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

Abstract

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 highnitrogen 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.

Plant nutrient supply determines competition between phytophagous insects

“…provide little support for a consistent direct effect of fertilizer type on herbivore populations (see also [21]), but a release from competition on plants supplied with synthetic fertilizers might indirectly allow greater population growth under conventional agricultural practices.…”

“…pekinensis cv Wong Bok) for several generations under the same environmental conditions as detailed above [30]. The infestation sequence (B. brassicae before P. xylostella) was chosen to mimic the order of arrival of these species on UK Brassica plants [21].…”

“…Samples were analysed by high-performance liquid chromatography on an Agilent 1200 series instrument equipped with a Phenomenex Luna 3 micron C18(2) (150 Â 2 mm) reversephase column. Desulphoglucosinolates were separated using a water-acetonitrile gradient (solvent A water, solvent B acetonitrile; 0-15 min, 25% B; 15-17 min, 70% B) at a flow rate of 0.2 ml min 21 . Retention times of known standards were used to identify desulphoglucosinolates; identification was confirmed by liquid chromatography-mass spectrometry.…”

“…Brevicoryne brassicae feeds predominantly on the plant apex and young foliage [20], whereas P. xylostella larvae feed mainly on older leaves (V. Chadfield & J. T. Staley 2009, unpublished data). The two species co-occur on crucifers during the spring and early summer in the UK [21], but are unlikely to compete through interference owing to their different feeding modes and sites.…”

Major Sulphur Compounds in Plants and their Role in Human Nutrition and Health - An overview

“…However, recently GSLs have been reported to be involved in mitigating cancer ((Beecher, 1994;Williamson et al, 1998;Das et al, 2000;Hayes et al, 2008) and consumption of large amounts of vegetables especially broccoli, Brussels sprout and cabbage is being encouraged. Staley et al (2010) reported that GSLs in cabbage (Brassica oleracea var capitata) were three times when grown on organic manure than when it was grown on chemical fertilizers. He also reported that the content of glubrassicin was much more than sinigrin.…”