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MAKING THE MOST OF IT: ANTS, BEES AND WASPS ON ARABLE FARMLAND M. E. EDWARDS Arable farmland – ecological desert – or so many instant responses would have it. But stop; is our standard response because we think in terms of pictures of idealised habitats, often botanically described, to which we give emotionally laden names – heath, chalk grassland, ancient meadow? If we can adjust our minds to think in terms of the resources required by the inhabitants of any landscape, rather than predetermined categories of so-called habitat, a rather different picture emerges. This is not to decry the inestimable value, if only to ourselves, of those places which we can describe in our favoured terms, nor to suggest that trying to conserve as many examples of these as possible is not a good end in itself, although I would argue that the practicality of preserving these over considerable time scales is not very realistic. Looking out over a typical view of the U.K. arable landscape, usually the first thing which strikes one is the expanse of open field, with a few hedgerows and strips of grassy growth. We are used to thinking in terms of uniform extents of ‘habitats’ and, consequently, tend to concentrate on the largest components of the landscape first – unless we are looking at a view over the sea, when the islands, the smallest parts, take our attention first. Think of the arable land, the source of our food, as the sea; you can now see the islands and pay attention to them. This model of habitable islands in a general sea is much more realistic for any inhabitant of a landscape. Most of the surrounding area is as inhospitable to any particular inhabitant, animal or plant, as an expanse of water – if it is land-dwelling; or land if it is water dwelling. The chances of all the factors required for the organism to grow and reproduce successfully being present in any one location are very low. Much publicity has been made of ‘linking corridors’, without actually thinking very deeply about what this ready phrase actually infers about the purported inhabitants and what they need from the landscape. In my opening paragraph I referred to the idea of resources, without actually defining what I meant by this term. A resource is any parameter which is required for the completion of an organism’s life cycle. It may be concrete, such as a patch of bare ground or a plant, or less tangible, such as a temperature or humidity envelope. Such resources may occur in a number of different landscapes, often associated with features, such as hedgerows or grassland, within the landscape. A collection of features is what is often referred to as a habitat, such as grassland – but does this include the scrub component.... should it? For the rest of this discussion I would like to confine my illustrations largely to the resources related to aculeate Hymenoptera (ants, bees and wasps) within the arable landscape and ways in which the availability of these resources may be improved in such landscapes. The aculeates are particularly interesting to me ecologically because they have much more in common with birds than many other insects, especially the predatory birds. Trans. Suffolk Nat. Soc. 44 (2008)
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All aculeates concentrate resources for their young in a protected environment, the nest, or they cuckoo the nests of other aculeates in a similar way to that of some bird species. The conditions which make nest sites particularly favourable for the development of the young will often not be those which are particularly favourable for the development of the required food sources, plant or animal, themselves. There may also be resources needed to build the nest itself which are found in yet a third feature of the landscape. In order to meet these conflicting demands it is necessary that the ability of the adult to move between the locations for the different resources is well developed and such movement goes beyond the simple need for a dispersal phase. The locations within which the disparate resources are found may well be non-contiguous and the term ‘partial habitats’ has been used in the past to describe these locations. At this point it is worth discussing the general biology of the three main groups of aculeates, the ants, bees and wasps. Although the development of social organisation within this group is the feature most often thought about by the non-entomologist, this just adds an extra layer of complexity to the basic division within the group – herbivore or carnivore? The wasps, at least in the U.K., are firmly carnivorous and include the most evolutionarily primitive lifestyles, some being little more than exteriorly feeding parasites of other insects, where the female makes sure that the prey item is safely out of the way of outside competition by hiding it in a primitive nest. Where only one prey item goes to make up the food for the larva it needs to be considerably larger than the eventual adult wasp in order to allow for the metabolic cost of conversion from prey to wasp. This is the situation in the Pompilid, or spider-hunting, wasps such as the widespread Anoplius nigerrimus. I have found this wasp hunting on the bare ground under crops and at the edges of arable fields; it is also, in my experience, a regular inhabitant of similar situations in gardens. Some species, however, get over the problem of transporting large prey items by collecting a number of smaller ones and caching these in individual brood cells for each larva, such as the sphecid wasp Cerceris arenaria, which collects vine weevils as prey. This development of accumulated provisioning allows greater separation between the individual required life-cycle resources as the load to be transported is individually smaller. The bees have taken this collection of individual, protein-rich, food resources to new levels by preying on the reproductive requirements of flowering plants. Although the word pollination is almost invariably associated with the word bee, it is quite clear that a number of bee species are more pollen thieves than active pollinators. Collection by some bee species, including the honey bee, involves being assiduous in not getting tangled up with such time wasting items as the plant’s stigma. Other species of plant and bee have, however, clearly evolved close associations of the plant reproductive parts and the bee’s pollen collection apparatus. Pollen has the advantage that individual packets of protein come in small units which can be readily collated into larger units from a number of different sources and then transported to the favoured nest site for consumption by the larvae.
