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Mae ymdeithiwr y derw yn frodor o dde Ewrop lle mae ysglyfaethwyr a ffactorau amgylcheddol fel arfer yn cyfyngu ar ei niferodd a lleihau unrhyw effaith mae'n ei chael. Er hynny, drwy gymorth symudiadau planhigion, mae ei gyrhaeddiad daearyddol wedi cynyddu tua'r gogledd dros yr ugain mlynedd diwethaf, ac mae wedi ymsefydlu mewn lleoliadau mor ogleddol â'r Iseldiroedd, Gwlad Belg a gogledd yr Almaen.
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Oak Processionary moth is a native of southern Europe where predators and environmental factors usually keep its numbers in check and minimise its impact. However, helped by movement of plants, its range has been expanding northwards over the past 20 years and has become established as far north as the Netherlands, Belgium and northern Germany. Climate change might be a factor - the reduced incidence of cold, wet weather during the spring-time larval emergence period in recent years might have enabled it to become established further north than would have been possible in the past.
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