An assessment of Vicia faba and Trifolium pratense as forage crops for Bombus hortorum

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1 An assessment of Vicia faba and Trifolium pratense as forage crops for Bombus hortorum B. BROWN* AND R. R. SCOTT Department of Entomology, P.O. Box 84, Lincoln University, New Zealand R. P. MACFARLANE DSIR Plant Protection, Private Bag, Christchurch, New Zealand ABSTRACT The suitability of Vicia faba and Trifolium pratense "Pawera" as forage for Bombus hortorum was investigated. Flower density, nectar volume per flower and pollen production were measured and the latter two compared with nutritional requirements of the bees at the measured bee population levels. Bombus hortorum was the numerically dominant bee species in V. faba compared with Apis mell$era in the later flowering T. pratense. Both crops supplied the nectar and pollen requirements of B. hortorum. Keywords: Bombus hortorum, bumble bee, Vicia faba, broad beans, Trifolium pratense, red clover, nectar, pollen. INTRODUCTION Bombus hortorum (L.), a long-tongued bumble bee species, is useful in increasing the seed yield of red clover crops, especially the late flowering tetraploid Trifolium pratense (L.) "Pawera" (Macfarlane & Griffin 1985). To maintain B. hortorum populations until T. pratense "Pawera" flowers, a succession of crops are needed to supply nectar and pollen *Current address: DSIR Plant Protection, Private Bag, Christchurch, New Zealand

2 New Zealand Entomologist, 1992, Vol. 15 requirements. The young queen requires protein from pollen for the full development of her ovaries, and nectar to supply energy for foraging. Larvae derive all of the protein, fats, vitamins and minerals necessary for their growth from pollen. Workers emerging from cocoons require pollen initially, but then exist solely on nectar. Energy from nectar is required by foragers to maintain their thoracic temperature between C. There are also high energy requirements when the nest is incubated (Heinrich 1979). Kcia faba L. (broad bean) and T. pratense "Pawera" (red clover) were chosen for this investigation because B. hortorum forages heavily on these crops (Macfarlane, pers. comm.). The objectives of this investigation were to assess if V. faba and T. pratense "Pawera" could supply the nectar and pollen needs of B. hortorum, to determine the species composition of the foragers on both crops, and to determine the factors that influence foragers on them. METHODS The 0.2 ha of broad beans was planted on 15 August 1988 on the DSIR Experimental Farm at Lincoln. Plant density was estimated on 24 November 1988 by counting plants in 15 permanent 0.25 m2 plots. The number of racemes per plant and open flowers per raceme in each plot were counted twice weekly from 24 November to 23 December The 0.36 ha of T. pratense, 4 km from Lincoln at Greenpark, was in its second season. The number of racemes per hectare was estimated weekly from 1 February 1989 until 7 March 1989, in 10 randomly located 0.25 m2 plots. On 20 randomly selected racemes in each plot the number of flowers and the number of open flowers were counted. The nectar secretion of both crops was measured from 10 flowers on randomly selected racemes, which were bagged for 24 hours before nectar was extracted with a 10 pl syringe. Bean nectar volume was measured 5 times per day on 4 days between 28 November and 9 December Clover nectar volume was measured 5 times daily on 5 days between 1 February 1989 and 7 March In both crops, the bee population was estimated in two permanent 30 m x 20 m strips, as described by Macfarlane et al. (1990). Temperature and relative humidity were recorded continuously on a thermohygrograph placed in a Stevenson Screen at crop level. Wind speed and cloud cover were recorded each time data were collected. To assess the pollen production, 14 immature, unopened flowers, were collected randomly from each crop, and placed in individual 10 ml vials, stoppered with moist cottonwool and incubated for 36 hours at 30 C to induce dehiscence (Traynor 1981). For pollen collection, "Tween 20" solution (0.1 %) was added to each vial, which was then agitated. The solution was removed by syringe and filtered through a 0.5 pm millipore filter. Pollen grains in 10 separate microscope fields of view were counted. Means were compared by standard analysis of variance methods. RESULTS Floral Biology and Bee Activity in Vicia faba Flowering lasted 39 days. Peak flowering, defined as the highest estimate of open flowers per hectare, occurred on 1 December. There were 28.8 million racemes and 