We found that (i) herbivory by aphids inhibits the production of floral volatiles in S. alba
, (ii) the inhibition was not caused by a chewing herbivore and was more pronounced in a specialist aphid than in a generalist, (iii) inhibition of floral volatile production did not have a major disruptive effect on pollinator visitation or on olfactory orientation of the aphids' natural enemies. Previous studies have reported that vegetative feeding by chewing herbivores can result in floral volatile emissions that are either enhanced 
, reduced 
or unaffected 
. Our study is the first to report that phloem-feeding herbivory affects floral volatile emission, and shows that the outcome of interaction between herbivory and floral chemistry may differ depending on the herbivore's feeding mode and degree of specialisation.
Rather than being a consequence of herbivory-related stress in general, inhibition of floral volatiles was dependent on the identity of the herbivore. The more pronounced effect of L. erysimi
compared with the highly polyphagous M. persicae
may indicate that the degree of aphid specialisation is also important. Differences in the way aphids interact with host plants at the molecular and biochemical level have been found between Brassica
specialists and generalists 
, and our results provide further evidence that plant responses to herbivory vary with the identity and feeding mode of the herbivore 
We did not investigate the mechanisms underlying the suppression of floral volatile production by aphid feeding but potential mechanisms can be proposed. Stress caused by aphid damage could compromise general plant health to an extent that floral volatile production cannot be supported. This is unlikely since the relatively severe damage caused by the chewing herbivore P. xylostella
had no negative effect. Another possibility is that aphids remove phloem sap, which could contain signalling molecules. The effect of L. erysimi
was more pronounced than that of M. persicae
and there is no reason to expect the specialist and generalist aphid to selectively remove phloem components, although through differences in feeding behaviour, the specialist may represent a stronger sink than the generalist. Nevertheless, the most likely explanation involves the interaction between aphids and plant defence pathways. Plant responses to phloem feeders differ from those by chewing herbivores 
; aphids cause less damage to plant tissues and activate salicylate-mediated responses more than those mediated by jasmonate signalling 
. The major compounds in the floral scent were volatile benzenoid compounds derived from the breakdown of phenylalanine by phenylalanine ammonia-lyase (PAL). This enzyme has been implicated in plant defence against aphids 
, and it is possible that aphid feeding resulted in diversion of either substrate or enzymes into induced defence and away from floral volatile production.
Piercing-sucking insects in particular have been shown to be able to evade, suppress or manipulate plant defences to their advantage 
, thus a specialist aphid such as L. erysimi
may be more highly adapted for disarming specific defence pathways found in its host plant, giving rise to the differences in volatile inhibition found for the two aphid species. We hypothesize that feeding by the specialist aphid L. erysimi
suppresses floral volatile production through modification of these interrelated defence pathways by the introduction, into the phloem, of salivary factor(s), and a role for phloem mobile chemical signalling.
A primary role for floral volatiles in S. alba
should be attraction of insect pollinators 
. Thus we hypothesised that the strong inhibition of floral volatiles caused by L. erysimi
in the laboratory would negatively impact the plant's ability to attract these insects. However, in our field experiment we did not observe a reduction in the number of visits to infested plants. Visitation was generally low, possibly due to the absence of S. alba
in the habitat. It is possible that in habitats where the plant is abundant, pollinator visitation will be higher and aphid damaged individuals will suffer a reduction in pollinator visitation. Although we did not find effects on insect pollinators here, this ecological interaction deserves to be explored further. We found that aphid feeding reduced the amounts of both major and minor components of the floral volatile blend. The major benzenoid components are often reported as pollinator attractants [e.g. 36]
but even floral compounds occurring in small amounts can be important in this regard 
. Interactions between herbivores and pollinators mediated by plant chemistry could be common in natural ecosystems and the effects could be highly dependent on the ecological context, varying with the pollinator community and availability of alternative resources.
The natural enemies of herbivores respond to changes in plant volatile emissions induced by herbivore feeding as a means of locating their prey 
. We tested olfactory orientation of the aphidophagous ladybird C. septempunctata
and the parasitoid D. rapae
which use aphid-induced volatiles in host location 
and may also use flower resources as food, with the hypothesis that inhibition of floral volatiles would allow aphids to become less chemically apparent to their predators.
Ladybirds were attracted to odour of S. alba
flowers but preferred the odour of flowers from undamaged plants over odour of flowers from L. erysimi
-infested plants. This suggests that the ladybird is attracted to floral volatiles and that attraction may be disrupted by the effects of aphid feeding, potentially allowing L. erysimi
to avoid detection. However, when presented with the entire plant, ladybirds were more attracted to infested plants and may have responded to the volatiles induced in vegetative tissues by aphid feeding or to the modified blend. Behavioural and/or electrophysiological responses of C. septempunctata
have been reported to (Z
)-3-hexen-1-ol and (Z
)-3-hexenyl acetate 
, methyl salicylate 
, which were either induced or increased in L. erysimi
-infested plants. Thus it appears that any reduction in ladybird attraction due to reduced floral volatile emission was overridden by volatiles associated with aphid feeding on vegetative tissues.
Ladybirds preferred odour of M. persicae
-infested plants over L. erysimi
-infested, possibly reflecting a preference for the generalist aphid over the Brassica
specialist which can sequester toxic glucosinolates from the host plant 
. There were qualitative and quantitative differences in the volatiles induced by L. erysimi
and M. persicae
that could potentially allow the ladybird to differentiate between the two blends. Several compounds were detected only from M. persicae
-infested plants, including the 3-butenyl- and benzyl isothiocyanates, whereas 2-tridecanone was detected exclusively from L. erysimi
infestation. When presented with vegetative parts or flowers alone however, ladybirds showed no preference for either aphid treatment, suggesting a more complex integration of volatiles from the two sources.
was strongly attracted to odour of flowering S. alba
, but preferred odour of flowers from L. erysimi
-infested plants over odour of flowers from uninfested plants. This suggests that this Brassica
specialist parasitoid can use volatile cues from the flowering parts of L. erysimi
-infested plants. The cue may be a reduced concentration of floral volatiles or changes in the ratio of floral components that could potentially signal the presence of aphids. Alternatively it could be induced volatiles from the vegetative tissues supporting the flowers that were below the detection limits in our analysis, or a combination of the above. Thus, while aphid natural enemies showed modified responses to aphid-induced floral volatile inhibition, we did not find strong evidence that aphids would become less chemically apparent as a consequence. Although D. rapae
may have an oviposition preference for M. persicae
over L. erysimi
, the parasitoid did not show an olfactory preference for plants infested with either aphid and this is in line with a previous study 
Flower chemistry and defensive chemistry have a common evolutionary origin 
, however most of the work relating plant defence and pollination has emphasised the dual role of floral chemistry in terms of attracting pollinators and reducing herbivory to reproductive parts [e.g. 43]
. Our findings suggest a more intimate link between plant responses to insect herbivory, in this case by a phloem-feeder, and the biosynthesis of floral volatiles. They also reveal differences in the way specialist and generalist aphids interact with plant biochemistry, and this discovery may eventually help to illuminate how aphids avoid or manipulate plant defences.