## 4 Discussion  
In the majority of Tephritidae species, mating is mediated by a combination of chemical, acoustic and visual cues. Pheromones play a key role during the mate-searching phase and, in general, they are produced by males to attract females [15; 23]. The results of our experiments showed that _R. batava_ antennae from both sexes react to a single compound, \(\delta\)-heptalactone, detected exclusively in the headspace emissions from males. No significant difference in EAG responses between sexes was recorded except testing the lowest \(10^{-5}\)\(\upmu\)g dose and males showed higher sensitivity than females. We assume that ability of males to sense lower dose of pheromone could allow them to detect single pheromone releasing males and to form groups of individuals for the purpose of mating, while females detect aggregations of the calling males perceiving higher concentrations of a pheromone. The responses of antennae to synthetic (-)- and (+)-enantiomers of \(\delta\)-heptalactone revealed that only (-)-\(\delta\)-heptalactone was EAD active while no response was registered to (+)-\(\delta\)-heptalactone. Usually, receptors activated by optically active compounds show preferential of varying degrees to one of the enantiomers or respond at a similar level to both enantiomers [24; 25; 26; 27]. Until date, no response to one enantiomer of an enantiomer pair has been reported in limited number of insect species [24; 28; 29]. Figure 7: Field catches of _Rugoletis batava_ in [PERSON] traps. Different letters indicate statistically significant difference (general linear model, F = 7.013, \(p\) = 0.027); intervals mark SE; \(n\) = 3 for each type; trapping period lasted from 30 July to 20 August 2019. The attractiveness of the electrophysiologically active compound was also registered for both sexes under field conditions, revealing that the compound functions as a long-range attraction pheromone. Considering the definition given by [PERSON], \"When male insects pheromone-call they often attract males as well as females; the pheromones concerned are called aggregation pheromones even if their original function was sex attraction\" [30], we argue that the pheromone is attributed to the aggregation pheromone type. Many Tephritidae species form leks [31]; however, males of some _Rhagoletis_ species (including _R. pomonella_, _R. rubicola_ and _R. mendax_) individually seek out females on the fruit surface [32]. During our previous project studying the seasonal flight and mating dynamics of _R. batava_ flies, 1847 pairs in copula were counted [21] and often a few mating pairs or a few individuals were observed at closer distance to each other rather than distributed evenly (Figure 8, personal report). [PERSON] and [PERSON] defined a \"lek\" as an aggregation of at least three pheromone-calling males in a clearly defined area, usually from adjacent leaves of a host or non-host plant species [33]. Further studies are needed to collect more detailed information about the spatial and temporal characteristics of groups formed in response to male-released pheromones in the natural habitat of the species. Pheromone of _R batava_ is a lactone formed by the cyclization of a hydroxyl ester [34]. To date, eight compounds bearing the lactone moiety have been reported in seven Tephritidae species, attributed to genera _Anustrepha_, _Bactrocera_, _Ceratitis_ and _Rhagoletis_ (Table 2). In all seven species, lactones were emitted exclusively by males (Table 2); hence, our data showing that (-)-\(\delta\)-heptalactone is a male-specific compound follows the pattern observed in other Tephritidae species. The biological activity of lactones produced by _Anustrepha_, _Bactrocera_, _Ceratitis_ and _Rhagoletis_ males were tested using either just females or the sex of the specimens caught in the traps was not determined; hence, information is insufficient to determine whether males respond to the lactones as well. Figure 8: A few _R.