## 4 Discussion
Another study provided evidence that individual Holstein-Friesian heifers were differentially attractive to _H. irritans_[21]. GC-MS analysis of volatiles collected from heifers revealed the presence of compounds previously reported as semiochemicals for cattle flies [87; 94; 95; 107; 108], including \(p\)-cresol and \(m\)-cresol. In Y-tube olfactometer tests, both \(p\)-cresol and \(m\)-cresol attracted horn flies at the highest concentrations tested (10 \(\mu\)L of 0.1 \(\mu\)g \(\mu\)L\({}^{-1}\) solutions), which is consistent with the attraction observed in the present study but here used just half of that concentration. Surprisingly, volatile blends collected from high and low fly-load heifers were not attractive in the previous work [21]. However, the sex of the flies was not considered. The behavioral responses of horn flies to \(\alpha\)-pinene, \(\beta\)-pinene, \(\beta\)-caryophyllene, and \(D\)-limonene were also tested at the current study. It is noteworthy to mention that racemic mixtures of \(\alpha\)-pinene, \(\beta\)-pinene, and \(\beta\)-caryophyllene and the (\(R\))-enantiomer of limonene have been used in this study. However, the absolute configuration of the terpen emitted from cattle dung was not determinate. Thus, there might be mismatch between the enantiomeric composition of terpen released from the dung samples and those of commercial samples tested for bioactivity. Nonetheless, all of these terpen were attractive to both sexes, male and female horn flies, with the exception of \(D\)-limonene, which was unattractive to females. The egg-laying also was influenced by the single terpen added on pieces of tuple covering dung. \(\alpha\)-Pinene elicited the oviposition of the female flies, while \(\beta\)-pinene acted as an egg-laying deterrent, and \(D\)-limonene and \(\beta\)-caryophyllene were not active. The olfactory responses of horn flies to \(D\)-limonene (\(R\)-(+)-limonene) have also been reported previously [64]. When \(D\)-limonene and a limonene-based insecticide (Orange Guard, Carmel Valley, CA, USA) containing 5.8% \(D\)-limonene steam distilled from citrus peels were tested, 74% to 90% of the horn flies were attracted to all concentrations of Orange Guard and the laboratory-grade limonene tested [64]. Although the sex of the flies was not considered, and concentrations were much higher than those tested in this study, this is in accordance with the attraction to horn flies observed here. The observed attraction of female flies to \(\beta\)-pinene, despite it provoking reduced egg-laying on dung treated with this compound, is an intriguing result. There is outdated information concerning the \(H\). _iritans_ ecology in the literature [3; 8; 14; 15; 79; 80; 81]. Twenty-five years ago, [PERSON] [15] observed in field that female horn flies obtained blood meals from the bovine host before oviposition. This would therefore suggest that female flies were attracted to \(\beta\)-pinene as a feeding signal. However, the feeding activity of \(\beta\)-pinene on _H. irritans_ has not been determined. Another plausible explanation is concerning to the effects of the particular compound compared with the effects of compounds in a complex mixture. In this scenario, [PERSON] et al. [81] reported that mixtures of 3-\(n\)-propylenol, 1-octen-3-ol, \(p\)-cresol, and acetone were partially attractive to _Glossina_ spp. populations compared to natural cattle host odor, implying that other attractants need to be identified in order to provide full activity. Additionally, it is well-known that the insect responses are often concentration-dependent. For example, [PERSON] et al. [87] proved that single compounds that showed EAG activity for cattle fly species provoked different responses in a wind-tunnel bioassay with face flies, _Musca autumualis_ ([PERSON]) (Diptera: Muscidae). The 1-octen-3-ol, 6-methyl-5-hepten-2-one, and 3-octanol showed significant attraction at certain concentrations, with 1-octen-3-ol attracting flies at very low levels.