Can beneficial insects help in the war against crop pests? Chloe Palmer speaks to the experts to find out more.
For many centuries, natural predators were the only weapon growers had against crop pests and weeds. Long rotations, diverse cropping patterns and small field sizes interspersed with mature hedges supported beneficial insects and helped the grower.
Now, with a range of crop protection chemicals available to control almost every pest and weed, it could be argued beneficial insects are redundant.
But as more product licences are revoked in the European Union, growers are looking at alternative forms of pest and weed control. Allowing insects to do the work of chemicals for free would appear a cost effective option.
Recent research has shown how provision of habitat within the farmed environment and adoption of other measures to encourage beneficial insects can result in increased yields and profitability.
Beneficial insect populations have declined over recent decades so it is critical growers take action to safeguard surviving species, according to Laurie Jackson of conservation charity Buglife.
“There are lots of reasons for the decline in the populations of many beneficial insects such as fragmentation of habitat, less mixed farming and larger areas of the same crop,” she says. “Many invertebrates are immobile and in some cases, may only move ten metres in a year so are very susceptible to habitat loss.”
Hedgerows are vital to insects in spring when other sources of pollen and nectar are limited, she explains.
“Species such as solitary bees have a nesting cycle of just a few weeks in early spring so rely on early flowering woody species. Annual trimming of hedges means species such as blackthorn and hawthorn are not able to flower because blossom only forms on second year growth.”
Ms Jackson would like to see growers creating diverse habitats on farms so insects can take pollen and nectar from many different species, improving their survival chances.
Dr John Holland, head of farmland ecology at the Game and Wildlife Conservation Trust, supports this view.
“The more diversity there is on a farm the more robust the control provided by beneficial insects. Retaining grassland within the farming system is also good for invertebrate species but not always practical,” he adds.
Relatively little is known about how effective many crop pest predators are, but Dr Holland’s research has shown flying predators such as hoverflies, parasitic wasps and long-legged flies are more successful when reducing aphid numbers compared with ground active species such as spiders and beetles.
“Hoverflies are important for aphid control because their larvae are predatory and some species have reduced significantly in recent years. Spraying large areas across a farm in the same day threatens these species as there are no refuges left for them,” says Dr Holland.
Investigating how important field margins are to beneficial insects is a key element of his research. Results show aphid control improved with larger areas of grass margin although some species such as long legged flies were only able to fly short distances from margins into the crop.
“There are also many other even less mobile species so creating beetle banks across larger fields or introducing grass strips and retaining unmanaged areas will help them to move into the crop,” says Dr Holland.
Many beneficial insects over-winter in the soil so minimising disturbance to the soil profile reduces mortality levels, adds Dr Holland.
“Minimum or no till systems cause the least disruption to soils so allow beetles and their larvae to persist over the winter. Reduced cultivations also help to maintain soil organic matter levels which foster detritus feeding insects, a source of alternative prey for natural enemies,” he explains.
The widespread occurrence of ‘impoverished soils’ with low organic matter content may be a factor in declining numbers of some insects so including livestock in rotations could help, believes Dr Holland.
Where this is not practical he suggests including leguminous crops, cover crops or mixed silage crops containing legumes. These will contribute organic matter and provide a nectar and pollen source for insects.
Ultimately, it is the availability of diverse habitats, rich in a variety of species which is most likely to meet different beneficial insects’ needs, says Dr Holland.
“Certain species such as hoverflies and some parasitoid wasps need to feed from simple, open flowers, such as yarrow, and members of the umbellifer family, such as cow parsley, because their proboscis cannot reach nectar in flowers of the pea family.”
Controversially, weeds are also important for many beneficial insects, providing cover and food in the form of seeds, pollen and nectar. Dr Holland admits it is tricky to retain weeds without running the risk of infestations, but says ‘scruffiness is good for biodiversity’.
“There is a balance between the cost of maintaining a clean crop and the value of effective biocontrol provided by non-target species. Retaining some unmanaged areas will harbour insects which can play an important part in pest control and leaving uncompetitive weeds can also save money,” he says.
“The loss of crop protection products, such as neonicotinoids, shows we need to look at alternatives to chemical control.”
Richard Butler farms at Eastwick Farm in the Vale of Pewsey on the Marlborough Downs. His farm extends to 700ha (1,729 acres) and comprises a dairy unit and arable operation. Soil types vary from light chalk downland to heavy clay soils in the vale.
Mr Butler is chairman of the Voluntary Initiative and believes passionately in the principles of integrated pest management (IPM) to ensure chemicals are used responsibly and to maximise the effectiveness of biological control.
He says: “Measuring pest thresholds is vital before spraying with insecticides. It is not only costly to use pesticides when pest numbers are low but it can also encourage resistance.
“In spring this year, levels of pollen beetle on winter oilseed rape were reduced due to the cold weather. So most crops did not need to be sprayed and this was a significant cost saving.”
Mr Butler believes availability of mobile technology to assess thresholds means every grower and agronomist could potentially adopt this approach prior to spraying.
“The technology means it is possible to identify the species of pest and estimate population numbers present on a field by field basis. Currently, 53 per cent of growers are using thresholds but we hope this figure will increase,” he says.
Using crop protection insecticides less frequently can only help beneficial insects, says Mr Butler. “It is vital we demonstrate to consumers we are acting responsibly. We must utilise the full range of control measures including the best genetics and appropriate rotations rather than simply resorting to chemical control.”
He urges growers whose ELS agreements are coming to an end to think carefully before ploughing out margins.
“Where there are margins next to woodlands, hedges and especially watercourses, it is very important to retain them, even if you are not putting your farm into another scheme. The margins provide habitat for beneficial insects but also help to prevent diffuse pollution of streams and rivers and help to meet LERAP requirements.”
Rotations containing legumes such as peas and beans, and flowering crops such as oilseed rape can all help to sustain pollinator populations and Mr Butler works closely with local beekeepers who bring hives to his farm each year.
Mr Butler is convinced the actions he has taken have encouraged wildlife and refers to independent surveys of farmland birds showing increases in red list species and rises in overall numbers on his farm. Many of the recorded birds and their chicks feed on insects so Mr Butler is confident there is a strong correlation.
He believes the industry should adopt best practice and IPM as a matter of urgency, saying the long term implications of not doing so could be very serious. “We have to defend the active insecticides we have and demonstrate good insect stewardship.”