While plants produce complex cocktails of chemical defences with different targets and efficacies, the biochemical effects of phytotoxin ingestion are often poorly understood. Here, we examine the physiological and metabolic effects of the ingestion of glucosinolates (GSLs), the frontline chemical defenses of brassicas (crucifers), on the generalist herbivore Helicoverpa armigera. We focus on kale and cabbage, two crops with similar foliar GSL concentrations but strikingly different GSL compositions. We observed that larval growth and development were well correlated with the nutritional properties of the insect diets, with low protein contents appearing to exacerbate the negative effects of GSLs on growth, pupation and adult eclosion, parameters that were all delayed upon exposure to GSLs. The different GSLs were metabolized similarly by the insect, indicating that the costs of detoxification via conjugation to glutathione (GSH) were similar on the two plant diets. Nevertheless, larval GSH contents, as well as some major nutritional markers (larval protein, free amino acids, and fat), were differentially affected by the different GSL profiles in the two crops. Therefore, the interplay between GSL and the nitrogen/sulfur nutritional availability of different brassicas strongly influences the effectiveness of these chemical defenses against this generalist herbivore.

Brassicaceae (Cruciferae) are ostensibly defended in part against generalist insect herbivores by toxic isothiocyanates formed when protoxic glucosinolates are hydrolysed. Based on an analysis of published host records, feeding on Brassicas is widespread by both specialist and generalists in the Lepidoptera. The polyphagous noctuid moth Helicoverpa armigera is recorded as a pest on some Brassicas and we attempted to improve performance by artificial selection to, in part, determine if this contributes to pest status. Assays on cabbage and kale versus an artificial diet showed no difference in larval growth rate, development times and pupal weights between the parental and the selected strain after 2, 21 and 29 rounds of selection, nor in behaviour assays after 50 generations. There were large differences between the two Brassicas: performance was better on kale than cabbage, although both were comparable to records for other crop hosts, on which the species is a major pest. We discuss what determines “pest” status.

Outcrossing rates and seed set were estimated at both the population and individual plant scale within four populations of Xanthorrhoea johnsonii. Sites differed with respect to habitat disturbance (time since ?re), plant density, mean plant height, and mean in?orescence length. Allozyme data from progeny arrays of 24 seeds per plant from ten plants at each site were used to determine maternal outcrossing rates, which were found to be consistently high at all sites (> 90%). Small but signi?cant levels of inbreeding were recorded at three of the four sites. There was close agreement between tm and ts measures, which indicated that most inbreeding was a result of within plant sel?ng rather than biparental inbreeding. Outcrossing rates were estimated within top, middle, and bottom thirds of spikes on six plants at a ?fth site. Signi?cant differences from complete outcrossing were recorded in the middle and/or bottom sections of three plants. Total seed set per plant and seed set per ?ower varied signi?cantly among the four sites. Xanthorrhoea johnsonii appears to be able to maintain relatively high outcrossing rates under different ecological conditions, although levels of both seed set per plant and seed set per ?ower are much more variable. 頲013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 335-345. ADDITIONAL KEYWORDS: ?owering density - fragmented populations - long-lived perennial - mating systems - plant age - seed set.Griffith Sciences, Griffith School of EnvironmentNo Full Tex