represents an average loss of 14% ± 8% live bees from the entire sample, or 16% ± 10% loss of live cells. In 2000, only 62 ± 3% of the cells emerged. This is a marginally significant difference from the % emerged in 1999. The average loss was 22 ± 5% of live cells, greater than last year’s average of 16 ± 10%.

Samples varied greatly both years in proportions of unemerged cells with dead adults (28% ± 24% in 1999; 25 ± 11% in 2000), pupae (57% ± 23% in 1999; 63 ± 16% in 2000) and larvae (15% ± 10% in 1999; 12 ± 10% in 2000), though the averages were similar both years (Table 3). The high percentage of pupae and adults among the dead cells in both years indicates that most mortality occurred after incubation began. Dead pupae would have died before the cells were brought to the field. Adults may have died in the incubator, or some may have died in the field.

Among the dead pupae and adults, males were 82% ± 6% in 1999 and 78 ± 10% in 2000 (Table 4).  These percentages are not significantly different. We estimate that the loss during cold storage, incubation and/or emergence was about 9% ± 7% of the total live female cells and about 20% ± 13% of the live male cells in 1999. The loss between incubation and emergence we estimate to be about 10 ± 5% of the total live female cells and 31 ± 6% of the live male cells. Therefore, male loss was somewhat higher in 2000 than in 1999. Disproportionately high male mortality may explain increases in % females in our sex ratio tests.

Discussion

Mortality during incubation may have been caused by insufficient energy reserves to complete development in some bees. The degree to which energy reserves are depleted may determine whether the prepupa, pupa or adult dies. Consistent with this hypothesis is the finding of Tepedino and Torchio (1982), that smaller Blue Orchard Bees (Osmia lignaria) were more likely to die overwinter than large bees. Because males are the smaller sex, a disproportionate number of them die. This appears to be true for the alfalfa leafcutting bee as well. In O. lignaria, females produced late in the season were smaller than females produced early in the season, and late season females were more likely than late season males to die overwinter. This is probably true for the alfalfa leafcutting bee, although we had no information on when cells were constructed (early vs. late season) in this experiment.

The energy reserve hypothesis is also consistent with Bosch et al. (2000), who found that the date of adult emergence, and longevity of O. lignaria are correlated with fat body reserves and temperature regime during development and overwintering. Energy reserves for overwintering are stored in the fat body of insects. As in O. lignaria, reduced energy reserves may be caused by prolonged exposure to warm fall temperatures (Stephen 1995) or by a reduced provisioning rate in the latter part of the season when pollen and nectar are low and aging reduces the efficiency of female bees (Torchio and Tepedino 1980,Tepedino and Torchio 1982).  Bosch et al. (2000) have demonstrated that reduced vs. full conditions of the fat body can be observed in x-ray images of adult O. lignaria. If this is also true of M. rotundata, then x-rays of emerged adults in sex ratio tests at various times during overwintering and/or incubation could be used as a screening test to predict what proportion of bees are likely to survive in grower samples. Comparisons could be made between prepupae from cells created early in the season vs. late in the season, and cells that are left in warm temperatures in the fall vs. moved to cold storage early. Quality and quantity of provisions could be an important factor in affecting overwintering mortality as well as pollen balls (Strickler and Freitas 1999) and chalkbrood (Goettel et. al. 1993, Vandenberg 1994), and thus affecting the sustainability of bee populations. Adjusting bee populations to the level of available flower resources should be a high priority for growers interested in improving bee yields from their fields.

References:

Baird, C. R. and R. M. Bitner. 1991. Loose cell management of leafcutting bees. In Alfalfa Seed Production and Pest Management, Western Regional Extension Publ. 12. 7pp.

Bosch, J., W.P. Kemp, and S. S. Peterson. 2000. Management of Osmia lignaria (Hymenoptera: Megachilidae) populations for almond pollination: Methods to advance bee emergence. Environ. Entomol. 29:874-883.

Fairey, D.T., L.P. Lefkovitch, J.A.C. Lieverse, K. Strickler, and D.W. Lundahl. 1996. A Comparison of X-ray Results From Three Labs. Newsletter of the Idaho Alfalfa Seed Commission. 

Goettel, M.S., J. D. Vandenberg, G. M. Duke and G. B. Schaalje. 1993. Susceptibility to chalkbrood of alfalfa leafcutter bees, Megachile rotundata, reared on natural and artificial provisions. J. Invert. Pathol. 61:58-61.

Rimbey, N. R. and R. L. Smathers 1999. 1999. Southwestern Idaho Crop Costs and Returns Estimate: Alfalfa Seed. University of Idaho Cooperative Extension System publication EBB2-AS-99. 4pp.

Stephen, W.P. 1995. How and where were they raised? – Critical management considerations in Megachile. 26^th Northwest Alfalfa Seed Growers Winter Seed School. 27-35.

Strickler, K. and S. Freitas, 1999.  Interactions between floral resources and bees (Hymenoptera: Megachilidae) in commercial alfalfa seed fields.  Enviro. Entomol. 28:178-187.  (See summary in slide show 2)

Tepedino, V.J. and P. F. Torchio. 1982. Phenotypic variability in nesting success among Osmia lignaria propinqua females in a glasshouse environment (Hymenoptera: Megachilidae). Ecological Entomol. 7:453-462.

Torchio, P. F. and V. J. Tepedino. 1980. Sex ratio, body size, and seasonality in a solitary bee, Osmia lignaria propinqua Cresson (Hymenoptera: Megachilidae). Evolution 34:993-1003.