Inference of adult female dispersal from the distribution of gypsy moth egg masses in a Japanese city

1 The native range of the gypsy moth Lymantria dispar (L.) spans the temperate forests of Eurasia. Across this region, a clinal female flight polymorphism exists; gypsy moth females in eastern Asia are mostly capable of directed flight, those in western and southern Europe are largely incapable of flight and populations distributed across the centre of the distribution exhibit a range of intermediate flight behaviours. 2 Although information exists about the timing and duration of female flight from laboratory and wind tunnel studies, little or no quantitative data are available on average distances flown by Asian gypsy moth females prior to oviposition in the field. This information is critical for estimating risk of contamination at specific ports and transit terminals, as well as for predicting the spread of populations that might someday invade currently uninfested regions of the world. 3 In the present study, an extensive visual survey of gypsy moth egg masses was conducted during a walk through streets and paths in a 3.92 x 5.76 km area in Kanazawa, Japan. This area consisted of a matrix of urban, agricultural and forest land uses. The distribution of egg masses relative to distances from host forests was used to infer the magnitude of pre-ovipositional female flight. 4 A total of 3172 egg masses was recorded from surveys conducted during the search of a path totalling 384 km. Within urban areas, egg masses were most abundant in the area < 1 km from the edge of forest land. 5 These results suggest that most female gypsy moth flight is limited to the area within 1 km of host forests. They also suggest that shipping containers and other parcels located > 1 km from forests are at a much lower risk of contamination with Asian gypsy moth egg masses.