Studies on the population dynamics of the green tortrix tortrix viridana lepidoptera tortricidae in lower franconia west germany

Horstmann, K.

Zeitschrift fuer Angewandte Entomologie 98(1): 73-95


Accession: 006520525

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The population density of T. viridana (L.) and of some other species of Tortricidae on oaks, and the parasitoids and diseases of T. viridana were studied in an oak forest in Lower Franconia over a 16 yr period. In this region, outbreaks of T. viridana of 4-5 yr duration, causing defoliation of the oaks, are separated by 7-yr periods of latency. During the time of investigation, 2 other species of Tortricidae (Tortricodes alternella (Den. and Schiff.), Zeiraphera isertana (F.)) each caused weaker outbreaks, not synchronized with the outbreaks of Tortrix viridana, and not reaching densities high enough to cause defoliation. The density of the other species of Tortricidae on oaks did not change appreciably. A total of 26 spp. of primary parasitoids (Hymenoptera [Ichneumonidae, Braconidae, Pteromalidae, Eulophidae] and Diptera [Tachinidae]) and 9 spp. of secondary parasitoids (Hymenoptera [Ichneumonidae, Torymidae, Perilampidae, Pteromalidae]) were reared from T. viridana, 6 of the primary parasitoids being the dominant species in at least 1 yr. The degree of parasitism varied between 7 and 67% (calculated for the 4th larval stage). Two frequent parasitoids of pupae are univoltine like the host, and act in a delayed density-dependent manner. Most of the other parasitoids of larvae and pupae are plurivoltine and parasitize other host species in summer and autumn. They act inversely density-dependent. Different factors are responsible for the control of the population density of T. viridana during the population cycle. In 1 case, the start of an outbreak was caused by the low efficacy of some important parasitoids. During the time of increasing host density, the degree of parasitism and the influence of diseases remained low. The population decline commenced as a consequence of food shortage after defoliation, followed by diseases (e.g., a microsporidiosis) and an increased efficacy of univoltine parasitoids. During the latency period, songbirds may control to some extent the density of T. viridana, acting on a background of a high and rather constant mortality caused by plurivoltine parasitoids. Migration is considered to be an important factor for the synchronization of density changes in an area.