**5. Conclusions**

The research of VOCs related to plant biotic protection is unequivocally dependent on modern gas chromatography-mass spectrometry (GC-MS), since only this technique allows detection and identification of compounds at such low levels. Due to sample complexity (plant tissue, soil), the use of gas-sampling techniques, which allow sample enrichment, connected with GC-MS, is thus mandatory [8, 12].

The responsiveness of EPNs, the biotic agents which are used for biotic plant protection, was tested in some studies [8, 9, 10, 44]. The studies have shown that damaged plant roots release chemical substances (β-caryophyllene, linalool) which can act as attractants for EPNs. Recent‐ ly, [53] reported positive chemotaxis of two EPN species to different VOCs such as heptanol, hexanol, methyl salcylate, etc. Interestingly, they showed that several volatiles repelled the EPNs. Similar effect of VOCs on the behavior of the EPNs was already observed by other researchers [99, 101].

Most VOCs that are involved in belowground tritrophic interactions remain unknown but an increasing effort is being made in this field of research. Understanding more of these complex interactions would not only allow a better understanding of the rhizosphere but could also offer ecologically sound alternatives in pest management of agricultural systems [44].

Biological control of plants is a way of controlling harmful organisms in agriculture and forestry by making use of live natural enemies (beneficial organisms). It aims to protect and stimulate useful organisms in nature, and to introduce targeted organisms outdoors or into places separated from nature. The application of biotic preparations requires the users to have considerable knowledge and greater ecological awareness. The preparations made biotically are ecologically more appropriate, their functioning is more specific, their formulation and application are important, as is the temporal precision of treatment. Their efficiency, on the other hand, is often lesser than that of chemical preparations, and such preparations are more expensive. The difference in price is due primarily to the relatively small market with biotic agents, which within the entire market of plant protection agents at the moment represent approximately 6% (according to the data of BCC Research), and is expected to rise at least to 8% until 2019.

The value of the global market of biotic agents was in 2013 assessed at approximately 1.8 billion \$ and it is expected to reach approximately 4.4 billion \$ until 2019. Globally, the USA is still the largest user of biotic agents; it is, however, expected that in the following years the fastest growing market with biotic agents will be Europe, whose strict legislation on plant protection products systematically stimulates ecologic production of food and fodder. Statistical data have shown that the sales of biotic agents between 2005 and 2015 rose by 44%. The expansion of the market is undoubtedly also a consequence of the raised awareness of environmental issues, which are included in the EU directives.

Knowing the communication between plants, herbivores, and their natural enemies is crucial for more efficient implementation and optimization of biological control in food production systems. The European Union has set five ambitious goals – in the fields of employment, innovations, education, social inclusion, and climate/energy – to be reached until 2020. The market with new, improved biotic agents would contribute to environmental protection, as well as to the expansion of economic activities.
