## 4 Discussion  
Using a comparative approach, we investigated the role of host plant-derived volatiles in the foraging behavior of adults of _E. scrobiculatus_ and _E. brandti_ via behavioral and antennal responses of both weevil species. Under field conditions, _E. brandti_ adults feed on the trunk. In the field study, we found many weevils aggregated on the bare phloem of healthy and injured trees, while they preferred to feed on the tree limb of _A. altissima_ in the large-arena experiment. In the Y-tube bioassay, female and male _E. brandti_ significantly preferred to volatiles from phloem, regardless of tree condition. Under field conditions, _E. scrobiculatus_, adults feed on 1-year-old branches, perennial branches, and petioles. In the field study, we found that weevils aggregated on the bare phloem of healthy and injured trees. Adult _E. scrobiculatus_ significantly preferred phloem volatiles over trunk volatiles in the large-arena experiment whereas in the Y-tube bioassay female and male weevils responded positively to all types of plant parts that they were exposed to. Hence, while both weevil species have the ability to discriminate against different plant parts, _E. scrobiculatus_ responses to host plant material seem to be more variable compared with _E. brandti_. Overall, our findings indicate that volatile compounds of _A. altissima_ may act as important olfactory cues in feeding behavior, particularly the volatiles from phloem for _E. brandti_. Volatiles of different parts of _A. altissima_ were collected by two methods (HS-SPME and dynamic headspace). It is known that plants may release more quantities of volatile chemicals when damaged by herbivorous insects [23]. In this study, the number of volatile compounds of phloem from weevil-damaged trees were greater than those identified from healthy trees, regardless of the collection method (HS-SPME or dynamic headspace method). When compared to volatiles from phloem, there were fewer volatile compounds identified in annual branches and seedling foliage. While volatile compounds emitted by _A. altissima_ have been analyzed before, there has been some variability in results. For example, [PERSON] and [PERSON] [24] analyzed the chemical composition of volatile compounds from fresh and dried leaves of young and old trees, and the main constituents were aliphatic C\({}_{6}\)-compounds and sesquiterpenes. Volatile compounds from leaves were extracted by simultaneous distillation extractor (SDE) and SPME, the component with the highest relative content was cyanophyllene, and there were more compounds being detected in SPME than SDE extracts [25]. In turn, [PERSON] [26] analyzed the volatile components of branches using the dynamic headspace method and found that \(\alpha\)-pinene was the most abundant component. [PERSON] et al. [27] analyzed the volatile components of leaves by HS-SPME, and the main components were (4_E_)-4-hexenyl acetate and (Z)-hex-3-en-1-ol. In this study, the main component in phloem from healthy and injured trees by HS-SPME was 1-hexanol and \(\beta\)-caryophyllene, respectively. The main component in phloem from healthy and injured trees, seedling foliage and annual branches by dynamic headspace method was \(\beta\)-caryophyllene, 1-tetradence, \(\beta\)-copaene, and \(\beta\)-copaene, respectively. Overall, the type of collection method seems to have a strong influence on the qualitative and quantitative composition of _A. altissima_ volatile compounds. In order to more precisely identify the volatile components of different parts of _A. altissima_, the methods of collection and analysis of volatiles need to be further refined. This investigation provided the first evidence that _E. scrobiculatus_ and _E. brandti_ respond positively to volatiles emitted by their host plant. Females and males of _E. scrobiculatus_ and _E. brandti_ showed a significant preference for healthy phloem over clean air in the Y-tube olfactometer.