Genetic differentiation in sympatric but temporally isolated pine bark bugs, Aradus cinnamomeus (Heteroptera)

The pine bark bug Aradus cinnamomeus is a Eurasian insect pest of Scots pine. Each generation lives for two years in most areas in Europe, but in the north and in the Åland archipelago the generation time is three years. In western Finland, the 2‐year bugs reproduce mainly in odd years, while in eastern Finland (as well as in eastern Sweden) the even‐year generation predominates. Odd‐year bugs are very rarely found in the even‐year area, and even‐year bugs are very rarely found within the odd‐year area. The existence of sympatric but isolated alternate‐year generations offers a possibility to study the effect of ecological genetic factors in a natural test situation since all environmental variables should be otherwise identical. In the north and in the Aland archipelago, the preponderance of one generation is not so pronounced, and three generations usually coexist in almost equal numbers. The Finnish parapatric odd and even‐year generations (which are in the majority) are genetically virtually identical, while the odd and even‐year minority generations deviate genetically (and morphologically) from the majority ones. In the three‐year area the sympatric but allochronic bug generations differ genetically from each other to about the same extent as the allopatric but synchronic populations differ from each other. The sympatric differentiation can be explained as being a result of within species competition, although several other possible explanations exist. Furthermore, the two‐year bugs collected from eastern Sweden appeared to diverge genetically, although they represent the locally predominating generation.