Magnitude and Structure of Lymantria dispar asiatica Infestations of Common Forest Steppe Tree Species in Northern Mongolia

Lymantria dispar L. is one of the most serious forest pests worldwide by infesting large forest areas involving massive tree dieback. Unlike outbreaks of Lymantria dispar L. in Germany that cause massive defoliation, observations of outbreaks of Lymantria dispar asiatica made in the research area Khonin Nuga, West Khentii, Mongolia during the past six years showed only punctual defoliation, and dieback was extremely scarce. However, information on the performance of Lymantria dispar asiatica and the corresponding reaction of host tree species in Khonin Nuga is rare. Therefore, this study concentrates on the examination of primarily affected tree species and the magnitude and spatial structure of Lymantria dispar asiatica infestations. The determination of Lymantria dispar asiatica pupal development was carried out on Betula fusca, Larix sibirica and Prunus padus asiatica. The magnitude and spatial structure of Lymantria dispar asiatica infestations was surveyed at two Larix sibirica island stands in the forest steppe. A forest inventory of the stands was conducted and the degree of foliation and the infestation with pupae was investigated. It was found that Lymantria dispar asiatica developed best on Larix sibirica as the pupae mortality was lowest here. In both Larix sibirica stands, trees of high vitality showed the highest number of pupae, whereas trees of low vitality had fewer pupae, suggesting a movement of larvae from defoliated trees to vital trees for pupation. The southern parts of both stands were most defoliated; from here on the larvae spread northwards.