Since the 1990’s, researchers have explored the viability of training bees and wasps to detect – and indicate the presence of – particular ‘target’ odours. Though there are numerous potential applications for this, most frequently of interest is whether insects can reliably detect trace concentrations of explosive materials in airports, warzones and at security checkpoints.
Insects have an extremely sensitive olfactory sense, which they use when foraging, avoiding predators and to trace the pheromones of potential mates. Bees can detect chemicals at concentrations as low as a few parts per trillion more effectively than ion mobility spectrometers (used in airports), and are sensitive to chemicals that canines find challenging. When exposed to an odorant, chemical receptors on the antennae generate odour-specific electrical signals which are sent to the antennal lobe. Bees, wasps and some species of moth can be trained to selectively respond to target scents.
The concept has been slow to gain traction in the scientific community, largely due to the logistical doubts. Challenges include creating a feasible handheld device, tracking free-ranging bees in large outdoor environments and the potential for the trained insects to generalize odours and register false positives.
How it’s done
The concept is based upon classical conditioning. For example, honeybees exposed to a stimulus – in this case the target odour, in conjunction with a reward – a sugar solution, extend their proboscis (‘proboscis extension reflex’). After a few exposures to the target odour, the bees extend their proboscis without the sugar solution.
At a research facility in New Mexico, honeybees are restrained in straw-like tubes within a small box, and individually trained. The handheld device is portable and contains a row of five bees. After training, a video camera linked to pattern recognition software is used to monitor the reaction of bees to puffs of air entering the box. The proboscis extension reflex is an unambiguous signal for the presence of the target odour.
British companies have expanded upon this by developing devices which can pack over 70 bees, allowing multiple chemicals to be screened for simultaneously. It is important that the bee’s exposure to air, and monitoring the associated reactions are not compromised by the volume of bees in the unit.
Contrastingly, some bees are not contained within a box, and are free-ranging when detecting odours. Bees move towards target scents, and their movements can be tracked using infra-red photographs taken every ten seconds, or by fitting individuals with radio transmitters. In 2014, European scientists developed this method of scent tracking for use in the Balkans, to pinpoint hidden landmines and explosives leftover from war in the region in previous decades. The mines were posing a renewed risk to farmers following flooding. However, the use of free-ranging bees is not appropriate for airport security – or other confined, public spaces – for obvious reasons.
The parasitic wasp species Microplitis croceipes use their olfactory sense to seek out hosts in which to implant their offspring, and for foraging. Wasps do not display a proboscis extension reaction, however, they do communicate behaviourally. Scientists are able to exploit these behaviours by training wasps to associate different scents with different rewards (and thus displaying different indicative behaviours). This results in an individual wasp being able to detect multiple odours.
In a device similar to that used to transport honeybees, wasps are able to fly within a ventilated, handheld box. Outside air is drawn in through a small hole linked to a video camera. If the wasps detect a target odour, they cluster around the hole, where the camera is able to record the reactive behaviour being performed. Field trials have found this method of chemical detection to be 94 times more sensitive to certain odours than mechanical devices.
The sensitivity of bees and wasps to almost any scent means that the potential applications for their training are numerous. Detectable odours include those of explosive materials (e.g. TNT, C4), illegal drugs, human remains and disease. Trained insects can indicate the presence of volatile compounds associated with human diseases, including skin cancers and tuberculosis, when exposed to a patient’s breath. In addition, in grain stores, wasps are sensitive to aflatoxin, which is produced by a fungus found on peanut and corn plants, as well as E.coli and Salmonella.
Could these trained bees and wasps replace sniffer dogs in high-risk contexts? Though opinion is mixed, if logistical obstacles are overcome, it is noted that insects would be cheaper, available in large numbers, flexible and easier to train than dogs. Honeybees can indicate the presence of TATP, an explosive compound which canines are unreliable in detecting. Insects may be most useful in situations, for example arson, where fumes can be toxic to dogs and handlers.
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