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Summary About half the number of stumps of recently felled Scots pine (Pinus sylvestris L.), selected so as to be evenly distributed over the plots concerned, were barked. Equal numbers of these and of the unbarked stumps were uprooted later, and the larvae, pupae and adults of Hylobius abietis L. and other insects in the stumps and roots were counted. Other readily obtainable data were also collected. In one experiment, cages were put over the stumps to trap the emerging H. abietis adults. On comparing the Hylobius populations in the two categories of stumps it was seen that barking at the beginning of the oviposition period or about 1, 2 and 6 months before it, did not result in a significant reduction of the numbers developing in and emerging from the stumps and roots. Even clearing the soil round the stumps and main roots, carefully barking the stumps to a greater depth than usual, replacing the soil and treading it down did not produce that effect. Nor was it obtained by barking at a time during the larval stage when the greatest possible number of larvae could be destroyed. The latter experiment was carried out in both thinned and cleared plots, the others in either of the two types of plots. It is concluded from a study of the literature on the subject and from these results that the efficacy of barking as a control measure against H. abietis is unproven and that some of the obvious modifications of the method are definitely ineffective. There was no consistent difference between the numbers of Hylobius developing in stumps in cleared or in thinned plots. The time required for development and emergence from the stumps and roots was 14–18 months, presumably corresponding to a two-year cycle from one egg stage to another. In some individuals the underground period was either 3–6 or 12 months. Most adults appear above ground in August. Less than 10 % of the eggs reached the adult stage on one cleared plot, the greatest mortality occurring in the early larval stages. Normally no H. abietis will be present in the stumps after the second summer after felling. The latter statement also applies to all Scolytids in clear-felled plots; in thinned plots small numbers may persist after that. Hylastes ater Payk. was the most numerous of these Scolytids. Where the adults of that species could choose between freshly barked and unbarked stumps greater numbers of them invaded the former, but the difference in numbers was offset later by the greater mortality in the brood in the barked stumps. It is believed, however, that the total mortality in the barked and unbarked stumps together remained the same, and that even the barking of all the stumps in a given area would not have affected the total mortality. Barking prevents pine-shoot beetles (Myelophilus piniperda L., Scolytidae) from breeding in the stumps, but not many are likely to do so under the conditions prevailing in the Netherlands; so the operation is not considered worth the expense. Other Scolytids regularly found in the stumps were Hylastes opacus Er., H. attenuatus Er. and Hylurgus ligniperda Fabr. The number of their offspring was not affected by barking. Cerambycid larvae found more or less regularly in the stumps were Acanthocinus aedilis L., Tetropium sp., Asemum striatum L. and Spondylis buprestoides L. Barking, at all events in late winter and early spring, reduces the numbers of the first two species. It is not clear what effect barking has on the others. Other species found more or less regularly in the stumps included the following Coleoptera: the Nitidulids Pityophagus ferrugineus L. and Rhizophagus ferrugineus Payk., the Elaterid Melanotus rufipes Herbst, the Tenebrionid Cylindronotus (Helops) laevioctostriatus (Goeze); and the following Diptera: the Asilid Laphria gilva L., and the Dolichopodid Medetera sp. The numbers of Elaterid larvae (including M. rufipes) and the number of larvae in the group called “various predacious Coleoptera” (including many Nitidulids) were significantly affected by barking in some experiments. It is not understood why they were not generally so affected. Zusammenfassung Vergleichende Untersuchungen an entrindeten und nicht entrindeten Kiefernstubben ergaben keine gesicherten Unterschiede. Die Entrindung ist somit als Bekampfungsmasnahme gegen H. abietis nicht geeignet. Die Entwicklungszeit von H. abietis in den Stubben auf Kahlschlagen oder auf durchforsteten Parzellen betrug in beiden Fallen in der Regel 14 bis 18 Monate. Die meisten Imagines schlupften im August. Die hochste Sterblichkeit wurde bei den fruhen Larvenstadien festgestellt. Normalerweise sind Stubben im zweiten Herbst nach ihrer Fallung von H. abietis frei. Die an Individuen reichste Art der Scolytiden war Hylastes ater Payk., der berindete Stubben bevorzugte. Durch Entrinden wurde der Waldgartner (Myelophilus piniperda L.) am Bruten in den Stubben verhindert. In den Niederlanden spielen Stubben als Reservoir von M. piniperda jedoch eine so geringe Rolle, das eine Entrindung ihretwegen nicht rentabel ist. Andere regelmasig in entrindeten und berindeten Stubben gefundene Scolytiden waren: Hylastes opacus Er., H. attenuatus Er. und Hylurgus ligniperda Fabr. Von Cerambyciden wurden regelmasig in den Stubben die Larven folgender Arten gefunden: Acanthocinus aedilis L., Tetropium sp., Asemum striatum L. und Spondylis buprcstoides L. Erstere zwei Arten wurden durch Entrinden eingeschrankt. Des weiteren wurden an Coleopteren gefunden: Pityophagus ferrugineus L. und Rhizophagus ferrugineus Payk. (Nitidul.), Melanotus rufipes Herbst (Elat.), Cylindronotus (Helops) laevioctostnatus (Goeze) (Tenebrion.) sowie die Dipteren Laphria gilva L. (Asilidae) und Medetera sp. (Dolichopod.). |