13.5 million open florets per hectare (Table 1). The greatest mean nectar volume per flower (0.86 pl) occurred on 5 December, 4 days after peak of flowering. The greatest standing nectar crop (7.1 litredha) occurred on 1 December when the number of racemes per hectare and the number of open flowers per raceme were at their maxima (Table 2). The time of day when nectar samples were collected did not affect volume (p < 0.9) but there were differences in nectar volume on different days (p < 0.001). Bee numbers and temperature showed no relationship (r2=0.074). Nectar volume and temperature showed no relationship (r2 = 0.04) and bee numbers and nectar volume also showed no relationship (r2 = 0.159). Total bee numbers also peaked (4831 bees) on 1 December. Over 50 % of the B. hortorum population was collecting pollen at each observation (Table 3). Apis rnellifera L. used the crop exclusively to collect nectar. The estimated number of pollen grains varied from 1.12 x lo5 to 7.87 x lo5 and differences between flowers were highly significant (p < 0.001).

3 44 New Zealand Entomologist, 1992, Vol. 15 Table The number of Viciafaba racemes and open flowers per hectare DSIR Farm, Lincoln, Estimated number Estimated number racemeslha flowers openlha ( x lo6) ( x lo6) 24 November 28 November 1 December 5 December 9 December 12 December 16 December 23 December Table 2: Floral Characteristics for Vicia faba between 28 November 1988 and 9 December 1988, DSIR Farm, Lincoln. Average Standing Nectar nectar volume/flower Flowerslha Croplha (PI) ( x lo6) (llha) 28 November 1 December 5 December 9 December Floral Biology and Bee Activity in Trifolium prateme "Pawera" Flowering lasted 42 days. Peak flowering occurred on 8 February approximately 1 week after flowering began, with 83 million open flowers per hectare available for nectar and pollen removal (Table 4). In contrast with V. faba, the number of flowerslraceme and open flowers contributed to this peak rather than the number of racemeslha. The greatest number of racemeslha occurred later on 24 February, when only 15% of the flowers were open. At peak flowering, there were approximately 11 1 flowers per raceme, with 57 % of these open (Table 4). Mean nectar volumes ranged from 0.06 to 0.27 pllflower. The differences between means were significant (p< 0.05). While mean flower nectar volumes were highest on 1 February, before peak flowering, the maximum standing volume of nectar (15.7 litredha) occurred when the numbers of open flowers peaked (Table 5). Total bee numbers peaked at 7885 bees on 24 February, 16 days after the open flower peak (Table 6). However, the B. hortorum numbers were highest on 17 February when this species comprised 29.5% of the bee fauna and this was the only occasion when B. terrestris (F.) was observed. From that date, A. mellifera was the,major bee faunal component. Table 3: Bee population composition (species and numberlhectare) and estimated percentage of population carrying pollen on Vicia faba at Brown's paddock, DSIR Farm, Lincoln, Bombus Bombus Bombus Apis hortorum terrestris ruderatus mellifera With With With With pollen pollen pollen pollen No./ha (7%) No./ha (%) No./ha (%) No./ha (%) 28 November December December December

4 New Zealand Entomologist, 1992, Vol. 15 Table 4: Trifolium pratense "Pawera" flower population data, Greenpark, Lincoln, 1989 Estimated numberestimated number Estimated number of racemes per of flowers per Average % of flowers open hectare ( x lo6) raceme flowers open hectare ( x 10 Ber ) 1 February February February February March The effect of temperature on bee numbers varied between species. Apis mll$ia numbers increased as temperature increased (r2 = 0.64). Temperature had no effect on B. ruderatus (F.) numbers (r2 = 0.195) or on B. hortorum numbers (r2 = 0.044). Temperature had some effect on nectar volume (r2 = 0.288). Nectar volume had no effect on bee numbers. The correlation coefficients were , and for B. hortorum, B. ruderatus and A. mellifera respectively. No other measured environmental factor showed any relationship with either bee numbers or nectar volume. The estimated number of pollen grains was 3.3 x lo4-1.3 x lo5 per flower and between flower differences were significant (p < 0.001). Analysis of variance showed that there were highly significant differences in the number of pollen grains produced per flower for both V. faba (p< 0.001) and T. pratense "Pawera" (p < 0.001). Values ranged from 112,000 to 787,000 grains per flower for V. faba and from 33,000 to 129,000 grains for T. pratense "Pawera". DISCUSSION There was