## 4 Discussion  
Based on our results, we suggest that the honeybee volatilome is impacted by atmospheric CO\({}_{2}\) concentrations. Specifically, elevated CO\({}_{2}\) concentrations lead to a lower diversity and quantity of honeybee VOCs. Acetaldehyde, isobutanol, 2-methyl-1-butanol and 3-methyl-2-buten-1-ol were only found from aCO\({}_{2}\) honeybee. Also, 3-methyl-1-butanol was more abundant in aCO\({}_{2}\) honeydew. To date, the volatile compounds released by aphid honeybee have been identified in three aphid species, including _A. pisum_[2; 9; 10]. Four of the molecules identified in the present study were also found by [PERSON] et al. [9]; namely 2-propanone, 3-methylbutanal, 2-methylbutanal, and 3-methyl-1-butanol. We identified additional VOCs, including 2-methyl-1-butanol, which is  
Figure 3: Mean (\(\pm\)SE) number of eggs laid by the horerfly females on plants smeared with crude honeydew from aCO\({}_{2}\) (**white bars**) and eCO\({}_{2}\) (**grey bars**) conditions.  
Figure 2: Percentage of time (\(\pm\)SE) that horerfly females expressed different behaviors on plants smeared with crude honeydew from aCO\({}_{2}\) (**white bars**) and eCO\({}_{2}\) (**grey bars**) conditions in a dual-choice assay. The random choice category (**black bar**) was shared for both CO\({}_{2}\) conditions.  
also released by the honeydew of the two other aphid species, _Megoura viciae_ Buckton [2] and _Aphis fabae_ Scopoli [10]. These volatile molecules are the results of direct modifications of amino acid-derived starter units, performed by bacteria [19]. We found additional molecules that were not previously detected in aphid honeydew, including acetaldehyde, 2-thiapropane, isobutanol, and 3-methyl-2-buten-1-ol. The observed differences in VOC emissions might be related to different rearing conditions, aphid strains and associated microbial flora.  
Whether the honeydew bacteria community (diversity and abundance) is impacted by CO\({}_{2}\) concentration needs investigation. Amino acids and sugars present in honeydew are considered as precursors for bacterial metabolism [10]. Phloem sap composition might be modified under increased CO\({}_{2}\) concentrations, due to the primary and secondary metabolism of host plants being modified [20]. However, the impact of CO\({}_{2}\) on phloem characteristics remains unclear, with some authors finding that free amino acid content decreased [21; 22; 23], showed no significant modification [24; 25], or increased [26]. Because honeydew composition reflects phloem content, its nutrient composition might also be affected by CO\({}_{2}\) concentrations.  
The volatile cues released by aphid honeydew have been shown to guide aphid predators toward their prey, and to induce oviposition [27]. In our behavioral assay, gravid females of _E. balteatus_ used both aphid-infested plants (aCO\({}_{2}\) and eCO\({}_{2}\)) as appropriate oviposition sites. The slight qualitative and quantitative differences in VOCs highlighted previously were not sufficient to allow ovipositing females to discriminate the two plants.  
We also hypothesized that the amount of excreted honeydew is impacted by CO\({}_{2}\) concentrations. Indeed, due to possible changes in phloem sap, aphids might adapt their feeding behavior, and thus the ingestion of sap, to reach their nutritional threshold. We found that aphid colonies (in terms of weight) reared under enriched CO\({}_{2}\) concentrations reduce the quantity of honeydew emitted, but do not influence the number and size of the droplets excreted by aphids. Contrasting results were previously obtained, suggesting that aphids reared under elevated CO\({}_{2}\) concentrations produce less [22] or more honeydew [28]. However, the species used and the physiological status of the host plant might influence the quality of excreted honeybee.