First Record of Dendrolimus pini Outbreak on Aleppo Pine in Croatia and Severe Case of Population Collapse Caused by Entomopathogen Beauveria bassiana

Abstract

Background and Purpose: The pine-tree lappet moth, Dendrolimus pini, is a widely distributed pest in Europe that can cause serious needle defoliation on pines, with outbreaks occurring over large geographical areas. In Croatia the presence of D. pini was recorded only in the continental part, but not in a high level of abundance, and the outbreak of the population has never been recorded so far. Materials and Methods: In autumn 2014 an unexpected and complete defoliation on Aleppo pine (Pinus halepensis Mill.) occurred in the vicinity of Skradin, near Šibenik, and was followed by defoliation in summer and autumn 2017 in Telašćica Nature Park, Dugi otok. Infested areas were inspected and overwintering larvae in soil surface were counted in order to estimate the population and assess whether the critical number of the population has been exceeded. Specimens were collected and transferred to the Laboratory for Entomological Analysis in the Croatian Forest Research Institute for further laboratory analysis. Results: Since more than 10 larvae per m2 of soil surface were found, it was concluded that an outbreak occurred at both localities. In both cases natural antagonists played an important role in lowering the pest population. In Skradin, entomopathogenic fungus Beauveria bassiana that occurred on the overwintering larvae broke the pest population, and in Dugi otok it was recorded, but to a much lesser extent and in combination with Drino inconspicua parasitoid. In both cases pines recovered very well in the following spring, with some bark beetle attacks mostly at the edge of the forest. Conclusions: The outbreaks of D. pini resulting with a total defoliation of Aleppo pine stands, and the fact that they occurred in the Mediterranean region which is not an optimal area for its appearance, makes these events unusual. Also, this is the first record of D. pini population breakdown by some antagonist.