Organic farming supports higher biodiversity. Research involving the Colorado potato beetle shows that this increased diversity can deliver a better ecosystem service in the form of more effective pest control.
The control of pests by their natural enemies is a valuable ecosystem service1: unpaid, and often overlooked, predatory insects such as ladybirds devour their voracious cousins that damage crop plants. But the insecticide sprays used in conventional farming are largely non-selective, and Crowder et al.2 (page 109 of this issue) now show that such insecticides disrupt the communities of those natural enemies — which, in turn, provide less effective pest control.
The intensification of agriculture is one of the main drivers of biodiversity loss. Organic management seeks to mitigate this loss by reducing chemical treatments, particularly through decreased spraying of insecticides to control pests. There is little doubt that organic farms generally support more biodiversity, with a higher abundance and greater species richness of many plant and animal groups3. Intensive management also disrupts food webs and can lead to communities dominated by one or a few resistant species, with most other species being rare4,5. However, ecologists have been challenged to provide evidence that the increased biodiversity on organic farms actually leads to a better ecosystem service in the shape of better pest control6.
Diversity itself has two components: species richness (the number of species) and species evenness (the relative abundance of those species). Species richness and evenness can vary independently, so communities can contain the same number of species but still differ in evenness. Communities dominated by a few common species have low evenness scores, whereas species are more equally represented in highly even communities. Reduced species richness and evenness can leave ecological niches unfilled or under-exploited, reducing ecosystem functioning and ecological services.
Central Washington state, where Crowder et al.2 carried out their study, is one of the regions where the infamous yellow-striped Colorado beetle feeds on potato crops (Fig. 1). The insect is native to North America but has spread around the globe, causing problems in both Asia and Europe. It can cause extensive damage, necessitating regular rotation of crops to avoid total defoliation by overwintering adults. The conventional response is to spray with a variety of insecticides, but the beetle has an extraordinary ability to develop resistance to these treatments, especially in regions in which spraying is intensive7. Thus potatoes, like other crops, can be victims of the pesticide treadmill8: escalating resistance leads to increased spraying with an ever-wider variety of chemicals, and spiralling costs.
Fortunately, the Colorado beetle is attacked by a variety of predatory bugs and beetles in the foliage of the plants, and pathogenic nematodes and fungi that inhabit the soil attack pupating adults below ground. In organic potato fields, communities of both above-ground and below-ground enemies exhibit high evenness, whereas in potato fields subject to conventional practice the evenness of both communities is much lower. A meta-analysis carried out by Crowder et al. confirmed that low evenness is a general property of natural-enemy communities on conventional farms when compared with organic ones. This suggests that the positive effects of organic farming on biodiversity could have been underestimated, as many surveys so far3 have included only a measure of species richness.
Unusually, Crowder et al. also examined the importance of natural-enemy evenness for successful pest control by setting up a controlled experiment. They grew potato plants in field enclosures together with the beetle pest, and independently manipulated predator and pathogen evenness above and below ground. They found that increased evenness of both predator and pathogen communities resulted in better pest control and, more importantly, larger plants. Higher evenness resulted in better control even though the different predator species all attack the same prey. This is probably because they forage on different parts of the plant, and, collectively, their complementary activities provide a better service.
This view is supported by the finding that predator survival was better in communities with high evenness, suggesting that they compete less with individuals of other species than with their own kind. Crowder and colleagues' research therefore provides direct evidence that greater diversity of natural enemies results in an improvement in the ecosystem service of natural pest control. Their study also raises an intriguing possibility for better biological control: the coordinated release of multiple predatory species rather than the traditional mass releases of single species7.
Clearly, we need to better understand the direct and indirect effects of pesticide use at the level of individual farms and on regional scales. Although pesticides can undoubtedly reduce populations of target insects, Crowder et al. demonstrate that the damage to natural-enemy communities leads to less effective natural control. Even subtle damage to such communities, such as changes in species evenness, can have large and measurable effects on crop performance. In turn, reduced natural control is likely to engender a cycle of continued reliance on pesticides and repeated spraying, reducing natural enemies still further. There is already evidence from China that the notorious brown plant hopper, a major rice pest, has achieved this status only thanks to the gradual elimination of its predators5,9. The reduction in natural control associated with pesticide use is probably one reason why the yields from organic farming are often comparable to those achieved through conventional farming10 — although one limitation of Crowder and colleagues' investigation is that they do not provide direct comparisons of potato yields for the organic and conventional farms that they sampled.
Their study nonetheless shows the need to take a broader approach to biological diversity and its preservation. It may not be enough, for example, to simply count species and document extinctions. Crowder et al. highlight the point that more subtle changes can also have serious consequences for the ecosystem services that natural communities provide, and on which we all rely.
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Intercropping cereals with faba bean reduces plant disease incidence regardless of fertilizer input; a meta-analysis
European Journal of Plant Pathology (2019)