Today’s blog for National Insect Week is by Senior Conservation Scientist, Dave Buckingham on the importance of farmland planting for insects – and birds

Understanding the ecology and requirements of insects is a key part of RSPB research into designing solutions to farmland bird declines. For most declining farmland bird species, insects are an essential food source in the breeding season, without which birds cannot rear their chicks. Chick malnutrition has long been commonplace, highlighting shortages in key insect prey.

Restoring healthy, diverse insect communities has been the focus of efforts to design conservation measures. Techniques such as extensive grazing of pastures or managing field margins are largely based on the needs of large-bodied, relatively long-lived insects such as grasshoppers and the caterpillars of moths, butterflies and sawflies.

The situation is very different in winter. Most of the farmland birds that conservationists are concerned about switch to feeding on seeds, so restoring seed-rich habitats has been the focus of conservation efforts. Bird populations have responded well to cereal stubbles and sown wild bird seed mixtures, though there is a recurrent problem with seed supplies dwindling in the late winter period, now known as The Hungry Gap.

Cirl bunting are one of the farmland birds which use respond well to cereal stubble (RSPB images)

Until recently, insects were not considered important to seed-eating birds in winter. Over many years of farmland bird research, we’ve only encountered rare examples of seed-eating birds exploiting insects (curiously, during the Hungry Gap). Examples include cirl buntings clambering around hedges infested with weevils (rather than habitually feeding on the ground) and reed buntings gorging on small beetles in seeded ryegrass plots, ignoring the hyper-abundant grass seed we had created for their benefit!

A new and surprising example from our work on cover crops (plants grown to cover the soil, rather than for harvesting) has got us particularly interested.

Hope Farm case study

At RSPB Hope Farm we’ve been growing cover crops for the last five years, as part of a move to integrate agricultural sustainability with biodiversity conservation. Cover crops are sown on arable fields after the food crop has been harvested. The resulting green cover protects the soil from erosion and losses of nutrients, while improving the soil condition, controlling weeds and improving later crop yields.

Cover crop at Hope Farm (c) Georgie Bray

Cover crops are an industry-led initiative designed to improve agricultural sustainability as well as reducing environmental impacts. They work so well they largely pay for themselves, even though they don’t receive environmental subsidies from the government.

We initially had concerns that cover crops might not provide much seed food for wintering birds and that the dense vegetation would exclude small seed-eating birds from foraging on the ground. As expected, the trial fields developed dense, seed-free swards but to our surprise, we started to find small flocks of seed-eaters in our cover crops, even during the Hungry Gap.

This is where the insects come into the story. When we inspected the areas where yellowhammers and reed buntings were feeding, we found no visible seed sources but large numbers of tiny flies and aphids. These insects are too small and/or active for birds to gather profitably in summer, but in cooler winter conditions they move a lot slower making them easier to collect in large numbers. If cover crops can routinely support wintering birds in this way, they may provide a valuable unplanned contribution towards supporting farmland bird populations.

To follow up these observations we spent the following winter visiting cover crops on 8 farms scattered across East Anglia, to test our Hope Farm observations. We measured bird usage, insect populations and the composition of vegetation on pairs of fields, each made up of a cover crop and a conventionally managed cereal stubble.

We’re still analysing the data, but we’ve been seeing the same patterns of denser, seed-free vegetation, enhanced insect densities, more earthworms and more birds on the cover crops. The main bird beneficiaries from the cover crops were insectivorous species such as meadow pipits and thrushes, which eat earthworms. Numbers of seedeaters were similar on both cereal stubbles and cover crops (i.e. cover crops were every bit as good as stubbles).

Next steps

The next stage of the study will be to look in more detail at the identities of the insects on the cover crops. Two numerous insect groups are of particular interest: the flies and the aphids.

It is relatively simple to identify flies to family level and this is sufficient to provide valuable clues to their ecology. Most of the flies we have examined so far come from the families Drosophilidae (fruit flies) and Lonchopteridae (spear-winged flies). These breed in spring/summer, long before the cover crops were sown. This points to two useful practical conclusions: insects use cover crops to hibernate in and that any aggregation of flies on cover crops will dependent on breeding habitats located elsewhere on the farm.

We use the excellent ID guide from the Dipterists Forum 

The concentration of aphids is more of a concern, particularly for our farm manager who instinctively hears the word “PEST!” when I say “aphid in the cover crop”. But is this true? There were no aphid problems in the crops that came after the cover crops – we checked. We needed to know exactly which species of aphids were involved: aphids have strict associations with specific food plants and only a few species feed on our crops and become problematic.

These tiny predatory Braconid wasps can cause havoc with aphid populations (c) Bernad Dupont (Flickr) 

Even if the aphids are grass-feeders that can attack cereal crops, the parasitic wasp populations that feed on them can help prevent infestations from developing (I’ve been shown the handiwork of these wasps by an aphid specialist – a couple individuals can wreak carnage) . We found parasitic Braconid wasps amongst the aphids from the Hope Farm cover crops, but their occurrence was patchier on other farms.

Understanding how cover crops support pest species and beneficial insects will help guide future work on tailoring cover crops to minimise impacts on the following crop and maximising wildlife benefits. To find out what we're doing at Hope Farm, check out our latest Annual Review

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