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Conservation Biological Control – making the most of natural enemies in farming landscapes

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There is great potential to manage agricultural landscapes to better support the naturally occurring predators and parasitoids of crops pests.  These natural enemies are an important component of integrated pest management (IPM), reducing the population growth rate of pests. This can help to prevent pests from reaching economically damaging levels, or delays the point at which economic thresholds are reached to beyond susceptible growth stages.  However, the impact of natural enemies can be limited by a lack of essential resource not available to natural enemies in crops. The targeted provision of these resources, known as Conservation Biological Control (CBC), aims to promote diverse, robust natural enemy populations to encourage a reliable and consistent contribution to pest management.

Conservation Biological Control – making the most of natural enemies in farming landscapes

Damaging infestations of arable pests, such as aphids, are difficult to predict and can cause substantial yield losses. Insecticides are a central component of the management of these pests as they are relatively easy to use, cheap compared to the potential impact of pests, and result in obvious reductions in insect population size when applied correctly. Nonetheless, insecticides are becoming victims of their own success.  Their intensive use over large areas is driving the development of resistance in target pests, and they are having chronic impacts on non-target farmland species, including natural enemies, which has led to increased restrictions on their use. The best approach is to use a combination of chemical and non-chemical strategies, including crop rotation, pest resistant crop varieties, and promotion of natural enemies, as part of an IPM programme.  Non-chemical measures often act by reducing the likelihood that pest numbers exceed economic thresholds and so reduce the need for insecticide treatment.  Less reliance on insecticides is pivotal to compliance with the Sustainable Use Directive and will also help to limit the development of insecticide resistance at a time when the size of our chemical armoury is ever declining.

Economic thresholds

Economic thresholds are a valuable method for assessing whether or not action is necessary to prevent pests from causing financial losses in a crop.  They are usually defined in terms of the number of a pest per unit area, per plant, or per part of plant, above which action should be taken.  Economic thresholds should account for the tolerance of the crop to pest injury, and many current thresholds are in need of updating1.  ADAS has reviewed the use of thresholds in arable crops and this has resulted in a revised threshold for pollen beetle, following concerns over the arrival of pyrethroid resistant populations in the UK.  We are now moving on to consider other pests or groups of pests such as stem borers of cereals (wheat bulb fly, yellow cereal fly, gout fly and frit fly).

Natural enemies of crop pests

Most pests of arable crops have a number of naturally occurring enemies.  These include generalist predators which roam the crop eating anything they can catch, as well as specialists that target particular prey.  In the absence of natural enemies, pest infestations can grow rapidly, but in their presence, growth is slowed, reducing the likelihood of economic thresholds being exceeded2.  Most natural enemies need a more diverse range of resources for their development than cropped landed provides.   If these additional resources are in poor supply the natural enemies will have less of an impact on pest numbers.  Providing habitats that include these resources will encourage natural enemies in much the same way that other beneficial insects, such as pollinators, benefit from planting floral field margins. 

Floral resources

In the insect world, adults often eat very different food from their offspring, to avoid competition and to make the most of seasonal changes in food availability.  The adults of many largely carnivorous larval species feed mostly or entirely on floral resources (Table 1).  For many, floral pollen forms an essential part of their diet in order to acquire the proteins they need for reproduction.  Although adult hoverflies, ladybirds and other predators are mobile, and can commute between crops and floral habitats, such habitats can be in short supply in agricultural landscapes.

Table 1. Examples of natural enemies of crop pests and the food sources required at different stages of their life-cycle.

Natural enemy

Larvae feed on…

Adults feed on….

Marmalade fly
Episyrphus balteatus

Various aphids

 

Pollen and nectar

7-spot ladybird
Coccinellia 7-punctata

Various aphids

Various aphids, pollen and nectar

Aphid parasitoids
Aphidiinae spp.

Various aphids

Aphid honeydew and nectar

 

Additional prey

Pest infestations in crops can be very damaging, but they can also be relatively short lived.  This can leave periods of the year during which prey may be scarce for natural enemies. Fortunately many predators can feed on alternative prey, and these may appear at different times in different crops and non-crop habitats.  For example, aphids appear on potato crops later in the season than on cereals, and many natural enemies easily move between these two crops. By increasing landscape diversity natural enemies are more likely to find suitable prey all year round, maintaining populations ready to respond to the next pest outbreak.

Overwinter habitat

Winter habitat is important for providing food and/or shelter for overwintering natural enemies, from which they can migrate into arable crops the following spring.  Undisturbed grassland is used by many beneficial insects and promoted in stewardship schemes in the UK, including the specifically designed ‘beetle banks’.  Tussocky grasses are especially important in providing shelter, though leaf litter, seed heads, dead herbaceous stems and bark are also commonly used. 

The impact of adding habitat around crops for CBC depends on the resources already available in the landscape.  If there are already a lot suitable floral resources, adding more may not make a difference. In addition, adding resources to very simple landscapes may not have an immediate effect as natural enemy populations take time to build up3.  Providing targeted additional resources across agro-ecosystems will support a robust and diverse natural enemy population, which in turn will contribute to suppression of pest populations.  

Arable landscapes are constantly changing over time, and environmental conditions play a large role in the population dynamics of both pests and their natural enemies.  This complexity makes it difficult to generalise on effective management options, but it should not discourage efforts to incorporate them into arable pest management. Ensuring farm landscapes have a diverse range of habitats, with flowering margins and a mixture of crops is a great start. Many predators and parasitoids have relatively small mouthparts, which means they are unable to access pollen and nectar from some flowers, so simple flowers such as coriander, bishop’s weed, buckwheat, corn chamomile, cornflower, and sweet alyssum are good choices. Growing these in field margins will encourage a diverse and robust natural enemy population.  

It is likely that the number of insecticides available will continue to decline, and this will present further challenges to sustainable food production.  Conservation biological control is one option among many that can be incorporated into IPM, and which will help farmers prevent damaging pest infestations from occurring.

Find out more

For more information on the role of natural enemies in arable crop protection, contact Mark Ramsden

References

  1. Ramsden M.W., Kendall S.L., Ellis S.A. & Berry P.M. (2017) A review of economic thresholds for invertebrate pests in UK arable crops. Crop Protection 96, 30-43
  2. Ramsden M.W., Menendex R., Leather S. & Wackers F. (2016) Do natural enemies really make a difference? Agricultural and Forest Entomology 19, 139-145
  3. Tscharntke T., Klein A.M., Kruess A., Steffan-Dewenter I. & Thies C. (2005) Landscape perspectives on agricultural intensification and biodiversity – ecosystem service management. Ecology Letters 8, 857-874

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