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Favour pollinators and natural enemies

Last changed: 15 April 2021
Photo of buff-tailed bumble bees pollinating faba bean plant

More crops into crop rotations to increase landscape scale crop diversity and flower strips along field edges. Retain the remaining forest and grassland patches. Avoid supplementation of honey bee hives in landscapes with scarce floral resources to avoid resource competition between managed and wild pollinators. Prioritize water withdrawal for agricultural irrigation in crops, which are free from herbivores. These are Chloë Raderschall´s recommendations based on her PhD thesis. She studied agricultural diversification strategies and how they affect pollinators and natural enemies of insect pests. Read her popular scientific summary.

How do we feed a growing and wealthier world population without sacrificing the planet’s well-being and the diversity of life forms we have today? This question has concerned humans for decades and remains one of the greatest challenges of our time. Over the past sixty years, modern technology succeeded in producing more food on the same amount of land and in doing so drastically reduced the number of people left hungry.

Agriculture has transformed the landscape

However, the paradigm of intensive agriculture sees humans and nature as antagonists, and the success of producing more food is largely due to the adoption of technologies to kill undesired natural actors such as insects eating crops, diseases or weeds. But intensive agriculture did not just combat organisms in crop fields, no it transformed entire landscapes. The traditional agricultural landscapes that were characterised by mosaics of small fields, flower-rich meadows and forest patches had to make way to expanding fields with monocultures of few crop types that dominate entire landscapes. Such simplified landscapes provide very few opportunities for organisms such as insects to nest, find shelter or food resources, all of which are essential for them to survive and reproduce.

During spring, yellow seas of oilseed rape provide an abundance of nectar and pollen for insects to feast on, but within a few weeks bloom is over leaving its flower visitors left hovering in front of empty plates. In addition to these land use changes, climate change is also negatively impacting many insects. For example, extreme weather events such as droughts and heat waves shorten the flowering time of crops and wild plants and so reduce food availability even further. As a consequence, insect populations have been plummeting over the past decades in both numbers and species diversity. With every species or population that disappears, interactions with other species are eroded risking the loss of ecosystem services – functions provided by nature that benefit us humans. Ironically, some of these ecosystem services, such as pollination, pest regulation or nutrient cycling, are pivotal for agriculture, and the loss of biodiversity and ecosystem services have started to take its toll in some regions by making crop yields more unstable.

Climate change

Crop yield stability is also increasingly impacted by climate change stressors. Extreme weather events subject crops to water or heat stress, and warming climates increase the consumption rates of insect herbivores, which can lead to pest outbreaks if their numbers are not regulated in time. Taken together, sustainable agricultural systems that work together with nature – not against it – are urgently needed to bolster crop production in the face of global change. Ecological intensification of agriculture seeks to intensify and diversify the functions of nature to produce more food. Diverse flower, shelter and nesting resources are integrated into agricultural landscapes to provide pollinators and predators of pests with the resources they need to fulfil their lifecycles and facilitate their movement between habitat patches to find mates and establish new populations.

Diversification strategies

In my thesis, I assessed how such diversification strategies, at landscape and field scale, affect insect and spider communities, and how these changes in community composition translate into ecosystem services delivered to crop fields. More so, I was interested in setting up an experiment that tests how insect pollination is affected by climate change stressors. Specifically I asked:

  • Can landscapes with more forest and pasture patches or more diverse crop types benefit pollinators, insect pollination of crop flowers and crop yield?
  • How do flower strips sown along field edges influence the communities of beneficial insects and pests in the crop and the wider landscape, and how are these effects impacted when honey bee hives are added, which potentially compete with wild insects for resources?
  • How do flower strips and honey bee hives influence ecosystem services and crop yield?
  • How does the insect pollination benefit get impacted by simultaneous water stress and insect herbivory, stressors that are expected to occur more frequently under climate change?

I used faba bean cropping as a model system to answer these questions.

 

Results

I found that landscapes with a higher diversity of crop types and with more forest patches and pastures enhanced bumble bee densities in faba bean fields. Flower strips enhanced carabid beetle species diversity and spider abundance in the strip and in the faba bean fields, probably by providing these ground predators with shelter habitat. Flower strips did not attract more bumble bees from the landscape to the faba bean field. However, flower strips did enhance bumble bee queen abundances in the landscape the following spring, but this positive effects of flower strips was lost in landscapes with many honey bee hives. I propose that both the failure of flower strips to attract more bumble bees into faba bean fields, and the significant positive effect of flower strips on bumble bee queen abundances in the landscape the following spring were in part driven by the unusually hot and dry summer in 2018. Due to the extreme weather, the flower strip and faba bean blooming period barely overlapped, and so the additional floral resources provided by the flower strip might not have been sufficient to attract more bumble bees from the wider landscape. In contrast, the nectar and pollen provided by the flower strip in late summer were particularly needed by bumble bees because the hot and dry weather hastened the bloom of many flowering crops, which starved the bees in late summer even more than usual. Faba bean yield was higher in insect pollinated compared with self-pollinated plants but the insect pollination benefit reduced with increasing proportions of forest and pasture patches in the landscape, but was independent of water stress and insect herbivory. Honey bee hive supplementation did not boost yield but enhanced aphid and ladybird beetle densities. Faba bean yield was higher in landscapes with more forests and pastures, but this effect were not driven by pollinator numbers but factors we did not measure. Finally, yield losses can be greatly reduced when climate change stressors, such as water stress and insect herbivory, are reduced simultaneously. Instead, minimal yield gains were realised when water stress was alleviated but plants were under insect herbivore attack.

Conclusions

My thesis shows, using observations and realistic experiments, that access to continuous and diverse floral and shelter resources across agricultural landscapes are key to support beneficial insect communities that are needed for crop production, particularly under climate change. Two of my field seasons were unfolding under extreme weather events associated to climate change. In 2017, weather conditions were unusually wet and cold, while 2018 measured the hottest and driest summer on record. You might be wondering if results that are collected under such 70 unusual conditions can be used to make general conclusions. I argue that my thesis offers valuable insight into how crop diversity and flower strips work under weather conditions that might soon become the norm given climate change. Based on my results, I recommend that farmers and landowners are encouraged and supported to include more crops into crop rotations to increase landscape crop diversity, and to sow flower strips, both of which provide extra food and shelter habitats in agricultural landscapes. In addition, remaining forest and grassland patches should be retained as important nesting and foraging habitats. I recommend that the supplementation of honey bee hives should be avoided in landscapes with scarce floral resources to avoid resource competition between managed and wild pollinators. I also recommend that water withdrawal for agricultural irrigation is prioritised in crops, which are free from herbivores. Agricultural landscapes should provide the resources required to maintain a diversity of pollinators, and of natural enemies of pests that provide resilient protection against herbivore outbreaks.

Text by Chloë Raderschall, chloe.raderschall@slu.se

Facts:

Chloë Aline Raderschall defends her thesis "Diversified agroecosystems for biodiversity and ecosystem services : Ecological intensification of faba bean cropping under land use and climate change" on 16 April 2021

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