little difference in the total flowering duration of the two crops. The red clover crop took 1 week from the beginning of flowering to reach its maximum number of flowers open per hectare, while the bean crop took 2 weeks. The beans' peak population of racemes per plant coincided with the maximum number of open flowers per raceme and maximum bee numbers. This is the ideal situation for the grower because, if most of these open flowers can be pollinated, the seeds ripen together and a single harvest carried out. In contrast, red clover had its greatest proportion of open flowers early in the flowering period, and the peak population of racemes occurred later. This observation Table 5: Floral Characteristics for Trifolium pratense "Pawera" between 1 February and 7 March, 1989 at Greenpark, Lincoln. 1 February 8 February 17 February 24 February 7 March Average nectar volume per flower Flowers open per hectare Standing crop of nectar (a9 ( x lo6) (Ifha) Table 6: Bee population composition (species and number per hectare) on Trifolium pratense "Pawera" at Greenpark, Lincoln, Bombus Bombus Bombus Apis hortorum terrestris ruderatus mellefera 1 February February February February March

5 New Zealand Entomologist, 1992, Vol. 15 Table 7: Estimated nectar volume per hectare removed by Bombus hortorum and Apis mellifera from Vicia faba and Trifolium pratense compared with the estimated volume available per hectare in summer B. hortorum A. mellifera Estimated field nectar harvest nectar harvest Total collected nectar volume (llha) (llha) (llha) (llha) Vicia faba 28 November December December December Trifolium pratense 1 February February February February March is consistent with previous evidence that major seed yield is produced during early and main flowering (Clifford 1979). The 2 week build-up to peak flowering in the beans gave B. hortorum a greater chance to find and identify the crop as a major food supply. The shorter time to peak flowering in red clover may mean that B. hortorum was still learning to manipulate the new food source and developing foraging routes to it. There were approximately six times as many red clover flowers open at peak flowering compared with beans, but just over double the amount of nectar available because of the lower production per flower. However, the important feature is whether or not the nectar production met bumble bee needs. Macfarlane & van den Ende (1990) considered that a foraging bumble bee might remove twenty 50 p1 nectar loads per day. Apis mllifera is estimated to remove eleven 30 pl loads per day (Dadant 1975). Of the estimated volumes removed by bumble bees and honey bees, only on 28 November did the nectar demand exceed the estimated supply (Table 7). Obviously, the foraging bees can only remove what nectar is present. Consequently, either the volume of nectar available was underestimated, the bumble bees were removing less than 50 p1 per trip, fewer than 20 trips were being made each day, the bee population was overestimated, or a combination of these factors operated. Honey bee numbers were such that the amount of nectar they removed was not significant. For red clover, the estimated volumes of nectar removed on 24 February and 7 March exceeded the estimated supply (Table 7). The numbers of honey bees present exacerbated the shortfall. An infestiation of leafcurl plum aphid, Brachycaudus helichrysi (Kaltenbach), may have also reduced nectar production. Heinrich (1979) estimated that a bumble bee colony's total daily pollen intake could be up to 20 g, with individual worker loads of g. In this investigation, pollen grain weights were not determined, but weight is not the sole determinant of pollen value to bees. Stace (1987) emphasised the importance of protein content and amino acid composition. Another factor is the digestibility which may be affected by the grain's surface characteristics; both K faba and T. pratense produce smooth pollen grains. If the mean pollen production for beans is taken as 340,000 grains and a bumble bee, visiting 50 flowers to collect nectar, picks up half the available grains, then 8.5 million grains would be collected per load. Assuming no flowers are re-visited, and 20 loads are transported per day then if, as observed, 56% of the B. hortoncm population was collecting pollen, 3.4 x 10" grains would be collected. This is significantly less than the estimated total production of 1.5 x 1012 grains. That the percentage of the population collecting pollen was always 50% or greater suggests that the beans were satisfying the bee colonies' protein demand.

