engleski

Spread and character of Tomostethus nigritus F. outbreaks in Croatia during the last decade

The larvae of Tomostethus nigritus F. (Hym.: Tenthredinidae) began causing severe defoliation on ash along avenues and tree lines in Zagreb, Croatia since 1997. The phenomenon of population outbreaks in periurban and urban environments is known but poorly documented in the literature ; the fact that it has not yet been recorded in Croatian forests has spurred authors to investigate further into this natural event. Populations achieved outbreak levels in the second year after defoliation was first recorded. Although only Fraxinus excelsior L. was attacked ; palatability tests indicate that larvae show no preference among several ash species, larval development in situ and their dispersion in the field suggest that phenology of the host tree was the dominant factor in host selection. Based on foliage availability, F. excelsior proved to be the most suitable species for females during their oviposition period. Detailed study of all developmental stages, especially the late larval and prepupal stages, confirmed earlier published knowledge on this sawfly but provided also some new facts and practical solutions that can be utilized for monitoring and managing this species. Parasitoid fauna have also been studied and, despite the relatively small impact that was recorded from laboratory rearings, it is assumed that parasitoids and floods are the most important natural controlling factors affecting this species in forests. Based on the spread and intensity of the recent outbreak and its similarity to European outbreaks, we presume that environmental factors, mainly the impact of late fall and early spring floods, are the most important trigger mechanism involved in this phenomenon.

Tomostethus nigritus; outbreak; population dynamics

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