The Crop Headland. Managing the Edges of Crops to Support Wildlife

Abstract

Much of the effort to conserve farmland wildlife concentrates on the non-cropped areas of land remaining on farms and usually taken up by hedgerows, other boundary features and the remaining strips of semi-natural vegetation found at the edges of crops. Relatively little attention has been paid to the outermost areas of the crop itself. It is viewed, like the rest of the crop, as an area of intensive management, not amenable to wildlife conservation. Yet many farmland wildlife groups interact between these two areas of the outermost edge of the crop itself and the hedgerow/boundary feature. For example, many species of insect pest natural enemies overwinter in hedge banks and tussocky perennial grasses at the crop edge before dispersing into adjacent crops in the spring (Coombes & Sotherton 1986). Many species of farmland bird nesting in hedgerows will forage for insect and plant food items for themselves and their chicks in surrounding crops (McHugh et al. 2016a, 2016b). Therefore, the management of crop edges, as well as the field boundary itself, may be an important part of the conservation of species inhabiting these edge habitats on farmland.

Much of the early research conducted on crop edges was undertaken by the Game & Wildlife Conservation Trust and its predecessors because of the conservation effort they focused on the grey partridge Perdix perdix, a species that nests in hedge banks and boundary strips but that feeds its precocious chicks in adjacent crops (Potts 1986). A nationwide decline in grey partridge abundance on farmland occurred from 1952 onwards from approximately 25 pairs/km² to fewer than 5 pairs in recent years, a decrease of over 90% (Hayhow et al. 2017; Sotherton et al, 2014). Declining populations have also been recorded in North America (United States and Canada) and in both eastern and western Europe (Potts 1986).

In Britain, the grey partridge is predominantly a species of lowland, arable landscapes, especially cereal fields. The most important changes that have occurred on farmland coincidentally with the partridge decline have involved the intensification of grain production, such as increasing field size (hedgerow removal), improved drainage, polarisation of farming systems, increased use of fertilisers, improved plant breeding and, especially, the increased use of pesticides. Since the 1950s, increases in the nurnbers of pesticide products used, increases in the areas of cereals sprayed, and the spectrum of activity of active ingredients have occurred (Ewald et al. 2015).

Detailed population studies on grey partridges found that the key factor most responsible for changes in a grey partridge population in southern England was  chick mortality and had clearly linked the observed national decline with the increasingly poor levels of chick survival (Potts 1970). Other studies linked chick survival to the availability of sufficient quantities of the preferred insects that are important in the diet of young chicks (Southwood & Cross 1969; Potts 1986). These insects include larval forms of Lepidoptera and Tenthredinidae (especially species of the genus Dolerus), several Coleopterans (Chrysomelidae, small diurnal Carabidae and Staphylinidae, and Curculionidae) and many members of the Heteroptera, including species more abundant at the edges of the cereal fields where grey partridge broods preferentially forage (Green 1984).

Pesticides can reduce densities of these chick-food insect species, especially in the short term. In the long term, insect populations can recover to their pre-treatment densities (depending on scale and frequency of pesticide use) following treatment. The issue with chick-food insects is that they are often sprayed in summer crops just as grey partridge chicks start to forage for these insects, usually in mid-June. Therefore, their food source can be depleted locally at a crucial phase of their growth. These pesticides include insecticides (Vickerman & Sunderland 1977; Sotherton 1990), insecticidal fungicides (Sotherton et al. 1987; Sotherton &  Moreby 1988; Sotherton 1989), and herbicides. Herbicide use has the most important effect because of its ability to remove cereal field weeds. These weeds are the host plants of many of these phytophagous chick-food insects (Southwood & Cross 1969; Sotherton 1982).

To test this experimentally, a series of field-scale experiments were devised whereby pesticide use could be excluded from small areas of crop, preferably adjacent to grey partridge nesting habitat, and compared to areas of crop fully sprayed as per normal farm practice. The part of the tractor-mounted spray boom closest to the field boundary was switched off as the outermost areas of the crop were being sprayed; at the time, the minimum length of the spray boom that could be switched off by the tractor driver in the cab was 6 metres. Hence, 6-m-wide sections of the boom were either switched off when spraying around the edge of the crop, avoiding the application of certain chemicals at sensitive times of the year, or these edges, known in farming terms as "headlands", were sprayed separately with products screened for their selectivity and "approved" for use (Sotherton 1992). The rest of the field was sprayed with the usual complement of pesticides, and only the crop margin (which, on average, accounted for 6% of the total field area) received these lower pesticide inputs. Fertiliser inputs were not manipulated. These selectively sprayed crop margins are termed "Conservation Headlands" (Oliver-Bellasis & Sotherton 1986).