Leaving a nasty taste in the mouth—but
perhaps not nasty enough
In the millions of years in which plants
and insects have co-existed many different types of interaction have evolved.
Many of these interactions involve some sort of mutual benefit. However, others
are distinctly one-sided, such as attracting pollinators by deceit or insects
using plants as food. In response to herbivory, many plants have evolved
defence mechanisms including the synthesis of anti-feedant chemicals. On the
other hand, many herbivorous insects have methods of dealing with anti-feedants
and some even accumulate them as their own defence against predators. Here we
focus on the work of Ruuhola and Yang (Turku, Finland, pp. 29–37) on the
herbivory by larvae of the autumnal moth (Epirrita autumnata) of
mountain birch (Betula pubescens subsp. czerepanovii) in Finland.
Fourth-instar larvae were allowed to feed on leaves on trees in the wild and on
branches in the laboratory; enzyme activities involved in plant defences were
assayed. Feeding induced increases in activities of polyphenol oxidases and
guaiacol peroxidase. Catalase activity, by contrast, decreased. In laboratory
experiments, the most rapid response observed was the accumulation of the
reactive oxygen species H2O2 around the wound site. This
is thought by the authors to be involved in the induction of the oxidase and
peroxidase activities. Overall, this looks like a classic anti-feedant response
with the added dimension that H2O2 may inhibit the entry
of opportunistic pathogens. However, there is more to this situation. Firstly,
the larvae clearly have some ability to overcome the anti-feedant ‘environment’.
Indeed, in years of heavy infestation, extensive defoliation and even death of
trees occurs. Further, in more normal years there is evidence of induced
resistance to herbivory occurring in the following season. This implies some
sort of systemic response, perhaps mediated initially by the diffusion of H2O2.
Investigation of this will surely be a fruitful topic for further research.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
