Pest management: comparing conventional and organic farming
Large, farm–scale studies show greater biodiversity does not lead to better pest control
16 June 2011
The fourth in a series of papers investigating the how biodiversity and species' network structures affect 'ecosystem services' such as in pest control between organic and conventional farms has been published online.
Around one third of the food ever grown is destroyed by pests or pathogens (ref 1). Part of the food security challenge is to grow food using less inputs, such pesticides, so a knowledge of which method works best in certain situations is essential. Synthetic pesticides are a formidable weapon against maurading pests and may ensure a more certain harvest; equally, biological control using natural predators can render chemicals unnecessary. But there are relatively few studies that directly compare both techniques at the farm level.
Hence, Professor Jane Memmott and colleagues at the University of Bristol decided to look at the differences in biodiversity between conventional and organic farms at the farm–scale, and then see which was better at controlling pests.
To do this, the BBSRC-funded researchers constructed food webs, a network of who-eats-who on twenty farms in southwest England; 10 were organic, 10 conventional, and they were paired for parameters such as size and location (ref 2). Constructing food webs for 20 different locations was a major project, and Memmott says these are the first comparable food webs at any scale looking at organic vs conventional farms. "The fact that they are at whole-farm level makes them particularly unique," she says. "Food webs tend to be made in natural habitats – rain forests, rocky shores etc., there are very few from managed systems."
Pest control: chemicals work, but so do natural predators.
Image: Stavros Markopoulos
Web of life
The results showed that organic farms have significantly more species at three trophic levels (plant, herbivore and parasitoid) and a significantly different network structure as measured by the proportion of links per species, or connectance (ref 3). It was therefore not surprising to find that herbivores on organic farms were attacked by more species of parasitoids than on conventional farms (parasitoids are insects much like the monster in the Alien films: they inject eggs into larvae which grow and consume the prey from the inside before bursting out, killing their prey in the process.)
However, despite the increased number of parasitoid species on the organic farms, the researchers found no difference in percentage parasitism for a variety of prey species on the organic compared to conventional farms - the greater diversity of parasitoid species did not lead to more biocontrol as expected (ref 3 and ref 4). This was a surprise observational finding, so the researchers decided to perform a manipulative experiment to compare pest control on the two farm types by introducing a pest to the farms and seeing how well, or badly, it fared.
To prevent any pest introduction creating a genuine pest outbreak, researchers used a surrogate pest species, the alien leaf miner Phyllonorycter leucographella which is already present (and parasitised by enemies) in the UK and is closely related to real agricultural pests such as the Liriomyza vegetable leaf-miners and apple blotch leaf-miner Phyllonorycter crataegella.
Damage from the leaf miner Phyllonorycter leucographella, which was used to compare pest control between farms.
Image: Jane Memmott
The surrogate Phyllonorycter pest is specific to the non-native shrub Pyracantha coccinea and 50 of these plants were placed in each farm and innoculated with the pest. "Our chosen pest invaded urban habitats very successfully," says Memmott. "Indeed so well it was a study organism in its own right."
Surprisingly, the introduced pest faired equally well on both farm types and was not attacked more on the organic farms where more species of natural enemies were found. "The general expectation would be that all pests will be better controlled," says Memmott. "We have also looked at aphids [greenfly] and found broadly similar results, no differences, but we do predict that differences may exist in some other [insect] groups."
Memmott think the result may be because there wasn't enough difference in the structure of the parasitoid networks between the organic and conventional farms to make a difference in provision for biocontrol, or ecosystem services as it is also known. She also thinks that different results might be obtained in other regions in the UK; the southwest of England is an organic farming hotspot and organic farms may act as reservoirs for natural enemies that benefit conventional farms.
Comparisons of organic and conventional farms at the farm scale are rare.
Image: Journal of Applied Ecology
Alternatively, the use of insecticides on conventional farms may keep pest populations in check to the benefit of organic farms – without more research it's impossible to tell the extent of the interaction in either direction.
The research shows that conducting large-scale experiments is crucial if big food security questions are to be answered. The global food security agenda demands that more food is grown using fewer inputs and with less environmental damage. In policy terms, it is much less a case of organic vs conventional farms, as many agree that both are needed, but fundamental bioscience research informs the policies that aim to tackle agricultural challenges.
- Environmental economic costs of the application of pesticides primarily in the United States
- Plant diversity and land use under organic and conventional agriculture: a whole-farm approach
- Do differences in food web structure between organic and conventional farms affect the ecosystem service of pest control?
- Parasitoid diversity reduces the variability in pest control services across time on farms
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