Possible influence of summer-autumn sums of effective temperatures on the dynamics of population density of the gypsy moth Lymantria dispar (L.) in forest stands on the northern border of the range

Almost simultaneously in 2016, along the northern border of the range of spongy moth Lymantria dispar (L.) over a huge area of more than 1000 km in the longitude direction from Yekaterinburg (N 56.5 °, E 60.4°) to the village of Kyshtovka (Novosibirsk region), (N 56.4°, E 76.6°) the outbreaks of the spongy moth have formed on hundreds of thousands of hectares. Such synchronicity in the occurrence of outbreaks can indicate the impact of a single modifying factor. The purpose of the study is to identify a single modifying factor that could provoke an outbreak of the spongy moth, based on an analysis of weather conditions in the growing seasons preceding the formation of outbreaks and during their implementation. Over the large area covering the Trans-Ural and West Siberian populations, in their northern part, the scenarios of weather conditions changes before and during the implementation of the outbreak of 2016–2018 differed significantly in terms of humidity conditions but were similar in terms of heat availability. For two or three years, the heat availability was significantly lower than normal, then become the season with heat availability significantly higher than normal and a sharp increase in population density that provide the severe defoliation of stands for the next year when the heat availability of the growing season returns to normal. That indicates the differences in modifying factors provoking outbreaks at the north of the range of the spongy moth in comparison with southern parts of the range of this species. Possibly, growing seasons with low heat availability lead both to the elimination of the offspring of individuals in the population with a long duration of development till the imaginal stage, and to a reduction in the duration of development at the larval stage of individuals that have received the necessary, but an extremely small amount of summer-autumn temperatures to complete early embryonic development and preparation for wintering. With the onset of the growing season with a heat availability significantly higher than normal, the elimination of individuals with a longer duration of development decreases, which results in the population density increase and significant defoliation in the next year.

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