## 4 Discussion
When stable flies were offered a choice between these two oviposition substrates, the flies always chose horse dung over cow dung. However, analyses of volatiles emanating from horse and cow dung by coupled gas chromatography-electroantennography (GC-EAG) with stable flies revealed no differences in the chemical cues released from both substrates. The major EAG active compounds in both substrates were butanoic acid, 1-octen-3-ol, decanal, octan-3-one, skatole, dimethyl trisulfide, \(\beta\)-caryophyllene, and \(p\)-cesol [97]. Behavioral responses of stable flies to volatile cues emanating from manure slurry have also been investigated [99]. \(p\)-Cresol, \(m\)-cresol, and phenol, among others, were detected by GC-MS of SPME captures from cattle manure slurry. Olfactometer assays showed that \(p\)-cresol, \(m\)-cresol, and phenol at the lowest dosages (10 \(\mu\)L of 0.1 \(\mu\)g \(\mu\)L\({}^{-1}\) solutions) elicited significant attraction, but at higher dosages, they appeared to be unattractive and even acted as repellents for stable flies. Mixtures of phenol and either \(m\)-cresol or \(p\)-cresol (10 \(\mu\)L) at 10 \(\mu\)g \(\mu\)L\({}^{-1}\) were attractive to stable flies. Additionally, traps placed in fields with an application of manure slurry caught over 10 times more flies than those placed away from sprayed sites [99]. In this study, five single compounds identified in volatile blends from cattle manure were selected to evaluate their effects on the olfactory response and oviposition of horn flies. Among the selected compounds, \(p\)-cresol was chosen due to it being the major component in the volatile blend from fresh cattle dung. Furthermore, it provoked significant EAG responses in antennae from stable flies [97; 99] and horn flies [21; 87], and it elicited the attraction of stable flies [99] and un-sexed horn flies [21; 87]. The sesquiterpene \(\beta\)-caryophyllene was chosen in the current study, considering its significant EAG activity provoked in _S. calcitrans_ antennae. The monoterpene \(D\)-limonene, also known as \(R\)-(+)-limonene, was chosen taking into account the effects on _H. irritans_ informed by [PERSON] et al. [64]. Moreover, considering the well-known anti-insect properties of terpenes [105], \(\alpha\)-pinene and \(\beta\)-pinene were also chosen, although these compounds were minor components in the volatile blends from cattle feces. To minimize the amount of chemical cues to apply, single compounds were tested using 10 \(\mu\)L of 0.05 \(\mu\)g mL\({}^{-1}\) in \(n\)-hexane solutions. This concentration was established at half that of the \(p\)-cresol concentration, tested by [PERSON] et al. [21], [PERSON] et al. [87], and [PERSON] et al. [99], on stable flies or horn flies (10 \(\mu\)L of 0.1 \(\mu\)g \(\mu\)L\({}^{-1}\) solutions). \(\beta\)-Caryophyllene, which was EAG-active in _S. calcitrans_ antennae [97], and \(p\)-cresol, which was EAG-active and attractive for stable flies [97; 99], acted as semiochemicals for horn flies in the present study but using half of the concentration used for stable flies. Here, \(\beta\)-caryophyllene was attractive to horn flies of both sexes, but it was not active in the egg-laying behavior. \(p\)-Cresol was attractive to mixed-sex flies, and it was also an attractant for females. Moreover, it elicited the oviposition. Related with the egg-laying bioassays using the chosen compounds, it is important to mention that aged feces were used as oviposition substrate, separating flies and dung with pieces of tuple; therefore, the egg-laying was influenced by the addition of specific compounds to the aged cattle dung. As mentioned above, the EAG and olfactory responses of horn flies to \(p\)-cresol were reported previously [21; 87]. Multiple active peaks in volatile extracts collected from the bodies of Holstein-Friesian heifers were recorded by GC-EAG using _H. irritans_ antennae. These compounds included \(p\)-cresol [87] and others previously reported to influence itsse fly behavior in field [89; 106; 107].