Vitali Zverev, Elena L. Zvereva and Mikhail V. Kozlov
Plants carry out many functions that provide humans with oxygen, food, fibre and fuel, but their effectiveness in performing these roles is affected by herbivores, like insects, that consume them. Therefore, understanding why some plants suffer more damage from insects than their neighbours is important.
Downy birch is a common deciduous forest tree found widely distributed across Eurasia. It shows appreciable size changes over its lifetime, developing from a seedling to a 20 m tall tree. These changes with age may affect its interactions with other organisms, particularly leaf-eating insects. We asked whether leaf damage by insects differs between small (juvenile) saplings and large (mature) trees.
One of two alternative hypotheses suggests that seedlings must use all their available resources for growth in order to win the competition against their neighbours. By contrast, mature plant individuals, which have already grown up, can afford to invest resources in the production of defensive chemicals and are therefore better protected from insects than are young plants. The other hypothesis argues that large plants are more apparent to insects, more easily found by pests and therefore more prone to damage than are small individuals. We compared the predictions of these two hypotheses by studying leaf damage by insects in birches of different ages in a latitudinal gradient of about 1000 km in length, spanning from St. Petersburg to Murmansk in north-western Russia.
Foliar damage to mature birch trees by insects was three times as high as damage to birch saplings, although the leaves of saplings were of higher nutritional quality for insects than leaves of mature birches. These patterns, as well as our findings on distribution of damage within and among birch individuals of different sizes, do not support the prediction based on changes in resource allocation to plant antiherbivore defence. However, our results do fit well with the predictions of the plant apparency hypothesis. Generally overlooked effects of apparency on plant damage by insects can explain, at least in some cases, the frequently observed lack of correspondence between the levels of plant chemical defences and of plant damage caused by insects.
Image caption: Larva of a tiny Eriocrania moth feeding within birch leaf. These larvae were used to study the leaf nutritional quality for insects. Photo provided by authors.