## 3 Discussion  
The VOCs that released from infested cabbage plants by _M. persicae_ showed many compounds comparing with uninfested plants and reported by previous studies [25; 26; 27]. In the current study, volatile compound profiles of uninfested and infested cabbage plants with _M. persicae_ were compared to show the differences between treated plants and used as identification tools for the infestation. [PERSON] et al. [27] reported that a comparison of volatile compounds identified from uninfested and aphid-infested plants from several _Brassica_ plants. The damage of cruciferous plants caused by aphids can emit many volatile compounds such as glucosinolate metabolites, phenolics and terpenoids [28; 29]. However, our results showed the _M. persicae_ preferred damaged _Brassica_ plants because the infested plant released different VOCs, such as alpha- and beta pineine, \(\ell\)-3-hexen-1-ol, myrcene, 1-hexanone, 5-methyl-1-phenyl, limonene, decane, gamma-terpinen and heptane, 2,_A_A_-trimethyl. This finding is consistent with [17], who reported that alpha- and beta pineine and limonene could increase in _Brassica_ plants infested by aphids. Some VOCs disappeared from uninfested plants, such as 3-hexen-1-ol-(E) and beta-pinene [17; 30]. The increase in (E)-3-hexen-1-ol, beta-pinene and decane in infested plants could be expected because these compounds are well known as green leaf volatiles and are involved in the attraction of natural enemies such as parasitoids and predators [22; 31]. The VOCs can be released by an intact and uninfested Brassicaceous plant in large amounts [32]. These compounds were found in the headspace of infested cabbage plants and can be involved in attracting beneficial insects as a response to the aphid infestation [22; 31; 33]. Thus, the selection of SPME in the extraction of volatile compounds from uninfested and infested cabbage plants with _M. persicae_ was based on the peak areas of all compounds identified in the treatments. The results of Y-tube olfactometer bioassays confirmed the results of aphids _M. persicae_ and the parasitoids _A. colemani_ and _A. abdominalis_ were influenced and attracted to volatiles produced by Cruciferous plants. These wasps significantly preferred, and were attracted to, volatiles from aphid-infested plants over uninfested plants. The use of the Y-tube olfactometer to test the response of aphid _M. persicae_ to the host plant, _B. oleraca_ var. _capitata_, indicated that _M. persicae_ was influenced by the volatiles released from _B. oleraca_  
Figure 4: Olfactory response of two parasitoids _Aphidius colemani_ and _Aphelinus abdominalis_ in Y-tube olfactometer experiments to volatiles released from infested and uninfested cabbage _B. oleraca_ (**a**) uninfested versus infested plants (**b**) uninfested versus clean air (**c**) infested versus clean air. All treatments presented with standard deviation (SD) bar. * indicates significant difference \(p<0.05\) (Chi-square test). var. _capitata_ and were significantly attracted to both uninfested and infested plants when compared with the clean air choice. Aphids can find their host visually and chemically, by chemical, color, size and the shape of the host, and this may be a useful guide to attracting aphids. This result confirms past studies [20; 34; 35] that show aphids find their host plants by plant odor as well as visual cues. Moreover, the attraction of aphids to the plant volatiles using olfactometer has been reported in experiments testing plant odor against aphids and their host-finding ability [34; 35; 36]. Our results showed that aphids tended to be attracted to both damaged and undamaged plants. Our observation is that plant compounds can explain the variance in attraction by aphids and also that plant volatile compounds can increase in response to feeding [37; 38]. The population of natural enemies can be increased when adding organic fertilizer [39]. Based on VOCs from cabbage, _M. persicae_ was attracted to seven different cabbage varieties in diverse ways. Additionally, the wingless _M.