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Ants employ a mixture of feeding strategies from both carnivore and herbivore, adding detritivore for good measure, according to species, but all have developed the process of accumulation of larval food resources in a central nest area to a high extent, largely through a major division of labour between individuals of the closely-related social group. Arable land, or more often the sparsely vegetated conditions around the edges, often provides ideal nest building locations, with plenty of sun to warm the brood mass. The critical queen inhabited areas are often well below the depth of light cultivation and some Myrmica nests on my allotment are remarkably longlived, despite regular digging. With the increasing use of minimum tillage at least some ant species are likely to be able to increase the parts of the arable system which they are able to utilise. Whatever the nature of the food items provided for the young, all nests have one over-riding resource requirement, the presence of a warm temperature envelope. Larval development is closely related to ambient temperature: the warmer it is, at least up to a point, the quicker the larva can develop. This is very important where you have a concentration of food resources just waiting for some other free-loading organism to come and take it over. In consequence nests are positioned in locally warm environments. This warmth is usually provided by the sun, although some species, notably the social ones, have a considerable ability to provide additional warmth through specialised metabolic activities. Typical nest sites in arable situations involve areas of bare ground in the sun or exposed sections of dead-wood. Both these conditions are readily provided on the edges of arable fields, trackways and the surrounding hedges, although the level of disturbance may be too severe for successful nesting at some times of the year. The flowers of the hedging plants and the herbaceous species of the margins also provide floral resources, both pollen (for larval development) and nectar (fuel for flight on the part of the adults). Such, otherwise unremarkable, hedges of Hawthorn or Blackthorn can support very large numbers of spring mining bees such as Andrena nitida or A. haemorrhoa. Arable crops also have potential to provide large sources of pollen, although with a short period of availability. Recent research at Reading University on the presence of wild bees within flowering oil-seed rape fields has shown a surprising number of species to use this resource. Cereal crop pollen is of very little interest to most bee species, but the aphids and small flies which infest them are readily taken by a number of wasps which collect these insects as prey items for their nest provisioning. One of the commonest wasps in such conditions is Crossocerus podagricus, nesting in small-diameter beetle burrows in the hedges and foraging for various small flies among the crop. One area where changes in the production of a former arable crop, clover ley and pasture, has had enormous influence on the fortunes of a group of insects is in the decline of many of our species of bumblebee. Up to World War 2 the distribution of these bumblebee species was fairly ubiquitous in the farmed countryside, although there were some species which showed clear
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north-south divides. Changes in farming practice, centred on the intensification of dairy farms, loss of working horses and increased availability of cheap inorganic nitrogen fertilisers, enormously reduced the proportion of land under cultivation with clovers. Legume pollen is high in protein and is highly favoured by many bumblebee species. Over the past fifteen years efforts to persuade farmers to grow small areas of red clover (particularly) on their arable fields are starting to bear fruit. One species, Bombus ruderatus (Plate 1), which was almost extinct in 1996, has shown a remarkable recovery in numbers and range which is closely linked with the availability of these forage areas. However, before getting too ecstatic we should not forget that it is also possible that changes in the temperature pattern, with longer, warmer late summers, may also have played a part. It is unquestionable that changes in farming practices have driven huge changes in the nature and numbers of organisms which can be found in the wider countryside. However, it is often as much the result of the political pressures which drive farming practices, as the practices themselves. The payment of farm subsidy based on the area of cultivated land drove much of the plough-up of marginal land, especially on steep hills and along hedge and woodland edges. In many of these situations the actual value of the additional crop was less than the cost of the effort required to cultivate it, being little more than a crude mechanism to provide farmers with top-up pay. Such a poor measure of farming success as payment by area of land under cultivation, whether it is productive or not, has been rightly replaced with one which considers the contribution to the overall environment. This is not by any means perfect at the moment, but using this approach Marek Nowakowski of the Wildlife Farming Company, in conjunction with Richard Pyewell and his team at the Centre for Ecology and Hydrology (CEH), have shown convincingly that by taking out the unproductive land (usually 5–10% of a farm) and creating wildlife resource areas for which the farmer is paid to create and maintain, it is possible to maintain farm profits and make vast improvements to the numbers and variety of wildlife present on farms. If this approach is to work we need to understand both the resources required by the target insects and how these might be provided within a farming system. Clearly we are not going to be able to re-create expanses of ancient meadow – whatever that might be, but we can look seriously at what makes new and disturbed environments good for particular species, often not ones which do particularly well on the more established botanical habitats. Very important in this context, and currently not at all well researched, are those plants which are annuals, biennials or short-lived perennials and their associated insects. All these plant types do badly in the long-term in permanent sward and woodland habitat types, relying on intermittent ground disturbance to provide opportunities for germination. Typical of this situation is the association between Weld Reseda luteola and the yellow-faced bee Hylaeus signatus (Plate 2). This bee only collects its pollen from the flowers of Reseda, with Weld being highly favoured. There is a further twist to this story as the dry old flowering stems of Weld are
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long-lived, often standing for a year after the plant has shed seed and died. These old stems are hollow, and because of the sort of places they grow in, well insolated. As such they are used by other wasp and bee species as suitable places to build their nests. An informed farming environment policy which takes into account situations such as this and utilises the opportunities created by the major use of the land – the growing of human food – to provide resources for wildlife can only be to the great advantage. In this context I feel that in conservation we have tended to waste a lot of effort in promoting systems which ask farmers to farm badly, making crops weedy: conservation headlands being a typical example. We should be asking farmers to grow good clean crops, both for human food and wildlife, not try to mix two basically incompatible systems. Both require effort and seriousness of purpose to realise. The message that a poor quality crop is desirable only leads to the assumption that a poor wildlife option is also acceptable. We are never going to have the area of land devoted to wildlife which is being devoted to food production and we should be asking for payment on delivery, with financial systems to reward good delivery in both cases. Mike Edwards Leaside Carron Lane Midhurst GU29 9LB
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M. Edwards M. Edwards
Plate 1: Bombus ruderatus, a scarce bumblebee whose recovery may be linked to the availability of red clover forage areas (p. 19).
Plate 2: The yellow-faced bee Hylaeus signatus, a solitary species closely associated with Weld Reseda luteola. (p. 19).