- 11 Jun 2014
This blog was created as part of the original Valuing Nature Network (2011 - 2014)
It was announced yesterday that English arable farmers will be able to fulfill their Ecological Focus Area requirement under the new Common Agricultural Policy by planting leguminous crops - peas, beans and other pulses – on 5% of their land. Ecological Focus Areas are one of the new compulsory greening measures, originally conceived as areas of land left out of production, or with low intensity production, managed or left alone to benefit wildlife. The inclusion of standard, conventionally managed peas and beans as Ecological Focus Areas is a long way from that, and to many a real disappointment.
The Government argues that these are flowering crops, so they will help bees and other pollinators, by providing nectar and pollen. It's a win-win. Farmers can produce food and help wildlife at the same time, right?
For mass-flowering crops in general (including oilseed rape), this is a valid argument. It has been the subject of a number of careful research studies in the last ten years. The picture that emerges is that oilseed rape seems to benefit early flying solitary bees, and to increase numbers
of bumblebee workers and colonies, but not necessarily the number of queens
those colonies produce for the following year. There is a much smaller amount of evidence on the effects of the leguminous crops included in Ecological Focus Areas. What there is shows conflicting evidence on bumblebees, and no clear benefit for solitary bees.
The effects of oilseed rape
In 2003, a study showed that increased amounts of oilseed rape in German landscapes was associated with increased numbers of bumblebees (Westphal et al. 2003). More detailed study in the same area using boxed bumblebee colonies showed that this effect did not translate into increased number of queens produced at the end of the year (Westphal et al. 2009) - in other words there was no effect on the reproductive success of bumblebees. This is probably because the oilseed rape adds a pulse of floral resource very early in the season (mostly in May), and the bumblebee colonies need to feed for a number of months after the rape flowers are over.
The increase in bumblebee worker numbers has since been found by other studies. For example Kovacs-Hostyanszki et al. (2013) found more bumblebees in German hedgerows just after oilseed rape flowering, compared to landscapes without oilseed rape. In Bavaria, sunflowers were visited by more bumblebees where there was more oilseed rape around in the spring (Reidinger et al. 2014).
It has also been shown that the increased numbers of short-tongued bumblebee workers resulting from the oilseed rape rush is associated with a decrease in numbers of long tongued bumblebee species visiting red clover (Diekotter et al. 2010), suggested to be a result of extra competition for nectar. The short tongued bumblebees, such as the buff-tailed Bombus terrestris, can’t reach the nectar in red clover and chew holes in the back to rob it without pollinating.
In the last couple of years, three studies, all in Germany, have shown that oilseed rape in the landscape had a positive effect on numbers of nesting red mason bees Osmia bicornis (Jauker et al. 2012; Holzschuch et al. 2013), or the number of species (but not abundance) of cavity nesting bees (Diekotter et al. 2014). Another German study showed no effect of oilseed rape on solitary bees caught in pan traps (Kovacs-Hostyanszki et al. 2013). These solitary bee species fly and forage for a much more limited time than bumblebees, coinciding with the rape flowering period.
So oilseed rape fields can benefit solitary bees that nest at the time it flowers. But neither sunflower nor oilseed rape are included in the Ecological Focus Areas list, and rape is very widely grown in the UK anyway. What of the leguminous flowers supported as Ecological Focus Areas? Unlike oilseed rape flowers, these tend to have larger flowers, with less accessible nectar and pollen, mostly used by larger bees or bumblebees.
Much less evidence for leguminous flowering crops
One four year study specifically looked at the effect of field bean Vicia faba, on bumblebee numbers (all species) in England (Hanley et al 2011). It found more bumblebees in margins around bean fields than around wheat fields during flowering. But the effect was short-lived and could not be detected two weeks after the beans had flowered. This implies that bean fields are used by bumblebees, but as for oilseed rape, the fields may not enhance their long term success.
In contrast, a one year study estimated colony density of the common carder bee Bombus pascuorum in the UK, and found that greater areas of oilseed rape AND field beans in the landscape were both separately correlated with increased estimated colony density in July (Knight et al. 2009). This implies that the number of bumblebee colonies could be enhanced by flowering leguminous crops. It is a really important finding and needs to be corroborated by other studies.
Field beans are unlikely to be used by most solitary bees, because the flowers are so big. Peas, lentils and other legumes could be a different story.
I know of one relevant study here. It compares solitary bees nesting in fields sown with a white clover, alfalfa and grass mix, with those nesting in set aside fields left to naturally regenerate (Gathmann et al. 2004). Set-aside fields had three times as many nesting species (12 bee species) as clover fields (4 species), and the clover fields had the same number of nesting species as barley fields. This shows that cavity nesting solitary bees don’t find clover and alfalfa crops particularly attractive.
