No flies on bio-diverse fields

aphidA recent article in Plos One highlights how organic farming favours insects, and therefore insect pollinated plants in meadows and wheat fields.  The study explored nine different landscapes under different levels of management intensity. In each landscape plots of organically managed winter wheat and meadow were compared with similar plots managed by a ‘convention’ management practices.  The results confirmed what might be anticipated;  that insect pollinated forb species benefited from the organic management more than non-insect pollinated species, and while there was variation between the plots this was the same for all the landscapes considered irrespective of the degree of management intensification in the surrounding area. This effect did differ significantly between pollination types for species richness in both agro-ecosystem types (i.e. wheat fields and meadows) but highlights how in both the wheat and meadow systems that organic management generally supports a higher species richness and cover of insect-pollinated plants, which is likely to be favorable for the density and diversity of bees and other pollinators and to encourage more insect predator species.

The significance of these results become clearer when we consider the results of a study reported earlier in PLOS One. It reported more sensational  and potentially significant results. There have been few explorations of how organic treatments affect predator levels compared with ‘conventional’ pesticide regimes  In this particular study 30 fields of the grain crop Triticale were investigated (Tricale is increasingly significant because of its relatively good tolerance to drought).  Of these 15 fields were managed organically and 15 using ‘conventional’ treatments. Five of the conventional fields were treated with insecticides while 10 were untreated. The study recorded in the plots the density over time of vascular plant species, pollinator insects, aphids and their predators. The organic fields were found to have a plant species richness five times higher, and an insect species biodiversity 20 times than that in the fields treated conventionally. Most significantly the insecticide treatment, which knocked back aphids only had a short term effect on the densities in the population of these plant-sucking aphids. In fact, later in the season aphid abundances were even higher and predator abundances lower in the insecticide treated compared to the untreated conventional fields.

The researchers concluded that the insecticide treatments kept aphid predators at low abundances throughout the season, thereby significantly reducing top-down natural control of aphid populations. They reported both plant and pollinator species richness as well as predator abundances and predator-prey ratios were higher at field edges compared to field centres; highlighting the importance of field edges for ecosystem services. The organic farming practices resulted in increased biodiversity, including important functional groups like plants, pollinators and predators which enhance natural pest control. Furthermore the preventative insecticide application in conventional fields had only short-term effects on aphid densities and in the longer-term had negative effects on biological pest control. The researchers proposed that conventional farmers should restrict insecticide applications to situations where thresholds for pest densities are reached.

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