Plants have a rich, even a rococo, chemistry. They elaborate many strange and complex substances — terpenes, alkaloids, phenols, and so on, widely exploited in herbal medicine. Yet this medicine rests on uneasy foundations. For all these plant metabolites have but one purpose — to discourage or kill anything that tries to eat the plant.

Fortunately, most plant metabolites are targeted, not at us, but at insects. They can even be released to counter a local insect threat. Cotton plants, for example, release a terpene vapour when attacked by armyworms. Some substance in the saliva of the insects, or generated by its attack, triggers the release of the vapour.

Daedalus sees this as a splendid new source of herbal fuel. Such fuels are being rapidly developed — witness ‘biodiesel’ oilseed fuel, and the recent Indian herbal fuel fraud. Plant terpenes, the constituents of turpentine, are an excellent hydrocarbon fuel. They are metabolically ‘cheap’, too — some tropical shrubs and trees release such copious terpene vapours that the scent carries for kilometres. There's a claim that it can be a chemical signal to other plants. So DREADCO biochemists are placing various insects on the leaves of terpene-releasing plants, to study the detailed chemistry of their mastication. They hope to identify the trigger compound that tells a plant that it is under insect attack, and must switch all its resources to terpene synthesis.

Once identified and synthesized, the terpene trigger will open the way to a new fuel technology. A plantation of shrubs or trees — pine, perhaps, or eucalyptus — will be enclosed in a huge plastic-film greenhouse. The terpene trigger will be pumped in, and every plant will start pouring out terpene vapour as if its life depended on it. The greenhouse air will be continuously extracted, and cooled or passed over absorbents to collect the terpenes, which will be re-formed into motor fuel. The level of terpene trigger will need to be carefully regulated. Just enough must be supplied to divert most of the plants' metabolic effort into terpene production, without sapping their vitality.

This elegant technology has all the ecological virtues. Unlike other biofuel schemes, it needs no labour-intensive planting and harvesting. The fuel ultimately comes from the carbon dioxide of the atmosphere; when burnt, it returns without augmenting it. And the plantations will not need expensive insecticidal spraying.