Pest control, naturally

Last Updated 19 July 2010, 11:56 IST

Organically grown food, which is to say, food grown without the help of synthetic fertiliser or insecticides, is feted as tastier and more nutritious. The claim is doubtful as the flavour and composition of a plant depend on its genes and not particularly the manner of cultivation. But, whatever the truth, organic food is considered expensive, as ‘unaided’ cultivation, for the benefit of the consumer, is said to need costlier tending and to have lower yield.

Recent work shows that this is not true. Organic farming is found to promote biodiversity, which, in turn, it is found, leads to natural pest resistance. All pesticides have a major negative attribute: that they get less effective and need to be used in progressively larger quantities. They also poison both the soil and groundwater. To find that organic farming is economical would thus have commercial importance.

Natural environments usually have many species living together. The relationship is symbiotic, with species mutually providing nutrition for or controlling predators of other species. Proliferation of one species at the cost of others then affects the growth of the proliferating species, which would reverse the trend. Over time, a balance of relative abundance is struck, including a balance in the rate of relative exploitation of soil resources, in keeping with the pace of their natural replenishment.

Agriculture, which is organised cultivation of a single plant variety, is a celebrated villain in disturbing the balance. With the loss of the safety net of balancing species, the cultivar needs special protection against pests and hence the need for chemical pesticides.  
But along with the pests, conventional farming also destroys the natural enemies and predators of the pests themselves. The use of the pesticide then creates more work for itself, apart from the resistance that the pests develop against the pesticide.

This effect is avoided in organic farming. While it is accepted that such farming promotes biodiversity, with higher abundance and greater species richness, there has still not been clear evidence that this increased biodiversity translates into better pest control. A study by entomologists at Washington and Georgia, USA notes that conventional farming methods degrade ecosystems in two ways - one by reducing the number of species and second by disturbing their relative abundance. This idea of relative abundance is a statistical measure of the distribution of different populations in an aggregate and has been formally studied in the field of information theory. There is then a unit of the level of ‘evenness’ of the populations, which helps measure the effectiveness of the different species acting together.

The study has looked into the effectiveness of ‘evenness’ of the pest reducing populations and it finds that the balance of populations as found in rain tropical forest or organic farming conditions dramatically increases the action of the natural pest control and also makes for more luxuriant growth of the subject vegetation!

Colorado beetle
The study, just described in Nature, was conducted in the state of Washington, whose potato crop faces extensive damage from the infamous yellow-striped Colorado beetle. The attacks are so intense that crops need to be regularly rotated, to avoid total destruction.  The usual remedy is spraying with a mix of insecticides, but the beetle keeps getting resistant. The sprays then need to be intensified and the range of chemicals widened, which increases costs.

The good news is that there is a variety of bugs and beetles that attack the Colorado beetle in the potato plant leaves and also worms and fungi that attack the beetle pupae below the ground.

But conventional farming, in a bid to control the Colorado beetle, also puts paid to the beetles’ natural enemies, and hence the farmers’ friends. But what is more significant is that apart from reducing the pest predator populations, these measures also affect their population distribution, which can make the surviving predators many times less effective.

The scientists divided the potato field into enclosures with equal infestation by the beetle pest, but with different combinations of different insects that prey on the beetle and pathogens that affect its breeding and growth. The effectiveness of the combinations was compared with the ‘evenness’  of the distribution of pest control agents.

The potato crop scientists found that increased evenness of both predator and pathogen communities resulted in better pest control and, more importantly, larger potato plants. Higher evenness of these agents leads to more efficient attack probably because they forage on different parts of the plant, and, collectively, they complement each other for better results.

The study shows that even subtle damage to pests’ natural enemy communities, like changes in evenness, can have large effects on crop performance. Reduced effectiveness of natural pest control leads to greater reliance on pesticides and further weakening of natural control. The study also shows the way to more effective biological pest control, by the release of several predatory species, rather than the traditional mass release of a single species.

Predators promote diversity        
Another study by the Smithsonian Tropical Research Institute in Panama, described in Nature a week earlier, finds that species-specific enemy organisms, which proliferate in the vicinity of an adult plant, actually limit seedlings of the same plant and promote relative abundance of other species. This behaviour, known as negative feedback, has been documented in forest ecosystems but how this comes about has not been clear.
In the STRI study, seedlings of different species were grown in the presence of different enemy organisms, for eg mammals, invertebrates and microbes, both in controlled green house conditions and in the field.
The results showed clear negative feedback effect of soil dwelling organism. Above ground enemies, like mammals, leaf eaters or leaf pathogens seemed to have little effect.
The study has revealed the importance of soil biota in maintaining abundance of plant varieties. 

The role of soil biota has not received attention so far and it was believed that the self limiting, ie, negative feedback effect was strongest in abundant species. But it is found that the opposite, in fact, is the case.

Studies that illuminate the mechanisms of diversity in ecosystems will enable sustainable farming, as demands on agriculture and forestry swell with increasing centres of consumption.

(Published 19 July 2010, 11:56 IST)

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