Factors favouring the development and maintenance of outbreaks in an invasive leaf miner Cameraria ohridella (Lepidoptera: Gracillariidae): a life table study
1 Life tables were constructed for seven generations of the invasive horse-chestnut leaf miner Cameraria ohridella in Switzerland, to assess the factors allowing the moth to build and maintain outbreak densities and to identify ‘gaps’ among their mortality factors that could be targeted by new control methods. The fecundity of females was calculated and the mortality factors affecting all developmental stages determined.
2 Females contained 106.6 eggs. In captivity, the mean number of eggs laid per female varied between 34 and 82 eggs, with a maximum of 184 eggs. Egg mortality was 18–75% and was mostly due to unknown factors.
3 The main mortality factors affecting larvae and pupae in the leaves were predation by birds and arthropods in all generation and leaf senescence in the autumn generation. Parasitoids killed less than 5% of the population.
4 Nearly 90% of overwintering pupae died in dead leaves, the majority of them being killed by earthworms and other leaf decomposers. As a result, the overwintering generations were the only ones showing a net rate of increase less than one. Non-overwintering generations had net rates of increase between four and 11, allowing populations to build up from spring to autumn.
5 The persistence of high outbreak densities in Europe is explained by a combination of at least three factors: (i) high fecundity; (ii) multivoltinism; (iii) the unusually low impact of traditional natural enemies of leaf miners and particularly parasitoids.
6 Yearly variations in population densities could be at least partly explained by differences in larval mortality due to leaf senescence and intraspecific competition in the last generation in autumn.