## 3 Results and Discussion
### Discussion  
The results are consistent with those reported by [PERSON] _et al._[11], with soybeans infested with _A. glycines_ produce a different profile of volatiles from non-infested leaves. Significant differences between our methods and those of [PERSON] _et al._[11] include (a) the method of volatile collection (passive sampling of enclosed leaf headspace with SPME fibers instead of Super Q absorbent), sorbent tubes and pump-aided gas sampling [11]), (b) the stage of soybean plants tested (reproductive here versus early vegetative stage in [11]), and (c) the use of field-grown soybean (in this research). Regarding the first significant difference, Carboxen/PDMS 75 \(\upmu\)m which targets compounds with a carbon number ranging from 2-12 was very useful in extracting a wide range of volatile and semivolative compounds emitted by leaves infested with _A. glycines_. We report 62 compounds that could be tentatively identified based on matching mass spectral profiles. More compounds could be detected and identified if different SPME coatings were used. Given their sensitivity and ease of use, experiments involving SPME to collect HIPVs in field conditions are warranted. Despite these differences, we also observed greater emissions of MeSA, farnesene and _cis_-3-hexen-1-ol acetate from _A. glycines_ infested soybean leaves. Like [PERSON] _et al._[11], we did not observe more _cis_-3-hexen-1-ol on vegetative stage (V1-4) plants infested with _A. glycines_. However, when plants were in the reproductive stage we did observe more emissions of _cis_-3-hexen-1-ol. These data indicate that insect herbivores, in this case _A. glycines_ and possibly thrips, are associated with greater MeSA volatile production from soybean leaves. From field-grown soybeans, we observed a greater difference in MeSA abundance between _A. glycines_ infested and non-infested soybean leaves. Although the two experiments were conducted under very different growing conditions, it is interesting to note that the average abundance of MeSA from soybean leaves was greater from vegetative stage soybeans grown within the greenhouse (\(\sim\)5.0 \(\times\) 10\({}^{6}\)) than from reproductive stage soybeans grown in the field (\(\sim\)0.45 \(\times\) 10\({}^{6}\)). This order-of-magnitude difference may have been due to the absence of thrips, which were not found on any of the soybean leaves selected from the field. In addition to MeSA, we also collected several other compounds that are considered components of plant defense against herbivores; _cis_-3-hexen-1-ol acetate, farnesene, caryophyllene and humulene. Only _cis_-3-hexen-1-ol acetate and farnesene were produced by leaves with _A. glycines_. Curiously, caryophyllene and humulene were produced in greater abundance by leaves without aphids. Our observations are consistent with a similar study of soybean response to _T. urticae_ ('red spider' mite) [4], in which the headspace was sampled from non-infested and infested leaves removed from greenhouse grown soybean plants. Overall volatile production was greater on _T. urticae_ infested leaves than non-infested leaves, with MeSA comprising 13% of the total volatile output. MeSA was produced by mechanically damaged leaves, but the response was 50\(\times\) as great on _T. urticae_ infested leaves. This difference suggests that MeSA is an indirect defensive compound with limited direct impact, based on the acceptance of _T. urticae_ for soybean [4] and the attractiveness of the predator _Phytoseiulus persimilis_ to MeSA producing plants. [PERSON] _et al._[15] has shown that the volatile response of corn to herbivore injury is not optimized in commercial varieties. To what extant breeding has removed or reduced the volatile response of soybeans to _A. glycines_ is not known. Our results were accumulated with a commercially available cultivar (Syngenta NK S24-K4 RR) that has not been breed for _A. glycines_ resistance.