The corn borer, Ostrinia nubilalis, is a butterfly whose caterpillars feed by boring holes in the stems and ears of maize, thus causing major damage and yield losses in most temperate countries. In the autumn, the caterpillars stop feeding and enter into diapause (a dormant phase of life), spending the winter sheltered by maize canes before they metamorphose into adults the next spring.
The corn borer appeared in western Europe about 500 years ago with the first imports of maize, before being introduced accidentally into North America at the beginning of the 20th century. It is likely that this pest species arises from Ostrinia scapulalis, its sibling species. The two species are said to be siblings because they are morphologically identical and so genetically close that only molecular studies, a few years ago, were able to make a distinction between them. However, one major difference is that O. Scapulalis does not attack maize but mainly artemesia, a non-cultivated plant. The corn borer is thus a species that has appeared through a "host change", the introduction of maize having engendered its own pest species.
For the corn borer, maize constitutes a new environment with its own predators and parasites. The most dangerous of these predators, by a long way, are humans. Because they were burned or used as forage or litter when harvesting was manual, and are now completely ground up since the generalisation of combine harvesters, the upper parts of maize canes have become a fatal hiding place for the caterpillars when they enter diapause. Indeed, corn borer caterpillars situated above the cutting height at the time of harvest are destined for almost certain death.
At the end of their growth, at harvest time, the caterpillars choose the site where they will spend the diapause period. This recently published study reveals that in fact, corn borer caterpillars travel down towards the soil and install themselves much lower in the canes than their sibling species, in both maize and artemesia. This behaviour, which occurs a few weeks before harvest time, is independent of environmental conditions and not caused by a search for food. The study has shown that it is determined genetically and implies a certain perception by the insects of terrestrial gravity and of their localisation.
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As a result, corn borer caterpillars are found in much larger numbers below the cutting height, which increases their survival rate by about 50% when compared with their sibling species.
Travelling down towards the ground thus ensures the excellent adaptation of corn borer. This behaviour probably results from selection operated by humans as harvests have passed. This modification to the position of larvae in diapause is thus an adaptive response to harvesting by man. If future studies confirm this interpretation, it would constitute a singular example of behavioural resistance to farming practices, as most of the cases known until now have involved resistance to insecticides. If maize has engendered its own pest, man has undoubtedly fashioned it in such a way that today, the corn borer looks on the "Grim Reaper" with much more serenity than its sibling species.
References:
Divergence in behaviour between the European corn borer, Ostrinia nubilalis, and its sibling species O. scapulalis: adaptation to human harvesting? Proc. Roy. Soc. B., 21 April 2010.
Vincent Calcagno123, Vincent Bonhomme1, Yan Thomas1, Michael C Singer4 and Denis Bourguet1.
1 Centre de Biologie pour la Gestion des Populations (CBGP), UMR INRA-IRD-CIRAD-Montpellier SupAgro, Campus International de Baillarguet, Montferrier-sur-Lez, France.
2 Institut des Sciences de l'Evolution (ISEM), UMR CNRS-UM2, Université de Montpellier II, Place Eugène Bataillon, Montpellier,France.
3 McGill University, Biology Dept, 1205 av. Docteur-Penfield, Montreal, QC, H3A 1B1, Canada
4 Integrative Biology, Patterson Laboratories, University of Texas at Austin, USA.
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