Bottom‐up effects of glucosinolate variation on aphid colony dynamics in wild cabbage populations

Abstract

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.

Plant responses to multiple herbivory : phenotypic changes and their ecological consequences

“…Moreover, the effect of B. brassicae-infestation on insect community waned when the season progressed in both 2012 and 2013. Seasonal variation in insect responses to plant chemical traits was also reported for the wild cabbage populations in their natural habitat in the UK (Newton et al, 2009). Plant-mediated effects on herbivores are not constant over the growing season and can be attributed to changes in herbivore densities and chemical traits of the plants .…”

“…the enhanced production of aphid-induced carnivore attractants. Controlled for genotypic variation, Newton et al (2009) reported a strong positive correlation between B. brassicae density and parasitism by Diaeretiella rapae, the most common parasitoid of B. brassicae. In the present study, there was no effect of cabbage population on the population dynamics of B. brassicae in either of the two years.…”

“…In the present study we have examined the effect of early aphid infestation on the arthropod community dynamics under field conditions. We used plants from three wild cabbage populations that display variation in both constitutive and herbivore-inducible foliar glucosinolate profiles (Mithen et al, 1995;Newton et al, 2009). Glucosinolates are secondary metabolites characteristic of the Brassicaceae (Fahey et al, 2001) and have been shown to play an important role in insect resistance ).…”

“…In a field experiment, we monitored the herbivore and carnivore community on plants originating from three wild cabbage populations that were either inoculated with the aphid B. brassicae L. (Hemiptera, Aphididae) or that were not inoculated with aphids early in the growing season. Brevicoryne brassicae is specialised on brassicaceous plants (Hughes, 1963) and is a common species on wild cabbage in its native habitat (Newton et al, 2009). In addition, we monitored parasitism of one of the most abundant chewing herbivores Plutella xylostella L. (Lepidoptera: Plutellidae) in detail by releasing and re-capturing larvae of this species and determine whether they were parasitized or not.…”

“…Insect herbivory may drive evolution of plant resistance and competitive ability in the field . For instance, the wild cabbage populations used in this thesis have been shown to differ in glucosinolate profiles, important secondary metabolites in brassicaceous plants, and to harbour different herbivore densities Newton et al, 2009). The investigation of natural variation in how plants interact with their insect community is important to understand the evolution of plant defences, i.e.…”