## 1 Introduction  
One of the most widespread and widely used medicinal plants worldwide is yarrow, _Achillica millelloium_ L. (AM), a perennial shrub belonging to the _Asteraceae_ family. The plant extract and essential oil possess numerous pharmacological properties such as antioxidant, antimicrobial, antispasmodic, and antitumor properties [1]. It is generally recognized that the essential oil content of the plant is low, and it may be influenced by harvest time and environmental conditions [1]. [PERSON] et al. examined 40 commercial _Achillea_ samples from different countries and reported essential oil contents ranging from 0.50% to 5.88% with an average value of 2.06% relative to the dry plant material [2; 3]. [PERSON] et al. reported a peak accumulation level of essential oil in the range of 0.230-0.334 mg/100 g, with the highest values in the white bud phenophase [4]. Similarly, taking into account the essential oil content, [PERSON] and [PERSON] identified the beginning of flowering instead of full flowering as the optimal developmental stage and reported 0.5-1.3% essential oil content [5]. According to [PERSON] et al. in their investigation on wild AM populations from Estonia, the highest essential oil content was obtained from the flower head (4 mg/g), approximately ten-fold more than the amount obtained from the stem and two times higher than quantity values obtained from the leaves. In terms of harvest period, July and August were more favorable in comparison to September [6]. [PERSON] et al. reported yields ranging from 0.32 to 1.01% for indigenous populations of yarrow distributed on salt-affected soils in Serbia. The authors emphasized the importance of different chemotypes derived from different genetic structures. Saline soil features did not represent an important influencing factor. Various cultivars of the plant have been developed to increase the essential oil content and to obtain a more favorable essential oil composition, but it is also generally accepted that in the case of AM, the genetic, ontogenetic, and environmental factors are very important for the composition of the essential oil [2; 3]. The monograph on _Millefolii herba_ by the European Scientific Cooperative on Phytotherapy (ESCOP) mentions the essential oil as a characteristic constituent, with a content between 0.2 and 5.9%, enumerating between components \(\alpha\)- and \(\beta\)-pinene, 1,8-cineole, sabinene, camphor, bomeol, \(\beta\)-caryophyllene, and germacene D, as well as proazulene- and non-azulogenic sesquiterpenes of the guaianolide type, chamaalene acrooxylic acid, flavonoids (mainly flavone- and flavonoid-O-glycos) and phenolic acids (mainly dicafeoylquinic acids) [7]. The European Pharmacopoeia 11 th edition yarrow (_Millefolii herba_) monograph foresees 2 mL/kg (dried drug) of essential oil and a minimum of 0.02% proazulene, expressed as chamaalene [8]. Formerly, it was considered that the proazulen accumulation in the plant is related to the ploidy of the species and that there are proazulene-free taxons; however, advancements in analytical techniques have since revealed that proazulene accumulation cannot be linked to ploidy [9], as proazulenes have been detected in species previously considered proazulene-free [10]. Azulenes and their derivatives play an important role in many of the therapeutic effects [11]. As sesquiterpene lactones are considered the most important bioactive materials of AM [12], efforts have been directed toward the isolation and identification of new compounds with this type of structure. [PERSON] et al. reported the isolation of three new sesquiterpene lactones [13]. More recently, [PERSON] et al. identified seven yet undescribed guaianolide-type sesquiterpene lactones (millefoliums) and five analogs [14] as well as six yet undescribed germacrene-type sesquiterpene lactones (millefoliums) and two known analogs [15].