All’s fir in love and war

Nigel Chaffey discovers there is a fountain of youth, but only if you're a firebug.

Having recently ‘inherited’ some new lecture topics in Ecology at my university, including having to supervise the herbivory practical in which snails are set upon unsuspecting plants [something a Botanist should not have to endure!], I wanted to boost the plant side of life in the Plant Defences lecture. Looking for suitable evidenced-based examples of plants using chemicals to thwart the herbivorous attentions of animals, my Googling (what we used to call literature-searching in the old days, but which is now a legitimate activity and a proper word!) led me to a charming paper from the 1960s (an era of much widespread experimentation in the effects of plant-based chemicals on animals, including humans – whether they be herbivorous or not…). Penned by Karel Sláma and Carroll Williams it documents their discovery of juvenile hormone activity by the balsam fir (Abies balsamea) against Pyrrhocoris apterus (‘firebug’ – Radomir Socha, Eur. J. Entomol. 90: 241—286, 1993).

Firebug (Pyrrhocoris apterus)
Firebug (Pyrrhocoris apterus). Photo: Alvesgaspar / Wikipedia.

Within insects, juvenile hormone [JH; Lynn Riddiford, General and Comparative Endocrinology 179: 477–484, 2012] is part of a suite of endogenous growth factors that act to ensure proper co-ordinated growth of the larva to adulthood. Production by the fir of a compound with JH activity, which leads to arrested development of the insect, ensures that they do not undergo metamorphosis through to the adult stage, which could be sexually-reproductive (and therefore lead to an increase in numbers of this herbivore which would attack more fir trees in future…). Its exploitation by the plant is one small part of the biochemical warfare that plants have waged with insects for millions of years. And there are many interesting facets to the balsamiferous JH activity story. How it was discovered for one, which was what one might call a ‘chance observation’* but which was recognised as having significance and followed up.

The firebugs were reared in paper towel-containing jars. In 10 years of firebug-rearing in Prague (at the Entomological Institute of Czechoslovak Academy of Sciences, Sláma’s home institution) the insects developed fully. It was only when these studies were pursued in the USA (at Harvard University) that the arrested development occurred. The reason for which odd behaviour turned out to be that the paper towels used in the States were composed largely of balsam fir pulp. From a historian of science perspective showing today’s scienteens what a science journal article looked like in the olden days we have a charming paper (it really is worthwhile reading it – it’s only short!), written in a style that doesn’t conform to today’s rigidly structured papers. It contains no data displays, has no statistical analysis, and no real clarification of the active compound (!), yet it was published in a highly respected journal with a 2014 Impact Factor of 9.674. Impressive!

But, there’s more.

Further work by the pair discovered that the JH-active compound (termed ‘paper factor’) also acted as a potent, and selective, ovicide against firebug eggs (arguably, an even better plant-produced form of insect ‘deterrent’). That work was published in an even more prestigious journal (which included a data display, but still no stats…), Nature (Sláma & Williams, Nature 210: 329-330, 1966) whose 2014 Impact Factor is 41.456. And that was followed a week later (23rd April, cf. 16th April)(!) by another paper in that same high-impact journal which announced that paper factor was also effective against Dysdercus koenigii (red cotton bug) – Kailash Saxena & Carroll Williams (Nature 210: 441-442, 1966). What distinguishes this from the original PNAS paper – apart from the inclusion of a data display (but still no stats…) – is that red cotton bug is a “considerable pest in India and North Africa” (and elsewhere e.g. Syed Ishfaq Ali Shah, Pakistan J. Zool. 46: 329-335, 2014), whereas the firebug is “a benign species which feeds by its sucking mouthparts on lime (linden) seeds” [well, balsam fir’s paper factor seems effective in protecting its owner!].

Paper factor** is one example of the thousands of so-called secondary metabolites or secondary plant chemicals (SPCs; e.g. Thomas Hartmann, Phytochemistry 68: 2831–2846, 2007) that are widely implicated in plant defence against insects (e.g. Gottfried Fraenkel’s The raison d’être of secondary plant substances, Science 129: 1466-1470, 1959) and which continue to be prime candidates for exploitation by mankind (e.g. Michael Balandrin et al., Science 228: 1154-1160, 1985). So, aside from the scientific writing comparisons with today’s journals,*** with that trio of papers from the 1960s one can see the beginnings of research into plant-produced SPCs. Not least of which is their relevance to development of biocontrol measures to control crop pests and diseases as explored by Johan Stenberg et al. (Trends in Plant Science). [Ed. – at last, the current material in this otherwise historical ‘news’ item!].

* This is an excellent example of the maxim – attributed to Louis Pasteur – that chance only favours the mind that is prepared.

** Paper factor is a sesquiterpene now called juvabione, the methyl ester of todomatuic acid.

*** And this series of papers would make a great student-centred teaching activity dealing with such matters as scientific writing, sufficiency of evidence, what determines which journal a discovery/investigation is appropriate for… One could also speculate as to whether the ovicidal nature of paper factor was known when the PNAS paper was written (and could reasonably have been include therein), but was omitted therefrom to generate a separate publication in a much more prestigious journal.