Identification of biogenic volatile organic compounds for improved border biosecurity

Abstract

Effective border biosecurity is a high priority in New Zealand. A fragile and unique natural ecosystem combined with multiple crop systems, which contribute substantially to the New Zealand economy, make it essential to prevent the establishment of invasive pests. Trade globalisation and increasing tourism have facilitated human-assisted movement of invasive invertebrates, creating a need to improve pest detection in import pathways and at the border. The following works explore a potential new biosecurity inspection and monitoring concept, whereby unwanted, invasive insects may be detected by the biogenic volatile organic compounds (VOCs) they release within contained spaces, such a ship containers. The brown marmorated stink bug, Halyomorpha halys Stål, is an agricultural and urban pest that has become widely established as an invasive species of major concern in the USA and across Europe. This species forms large aggregations when entering diapause, and it is often these aggregations that are found by officials conducting inspections of internationally shipped freight. Stink bug species are known to emit defensive odours, making H. halys a suitable candidate as model species for this study. Undisturbed aggregations of diapausing H. halys were found to emit tridecane and (E)-2-decenal. Mechanical agitation of diapausing H. halys was used to induce emissions of defensive odours, and the full VOC profile was confirmed through GC-MS analysis as: tridecane (41.7 ± 11.8 µg per bug), (E)-2-decenal (18.2 ± 4.2 µg), 4-oxo-(E)-2-hexenal (15.8 ± 6.3 µg), and dodecane (1.5 ± 0.6 µg). Testing the role of conspecific bugs on VOC release, it was found that H. halys required the presence of another bug as well as mechanical agitation to elicit a defensive odour response. From this, the effect of conspecific defence compounds were individually tested on single H. halys. One component, 4-oxo-(E)-2-hexenal, was found to cause individual bugs to both move further distances after exposure, and also release their own defensive odour. Thus, the agitation of aggregations, as it might occur during freight shipping, could facilitate an amplification effect for release of odours; were one bug to emit defensive VOCs in an aggregation, more would be likely to emit. This may increase the likelihood of detection of these VOCs within an enclosed space such as shipping containers. Experiments were performed to simulate the effects of two variables introduced by the act of shipping, ship movement and journey temperature fluctuations, upon aggregations of diapausing H. halys. Aggregations exposed to simulated shipping movement, using a 6-axis VS-6577G-B Denso robot arm, were not found to be any more likely to release VOCs than aggregations which remained stationary, nor did it cause any bugs to become mobile. Simulated temperature changes as they would be experienced during a voyage over 26 days from a port in the north-east USA to New Zealand were found to have a significant effect on the mobility of H. halys. However, towards the end of the simulated voyage, most H. halys died, probably from a lack of food or moisture in the shipping scenario. The high mortality observed in these aggregations prompted the collection of headspace samples from dead H. halys over the same time period and experiencing the same temperatures. This revealed that dead and decaying H. halys release the full VOC profile of tridecane, (E)-2-decenal, 4-oxo-(E)-2-hexenal, and dodecane over three weeks, although in smaller quantities than when actively releasing defensive odours. Theoretical calculations showed that the GC-MS analytical method combined with active sampling volatile collection traps was not sensitive enough to detect volatiles released by aggregations of living, dead, or combined H. halys within a 20 foot (38,000 l) shipping container. However, there are more sensitive technologies available which can detect VOCs to the parts per trillion level, which would be capable of detecting the expected VOCs concentrations associated with the presence of H. halys in shipping containers.