Diversity in Shelterbelts and Hedgerows

Research is showing that careful choices of hedgerow and shelter plants can have the additional benefits of attracting pollinating insects and predators for insect pests.

When farmers plant hedgerows their main concern is with shelter for livestock and crops. Over many generations they have discovered which plant species flourish in their region and are effective in reducing the impact of weather extremes.

However, establishing shelter belts can have unintended consequences, both positive and negative. Some species harbour birds that may damage crops; others may host and provide a reservoir of insect pests. Tall species may shade pasture and crops and in recent years many high hedges and trees have been pulled out to make way for overhead irrigation equipment.

On the positive side, some hedge species provide a good habitat for beneficial insects such as pollinators and the natural predators of insect pests. With the decline in honey bee populations, the cost and toxicity of sprays and the overall trend towards sustainable management practices, the potential benefits of designer hedges is receiving a good hard look.

A group of scientists from Plant & Food Research and the Foundation for Arable Research has received SFF funding to investigate appropriate hedgerow species, and to design and plant hedgerows and monitor the results. Initially they looked at what was already known about the associations between hedge plants and birds, slugs, pollinators, and insect pests and their predators.

Dr Brad Howlett, a Plant & Food entomologist and specialist in pollinators, says that insect pollinators complement the activities of honey bees because they are often active when honey bees are not and also move pollen in different ways. They can be responsible for more than 50% of pollination success in some areas.  “Recent work suggests that wild pollinators actually increase yields for a whole range of crops over just relying on honey bees, so any strategies that we can use to boost their numbers are likely to have beneficial economic impact,” he says.

“Also, many crop growers rely heavily on pesticides to control insect and bird pests to protect crops so it is also important to foster the natural enemies of pest species without significant increases in pest species themselves.”

About ten plant species have been identified as being potentially beneficial, including manuka, kanuka, hebe species and oleria. They have been used in plantings on four properties – three in Canterbury and one in the Wairarapa.

FAR designed the plantings in conjunction with the farmers, who decided on the area involved and the purpose each planting. An expert in plant establishment helped with choosing species that were suited to the locality and with their location within the planted area – in a long narrow hedge some plants may be more prone to wind damage than others.

“Most of the plants are quite slow-growing and at this stage the density is low and the biggest block is no more than an acre. Despite this we have seen an increase in important pollinators in the past year and also an increase in the important natural enemies of pest species but no significant increase in pest species themselves,” says Brad.

“So the results are following along the lines of what the theory suggests – choosing the right plants fosters the growth in beneficial insect populations.”

“We have been monitoring about 20 species in each area using water traps and pitfall traps and comparing the counts with those from bare fencelines and from different farms and different treatments and seeing how abundances change over time.”

In the past decade researchers have measured the pollination efficiency a number of flies, native bees and bumble bees and determined that about eight species are important for brassicas and perhaps 14 species are important for onions, and it appears that multiple species are effective over a range of crops. This means that it should be possible to tailor hedgerow composition to suit multiple crops in a particular location.

“On an arable farm they tend to grow a number of different seed crops like onions or carrots or brassicas, and generally the types of pollinators on all those are quite similar so a hedgerow that supports those key insects over their life cycle would be ideal,” says Brad.

“Crops flower at different times and so a variety of crops will assist with maintaining populations of those insects, but the aim of the mixed species hedgerows is that they will support pollinators through their entire life cycle so you don’t get populations crashing and not being available for pollination when crops come into flower.”

Dr Melanie Davidson is an entomologist with Plant & Food Research with a special interest in insect pests. She says that the species that they have chosen for the experimental hedgerows have been shown to provide a good habitat for beneficial species.

“So far we have seen an increase in the predator species on hedgerow plants. They include native lacewings and hover flies as well as introduced ladybirds and beetles,” she says.

“The next step is to see whether they will become numerous enough to migrate into the crop, whether it be pasture or arable, and predate insect pests.”

Melanie has also been involved with research into bird species that commonly predate crops, and found that they prefer nesting in hedgerows of pine and macrocarpa rather than mixed species hedges. This implies that the plantings of manuka, kanuka etc described above will attract fewer of these birds and result in less bird damage to crops. She points out that about 300,000km of pine, macrocarpa and gorse hedgerows were planted over the past century in Canterbury alone, and while they have provided reasonable shelter many could and perhaps should be replaced with more beneficial species.

“The drive by central and local governments to clean up waterways by increasing riparian planting also represents an exciting opportunity to use the right species that build habitats for beneficial insects,” she says.

“While such plantings create habitats for insects that can provide benefits to farmers, they are also a way of integrating our native fauna and flora into agricultural ecosystems.”

Although the experimental hedgerows are still young it appears that careful selection of hedge species will certainly result in a boost to beneficial insect populations and crop yields and a drop in pesticide usage.

SFF for the project funding runs out in June 2015 but the team hopes to be able to continue to monitor the hedgerows and document the life cycles of insects on various plant species and under various conditions. Ultimately they want to fine tune strategies farmers can use to build reliable populations of beneficial insects and become less dependent on honey bees and pesticides.