Insect olfaction as an information filter for chemo-analytical applications

Abstract

The olfaction of insect species is adapted to the requirements of the specific ecosystem they are living in. Volatiles perceived by insects can transport information on the location of an oviposition place or the physiological status of a host plant. As the olfaction of insects has adapted to the perception of distinct volatiles since millions of years, these volatiles can be regarded as marker compounds, which enable a reliable assessment of an ecological status. The identification of such marker compounds and the evaluation of their usefulness for trace analytical applications is the aim of this thesis. Three topics were selected to evaluate the utilization of insect olfaction for sensor applications: Meat spoilage, post mortem interval (PMI) estimation, and the volatile based detection of wood fire before ignition. Trace analysis (GC-MS), electrophysiology (EAG, GC-MS/EAD), behavioral studies and field tests were used to understand the volatile based interaction of the blowfly Calliphora vicina and the fire beetles Melanophila cuspidata, Merimna atrata, and Acanthocnemus nigricans with their environment. The results gained by these methods allowed the selection of volatile marker compounds that fulfilled three criteria: high abundance, reliable emission, and unique emission from the substrate and not from any other sources. These criteria should ensure that the marker compounds`s emission is not only reliably correlated to a physiological state of the substrate, e.g. an increasing spoilage of meat, a time point after death of a vertebrate, or a distinct temperature of wood prior to ignition, but that the marker compound can be sensed by technical sensor systems, as well. In case of meat aging, the predominant emission of nonanal under warm conditions was an indicator for the correct maturation of the meat to ham. Dimethyl trisulfide, phenol, and indole indicated moisture induced spoilage under warm conditions. Under cold conditions, increasing 2,3 butanediol emission and decreasing nonanal emission was a spoilage indicator for both the dry and the wet samples. However, 2,3 butanediol was not perceived by the fly, because its activity is restricted to the warm vegetation period, where dimethyl trisulfide, phenol, and indole are emitted. Nonanal, hexanal, dimethyl disulfide, dimethyl trisulide, butan-1-ol, and phenol are useful volatiles to estimate the PMI of vertebrates. The aldehydes are emitted in the early stages of vertebrate decay, followed by butan-1-ol and the sulfur compounds. Phenol is predominantly emitted in the later stages of decay. However, butan-1-ol and phenol are not perceived by the fly, which is a generalist among carrion visitors and does not differentiate between the early stage, the bloated stage, and the active stage of decay. The olfaction of other necrophagous insects, which prefer a distinct stage of decay, might lead to new insights into the time dependent volatile emission during vertebrate decay. The results gained by the investigation of the odor profile of decaying vertrebrates allowed a correlation between PMI and volatile profile that can be used by police forces as an additional indicator of the PMI. Terpenes, aliphatic aldehydes, furfural, and methoxyphenols are emitted from heated wood flakes. All of these compounds are perceived by M. cuspidata and M. atrata, as they can be used to assess the heating stage of a stem in order to find appropriate oviposition places. A. nigricans predominantly prefers methoxyphenols, while furfural is perceived by M. acuminata and M. atrata. These results complement the knowledge on the behavior of these beetles in their ecosystem, as A. nigricans oviposits on smoldering logs, which emit high abundances of different methoxyphenols, and M. atrata and M. cuspidata oviposit on heated but unburned stems, which emit, among other compounds, furfural. Semi conductor gas sensors perceiving furfural with a sufficient selectivity can predict a wood fire before it ignites. This would enable a new generation of early fire warning systems that can be used in the wood processing industry, wood land fire warning, or households. These three examples show that insect olfaction can be utilized for the development of concepts for technical sensor applications, but that the ecological background of the insects has to be considered when selecting marker compounds. The restricted selectivity and sensitivity of technical sensors in comparison to the highly selective and sensititve biochemical perception mechanisms of insects have to be considered, as well. Therefore, the direct transfer from insect olfaction to technical solutions is not always possible.