Early shoot growth termination in Betula pendula is associated with the number of overwintering aphid eggs on boreal birches
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Many autumn migrating and phloem-feeding insects, like aphids, use leaf reflectance to distinguish senescing foliage. They consume resources in autumn and lay eggs that hatch in spring when trees need phloem nutrients for developing leaves. The herbivores may thus drive the evolution of tree senescence and autumn leaf colours. In accordance with this, the yellowing birch leaves attract aphids (Euceraphis betulae) and the genotypic variation in the timing of autumn leaf colouration is associated with the abundance of oviparous wingless female aphids in a natural Betula pendula population. Currently, however, there are no published studies investigating the potential association of autumn senescence and the number of overwintering aphid eggs. We examined a local B. pendula population in 2001 and 2002 for genotypic differences in the timing of shoot growth termination and for the abundance of overwintering birch aphid eggs. There were heritable differences in the timing of shoot growth termination among B. pendula genotypes. The genotypes that terminated shoot growth early had higher aphid egg loads after a short but not after a long autumn. We suggest that this is because the egg laying period was cut off due to a rather sudden and substantial change in the temperature in the shorter autumn. We are the first to find genotypic differences associated with the overwintering offspring of autumn-migrating aphids on a widely distributed temperate broadleaved tree. As shoot growth termination directly precedes leaf senescence, autumn-migrating aphids may have an additional potential to influence indirectly but significantly the onset of autumn leaf senescence at an evolutionary scale.
KeywordsGrowth termination Senescence Aphid eggs Genetic variation Heritability Genetic correlation Deciduous tree
We thank the Punkaharju Research Unit of the Finnish Forest Research Institute for facilities and help. Hanni Sikanen, Mari Tuominen and Pirkko Pasanen are thanked for the field assistance, Boy Possen for digging quickly up the long term weather data for us and Simcha Lev-Yadun and anonymous reviewers for many instructive comments on the earlier versions of the paper. This study was supported by the Academy of Finland, and funding granted to T.S. by Kone and Niemi foundations.
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