I parassiti ringraziano l’agricoltura biologica

24 Nov 2009
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Il titolo gia’ sembra una risposta alle tre organizzazioni del biologico che mescolano insetticidi ed erbicidi sotto l’unica categoria dei pesticidi (http://www.salmone.org/2009/11/19/lagricoltura-biologica-gli-ogm-del-tipo-bt-riducono-luso-dei-pesticidi/ ). Qui come e’ ovvio i Pests sono insetti ed e’ quindi ovvio che i pesticidi sono gli insetticidi e solo in questo modo useremo su questo sito la parola pesticidi.

Science ( http://sciencenow.sciencemag.org/cgi/content/full/2009/1113/4?etoc ) riferisce di un articolo che mostra come alcuni parassiti preferiscano le coltivazioni biologiche ed altri quelle che usano fertilizzanti sintetici. Ancora una dimostrazione che la Natura non e’ buona e non pensa a nutrirci producendo frutti per noi, che naturale non vuol dire ne’ sicuro ne’ salutare, ma e’ solo (come direbbe Pascale nel suo ultimo libro) una parola-ameba.

Some Pests Prefer Organic
By Phil Berardelli
ScienceNOW Daily News
13 November 2009
Contrary to claims made by some proponents of organic farming, natural fertilizers are often no better than chemical fertilizers at defending crops against insects–and sometimes they’re worse. That’s what British researchers found over the course of a 2-year trial. The results suggest that farmers should tailor fertilizing to individual plant varieties.
Organic farming has gained popularity in recent decades because of its use of natural ingredients. Proponents say that cow manure, for example, is far less harmful to the environment than petrochemical-based products. Some advocates have also claimed that organic fertilizers help plants resist insect pests better than synthetic varieties do. That’s because plants absorb the nitrogen and other nutrients from organic fertilizers more slowly, and the pest larvae that rely on those nutrients have a tougher time gobbling them up.
Previous research on the topic proved inconclusive, so researchers at Imperial College London and two other institutions in the United Kingdom studied how three pests–two types of aphid and one species of moth–responded to the application of natural and synthetic fertilizers on cabbage plants. The team used chicken manure and other green fertilizers derived from beans and alfalfa, plus commercially produced ammonium nitrate, and applied all in both high and low concentrations. The experiment spanned two growing seasons at multiple field sites.
The team got surprisingly mixed results. The moth, Plutella xylostella, favored the conventionally fertilized plants, laying its eggs about four times more frequently on them than on organically fertilized cabbage. One aphid, Myzus persicae, also preferred the commercial fertilizer, laying eggs on ammonium nitrate-fed plants twice as often as on plants fed with organic fertilizer. But the other aphid, Brevicoryne brassicae, preferred the organically grown variety by about a three-to-one egg-laying margin, the researchers report this week in the Proceedings of the Royal Society B.
The lesson here, says entomologist and co-author Simon Leather, is that the complex chemical interactions between fertilizer and plant can be unpredictable, repelling some pests but attracting others. “One size does not fit all,” he says.
Biologist Gordon Port of Newcastle University in the United Kingdom agrees, calling the work a “robust” study. What remains to be seen, he says, is how the natural enemies of the pests (such as ladybugs for the aphids and wasps and spiders for the moths) react to the two different types of fertilizers.

Varying responses of insect herbivores to altered plant chemistry under organic and conventional treatments
1. Joanna T. Staley1,*, Alex Stewart-Jones2, Tom W. Pope2,Ý, Denis J. Wright1, Simon R. Leather1, Paul Hadley3, John T. Rossiter1, Helmut F. van Emden3 and Guy M. Poppy2
Author Affiliations
1. Division of Biology, Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire SL5 7PY, UK
2. School of Biological Sciences, University of Southampton, Hampshire SO16 7PX, UK
3. School of Biological Sciences, University of Reading, Whiteknights, Reading, Berkshire RG6 6AH, UK
1. *Author for correspondence (j.staley@imperial.ac.uk).
The hypothesis that plants supplied with organic fertilizers are better defended against insect herbivores than those supplied with synthetic fertilizers was tested over two field seasons. Organic and synthetic fertilizer treatments at two nitrogen concentrations were supplied to Brassica plants, and their effects on the abundance of herbivore species and plant chemistry were assessed. The organic treatments also differed in fertilizer type: a green manure was used for the low-nitrogen treatment, while the high-nitrogen treatment contained green and animal manures. Two aphid species showed different responses to fertilizers: the Brassica specialist Brevicoryne brassicae was more abundant on organically fertilized plants, while the generalist Myzus persicae had higher populations on synthetically fertilized plants. The diamondback moth Plutella xylostella (a crucifer specialist) was more abundant on synthetically fertilized plants and preferred to oviposit on these plants. Glucosinolate concentrations were up to three times greater on plants grown in the organic treatments, while foliar nitrogen was maximized on plants under the higher of the synthetic fertilizer treatments. The varying response of herbivore species to these strong differences in plant chemistry demonstrates that hypotheses on defence in organically grown crops have over-simplified the response of phytophagous insects.

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