Peak Years and Regulation of Numbers in the Aphid Neomyzaphis abietina Walker

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Abstract
In the geographical areas considered here (oceanic NW-Europe) Neomyzaphis abietina is an introduced species. It is likely that it has not until recent time reached these areas. There is some evidence, that its original host is Norway spruce. In this zone the Neomyzaphis population is quite predominantly regulated by climate. The population tends to mass-outbreaks after mild winters, and most often in the winter-mild regions. This is because overwintering in the regions considered here does not - or only to a small degree -take place in a diapause stage, but on the contrary in a stage able to reproduce at very low temperatures. Enemies and diseases are unimportant at this season. It is possible to regard the regulation of Neomyzaphis numbers as a double one: a one-sided density-dependent regulation acting on the spring population maximum and a climatic regulation acting on the early winter maximum. It is climate and not the density-dependent agents which create the mass-outbreaks. The long term population trend might in the winter-mild areas be called regular fluctuations and they have even been called "periodic cycles". Seen on a diagram they may appear similar to those of many so-called cyclic species. A more correct term would in this case - and possibly also in some of the others - be "frequent fluctuations", because they are simply a reflection of fluctuating climate. In the winter-colder areas only erratic fluctuations occur.

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