Flowering plants play a primary role in nature. Not only do they provide additional food sources, such as nectar or pollen, to a diversity of animals (like insects, birds, and spiders), but they also provide shelter for those creatures.
The decline of insect populations, recently documented in parts of the world, has raised an alarm to the community. In several places, like the United Kingdom and the Netherlands, flowers are planted to establish and promote “homes” for insects, especially beneficial insects, to revive their numbers in the environment.
In crop production, farmers have faced many problems caused by insect pests. Chemical pesticides are often used to provide an immediate effective insect killing method, but are toxic to the environment and human health.
While pesticides show their prompt efficacy in insect pest management, they simultaneously also kill other beneficial insects. This status leads to the loss of ‘balance’ in agro-ecology, and farmers may spend more money controlling insect pests.
Planting flowers is a habitat management method that can enhance the presence of natural enemies in controlling insect pests. Natural enemies include parasitoids and predators, both of which feed on insect pests as their primary source of food and energy. Flowers, as a part of conservation biological control, establish habitat for natural enemies by providing them shelter, nectar, alternative host/prey, and pollen (SNAP). All it takes to add conservation biological control to a crop is planting strips of appropriate flowers among crops or along field margins.
Which flowers work best in conservation biological control to attract and sustain natural enemies of insect pests in different crops? Most flowers that attract natural enemies have bright colors and opened structures with exposed stamens and pistils, which help the searchers quickly locate and access the food source.
Scientists have studied and selected some potential flowers that enhance the presence of natural enemies: buckwheat (Fagopyrum esculentum), sweet alyssum (Lobularia maritima), and phacelia (Phacelia tanacetifolia). Buckwheats attract parasitoids, while sweet alyssums and phacelias invite more predators to the area where they are present. Therefore, these floral agents have shown their roles in establishing natural enemy populations for controlling insect pests in different crops. For instance, buckwheat is used in apple orchards, vineyards; phacelia in cereal fields, and alyssum in broccoli fields, and collard green fields.
Using non-crop floral plants as SNAP providers for natural enemies has been successful (review articles in temperate areas, and tropical areas). In agricultural landscapes, the success of this method depends on the complexity of the landscape where it is applied.
A study from a Lincoln University postgraduate student, Mattias Jonsson, has shown that planting flowers in simple landscapes is more effective in controlling insect pests than in complex landscapes.
Jonsson and his colleagues compared two types of flower model (with and without buckwheat strips) on annual kale crop fields in different landscapes. These kale crop fields were grown for winter feed for cattle and sheep. The whole experiment was conducted on twenty-one 1800 meter squared plots and located in different fields, which were 6 km apart from each other in the Canterbury area, New Zealand. The percentage of annual kale crop cover, grassland cover, and wooded habitat on each field were determined to assess the complexity of the landscapes.
Brassica aphids (Brevicoryne brassica) and diamondback moths (Plutella xylostella) are two key insect pests on kale crop. In nature, they are attacked by both parasitoids and predators. In this research, Jonsson and his colleagues focused on wasps that parasite on nymphs of aphids and larvae of diamondback moths. They assessed which landscape types were more compatible with planting flowers method.
The authors found that planting buckwheat worked better in simple landscapes than in complex landscapes. However, they were not clear on why additional flowers made any difference in complex landscapes.
One explanation might be related to the odor from different plant species in complex landscapes that cause difficulties for natural enemies to identify the food source. Another explanation might be that other food sources in complex landscapes provides enough food for natural enemies, and the buckwheat strips became redundant in this situation.
Jonsson’s study has confirmed the effectiveness of using flowers as a habitat management method in controlling insect pests in crop production. We can see habitat management using flowering plants is a useful way to manage insect pests instead of or as well as applying chemical pesticides. And it makes the landscape look prettier!
Mattias Jonsson, Cory S. Straub, Raphael K. Didham, Hannah L. Buckley, Bradley S. Case, Roddy J. Hale, Claudio Gratton, and Steve D. Wratten. Experimental evidence that the effectiveness of conservation biological control depends on landscape complexity. Journal of Applied Ecology 2015, 52, 1274 – 1282.
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Kieu Nguyen is a postgraduate student completing a Master of Science. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.