Needle asymmetry, pine vigour and pine selection by the processionary moth Thaumetopoea pityocampa

Developmental stability reflects the ability of a genotype to control stable development of a specific phenotype under a wide range of environmental conditions. Developmentally unstable phenotypes can be recognised by deviations from bilateral symmetry in bilaterally symmetrical traits and, because asymmetry might reflect nutritional quality of leaves for phytophagous insects, they therefore may base plant selection depending on leaf asymmetry. In this article we study such hypothetical relationships occurring between Aleppo pine (Pinus halepensis) and pine-host selection by the pine processionary moth Thaumetopoea pityocampa (Lepidoptera: Thaumetopoeidae). Needle length of Aleppo pines indicated directional asymmetry and, as the hypothesis of developmental stability predicts, relative asymmetry was negatively related to needle length and positively to pine growth in height. Moreover, relative asymmetry proved to be negatively related to concentration of limonene, a defensive monoterpene that affects pine selection by adult female moths. In terms of growth, pine variation in needle length can be explained by the increase in volume of the pines from one to the next year, with smaller needles appearing in the pines that most increased their volume and those that least increased their height. Finally, as expected from a phytophagous insect that selects plants in relation to nutritional characteristics and level of chemical defence against herbivorous, the pine processionary moths selectively oviposited in the trees with the largest and most asymmetric needles. With these results, two of the main hypotheses that explain plant selection, plant-stress and plant-vigour hypotheses are discussed.