偏爱天然
这种有翅膀的蚜虫（Myzus persicae）对添加了有机肥料的卷心菜不感兴趣。
图片提供: 美国农业部

与某些有机农业的支持者所持的观点恰好相反，与化肥相比，天然肥料在保护农作物免遭害虫侵袭方面并不一定做得更好，有时甚至要更糟。这正是英国研究人员在一项为时两年的实验中获得的发现。这一研究成果表明，农民需要根据不同的植物类型有选择地使用各种肥料。

由于使用了天然成分，有机农业在最近的几十年中变得越来越流行。例如，这种耕作方式的支持者认为，与石化产品相比，牛粪对环境的危害远比前者要小得多。一些提倡者也曾宣称，与人造肥料相比，有机肥料能够帮助农作物更好地抵御病虫害。这是因为植物从有机肥料中吸收氮和其他营养物质的速度更为缓慢，因此以这些营养物质为食的害虫幼虫则不得不需要面对一段艰苦的时光。

之前对此问题进行的研究被证明是缺乏说服力的，因此英国伦敦皇家学院的研究人员与英国其他两家研究机构的科学家联手，就3种害虫——两类蚜虫及一种蛾子——如何对在卷心菜上施用的有机肥料和人造肥料作出反应进行了研究。研究小组将鸡粪以及来自豆科植物和苜蓿的其他绿色肥料，还有商业生产的硝酸铵，分别以同样的浓度施加在卷心菜上。这项实验在多个田间试验点持续了两个生长季节。

研究人员最终取得了令人惊讶的结果。与被施用了有机肥料的卷心菜相比，一种蛾子（Plutella xylostella）更喜欢施加了人造肥料的植物，它们在后者上产的卵通常是前者的4倍。一种蚜虫（Myzus persicae）同样也喜欢商业肥料——与添加了有机肥料的植物相比，它们在使用了硝酸铵的卷心菜上产的卵是前者的两倍。然而还有一种蚜虫（Brevicoryne brassicae）却喜欢用鸡粪浇注的卷心菜，它们在这些蔬菜与用化肥培植的蔬菜上的产卵比为3∶1。研究人员在最新出版的英国《皇家学报B》上报告了这一研究成果。

参与此项研究的昆虫学家Simon Leather指出，这项研究为我们带来的教训便是，肥料与植物之间复杂的化学反应是很难被预测的，某些肥料可能会击退一些害虫，但同时却会吸引另一些害虫。他说：“一个尺码不可能适合所有的身材。”

英国纽卡斯尔大学的生物学家Gordon Port对此表示赞同，他称这项研究是一项“坚实的”工作。Port认为，接下来需要搞清的便是害虫在自然界的敌人——例如蚜虫的天敌瓢虫，以及蛾子的天敌黄蜂和蜘蛛——如何对这两种不同的肥料产生反应。

原始出处：

Proceedings of the Royal Society B November 11, 2009, doi: 10.1098/rspb.2009.1631

Varying responses of insect herbivores to altered plant chemistry under organic and conventional treatments

Joanna T. Staley1,*, Alex Stewart-Jones2, Tom W. Pope2,?, Denis J. Wright1, Simon R. Leather1, Paul Hadley3, John T. Rossiter1, Helmut F. van Emden3 and Guy M. Poppy2

1Division of Biology, Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire SL5 7PY, UK
2School of Biological Sciences, University of Southampton, Hampshire SO16 7PX, UK
3School of Biological Sciences, University of Reading, Whiteknights, Reading, Berkshire RG6 6AH, 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.