Patches of semi-natural habitat are always beneficial
In all the above studies where it was considered, and in many other studies (see Ricketts et al., 2008 for a review), the proportion of semi-natural habitat was strongly positively correlated with numbers of wild bees. Semi-natural habitat includes patches of rough grassland, fallow land, field margins, hedges, woodland, forest edges and scrub. Increasing these habitat types was the original plan for Ecological Focus Areas. Authors of all the studies stress the importance of these habitats, particularly once crop flowers are over.
The effects of crop protection
There is another reason why legume crops managed conventionally may not help bees. I spoke to a farmer growing peas a couple of years ago, and he told me that he sprayed his peas every ten days to two weeks. It was not clear whether he was talking about fungicide, to control powdery mildew and other fungal diseases, or insecticide to control pea midge, aphids, or pea moth. I do not know how widespread such frequent spraying is, but at least one chemical company advises that fungicides are sprayed on peas and beans at early flowering, and another clearly recommends treating peas and beans with insecticides during flowering if pest insects are seen. Fungicides are suspected to have sublethal effects on honey bees (see Pettis et al. 2013 for example), particularly in combination with insecticides, although there is a good deal of work still to do on these effects.
As far as I can tell, there is nothing in the Ecological Focus Areas requirement that restricts the use of insecticides or fungicides on these crops.
Weighing up this evidence
To summarise, one piece of evidence suggests a benefit to bumblebees from planting more beans, while another study shows the effect on bumblebee numbers can only be detected during flowering (small amount of evidence, not in agreement). There is a clear but uncalculated risk to both bumblebees and solitary bees from fungicide and insecticide use on flowering peas and beans. There are no particular benefits to other wildlife, such as mammals, birds or plants, from planting peas and beans.
In contrast, there is ample evidence that increased area of some of the alternative suggestions for Ecological Focus Areas – semi-natural habitats – in farmed landscapes is associated with finding more bees (lots of evidence, agreement). These habitats also benefit other wildlife, including birds.
Which would you choose?
This is a continuation of a discussion I started with the Agriculture Minister George Eustice, on the World at One, BBC Radio 4, 10 June 2014.
References
Diekotter, T., F. Peter, B. Jauker, V. Wolters, and F. Jauker. 2014. Mass-flowering crops increase richness of cavity-nesting bees and wasps in modern agro-ecosystems. Global Change Biology Bioenergy 6:219-226.
Diekotter, T., T. Kadoya, F. Peter, V. Wolters, and F. Jauker. 2010. Oilseed rape crops distort plant-pollinator interactions. Journal of Applied Ecology 47:209-214.
Gathmann, A., H. J. Greiler, and T. Tscharntke. 1994. Trap-Nesting Bees and Wasps Colonizing Set-Aside Fields - Succession and Body-Size, Management by Cutting and Sowing. Oecologia 98:8-14.
Hanley, M. E., M. Franco, C. E. Dean, E. L. Franklin, H. R. Harris, A. G. Haynes, S. R. Rapson, G. Rowse, K. C. Thomas, B. R. Waterhouse, and M. E. Knight. 2011. Increased bumblebee abundance along the margins of a mass flowering crop: evidence for pollinator spill-over. Oikos 120:1618-1624.
Holzschuh, A., C. F. Dormann, T. Tscharntke, and I. Steffan-Dewenter. 2013. Mass-flowering crops enhance wild bee abundance. Oecologia 172:477-484.
Jauker, F., F. Peter, V. Wolters, and T. Diekotter. 2012. Early reproductive benefits of mass-flowering crops to the solitary bee Osmia rufa outbalance post-flowering disadvantages. Basic and Applied Ecology 13:268-276.
Knight, M. E., J. L. Osborne, R. A. Sanderson, R. J. Hale, A. P. Martin, and D. Goulson. 2009. Bumblebee nest density and the scale of available forage in arable landscapes. Insect Conservation and Diversity 2:116-124.
Kovacs-Hostyanszki, A., S. Haenke, P. Batary, B. Jauker, A. Baldi, T. Tscharntke, and A. Holzschuh. 2013. Contrasting effects of mass-flowering crops on bee pollination of hedge plants at different spatial and temporal scales. Ecological Applications 23:1938-1946.
Pettis, J. S., E. M. Lichtenberg, M. Andree, J. Stitzinger, R. Rose, and D. vanEngelsdorp. 2013. Crop Pollination Exposes Honey Bees to Pesticides Which Alters Their Susceptibility to the Gut Pathogen Nosema ceranae. PLoS ONE 8:e70182.
Ricketts, T. H., J. Regetz, I. Steffan-Dewenter, S. A. Cunningham, C. Kremen, A. Bogdanski, B. Gemmill-Herren, S. S. Greenleaf, A. M. Klein, M. M. Mayfield, L. A. Morandin, A. Ochieng, and B. F. Viana. 2008. Landscape effects on crop pollination services: are there general patterns? Ecology Letters 11:499-515.