Natural plant populations often show substantial heritable variation in chemical structure of secondary metabolites. Despite a great deal of evidence from laboratory studies that these chemicals influence herbivore behaviour and life history, there exists little evidence for the structuring of natural herbivore communities according to plant chemical profiles. Brassica oleracea (Brassicaceae) produces aliphatic glucosinolates, which break down into toxins when leaf tissue is damaged. Structural diversity in these glucosinolates is heritable, and varies considerably at two ecological scales in the UK: both within and between populations. We surveyed herbivore attack on plants producing different glucosinolates, using 12 natural B. oleracea populations. In contrast to the results of previous studies in this system, which suffered low statistical power, we found significant differential responses of herbivore species to heritable glucosinolates, both within and between plant populations. We found significant correlations between herbivore infestation rates and the presence or absence of two heritable glucosinolates: sinigrin and progoitrin. There was variation between herbivore species in the direction of response, the ecological scale at which responses were identified, and the correlations for some herbivore species changed at different times of the year. We conclude that variation in plant secondary metabolites can structure the community of herbivores that attack them, and propose that herbivore-mediated differential selection deserves further investigation as a mechanism maintaining the observed diversity of glucosinolates in wild Brassica.

gross domestic product, purchasing power parity); (b) percent of the GDP invested in research; (c) Social Progress Index (SPI, data from the NGO Social Progress Imperative); (d) total population size; and (e) the proportion of English speakers. Our choice of country-level indices is underpinned by the following hypotheses: (a) economic growth relates positively to the valuation and production of applied ecological research; (b) countries that invest more in research are more likely to produce and read applied ecological research; (c) social progress relates positively to research production and valuation; (d) all else being equal, more populated countries will have a greater number of people reading applied ecological research; (e) people in countries with an official language that is not English are less likely to read and produce applied ecological research in English-language | 5 Journal of Applied Ecology EDITORIAL

1. There is an ongoing debate about the relative importance of top-down and bottom-up regulation of herbivore dynamics in the wild. Secondary metabolites, produced by plants, have negative effects on survival and growth of some herbivore species, causing bottom-up regulation of population dynamics. Herbivore natural enemies may use plant secondary metabolites as cues to find their prey, but their survival and reproduction can also be influenced by the upward cascade of secondary metabolites through the food web. Thus plant chemistry might also affect herbivore populations by mediating top-down regulation.2. We investigated the influence of heritable variation in aliphatic glucosinolates, a class of secondary metabolites produced by Brassica plants, on the relative importance of top-down and bottom-up regulation of Brevicoryne brassicae (mealy cabbage aphid) colonies in natural Brassica oleracea (wild cabbage) populations. We manipulated natural enemy pressure on plants differing in their glucosinolate profiles, and monitored aphid colony growth and disperser production.3. Aphid colony sizes were significantly smaller on plants producing sinigrin, compared with plants producing alternative aliphatic glucosinolates. Aphid natural enemy numbers correlated with aphid colony size, but there was no additional effect of the plants' chemical phenotype on natural enemy abundance. Furthermore, experimental reduction of natural enemy pressure had no effect on aphid colony size or production of winged dispersers. 4. Our results provide evidence for glucosinolate-mediated, bottom-up regulation of mealy cabbage aphid colonies in natural populations, but we found no indication of top-down regulation. We emphasise that more studies of these processes should focus on tritrophic interactions in the wild.

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