6 New Zealand Entomologist, 1992, Vol Fewer bumble bees were observed collecting pollen on the red clover. During February and March brood rearing is past its peak and there is less demand for pollen. The presence of Bombus hortorum males observed on the crop supports this conclusion, because males and queens are only produced once the colony reaches maturity (Macfarlane 1984). The estimate of pollen demand (2.5 x 10'' grains) at the time of peak bumble bee population is again significantly less than the pollen available (1.98 x 1012 grains). There were almost twice as many bumble bees observed on the beans (4748) as there were on the red clover (2407), of which 3582 and 1494 respectively were B. hortorum. Various factors may have caused these differences. There may have been a lack of secure nest and hibernation sites or spring forage near the red clover; more spring forage may have been available in neighbouring domestic gardens near the bean paddock; or the Canterbury drought may have affected nectar production (resulting in colony collapse before the red clover flowered). A further possibility is that bumble bee density in the environs of the two crops may have been comparable, but the bumble bees did not frequent the red clover. This may have been a result of interspecific competition since honey bee numbers were much higher in absolute terms on the red clover. However, other flowers may have been more attractive. CONCLUSIONS Both the I.: faba and the T. prateme "Pawera" crops produced sufficient nectar and pollen to meet the needs of B. hortorurn. This species was the dominant forager on V. faba, while A. rnellifera dominanted on T. pratense. Bombus hortorum was the most common bumble bee species on the red clover. Temperature had little effect on bee numbers on the beans but on red clover increased temperature resulted in increased A. rnellifera numbers. REFERENCES Clifford, P. T. P., 1979: The effect of closing date on seed production of red clover cultivars. New Zealand Journal of experimental agriculture 7: Dadant, C. P., 1975: The Hive and the Honey Bee. Hamilton, Dadant & Sons. 740 pp. Heinrich, B., 1979: Bumble Bee Economics. Cambridge, Harvard University Press. 247 pp. Macfarlane, R. P., 1984: Bees and pollination. Pp In : Scott, R. R., (Ed.), New Zealand Pest and Beneficial Insects. Lincoln, Lincoln University College of Agriculture. Macfarlane, R. P.; van den Ende, H. J., 1990: A summation of factors influencing red clover (Trifolii pratense) seed yield in New Zealand. DSIR Plant Protection internal report. 41 pp. Macfarlane, R. P.; van den Ende, H. J.; Griffin, R. P., 1990: Pollination needs of "Grasslands Pawera" red clover. Proceedings of the International Symposium of Pollination 6: in press. Macfarlane, R. P.; Griffin, R. P., 1985: Assessment ofbumble bees and honey bees as pollinators of red clover. Proceedings ofthe 4th Australasian Conference on Grassland Invertebrate Eco1o.g: Stace, P., 1987: Pollen quality, honey production and strong bees. Australian beekeeper 88: Traynor, J., 1981: Use of a fast and accurate method for evaluating pollen production of alfalfa and almond flowers. American beejournal (January):