Nearctic Chymomyza amoena is the only drosophilid to breed in fallen parasitized substrates (plums, unripe and ripe apples, crabapples, acorns, black walnut hulls). Minimum generation time is 30 days. The breeding season extends from May into October. The species is coldhardy; 3rd instar larvae overwinter in fruits or nuts. Chymomyza amoena was discovered in the former Czechoslovakia in 1975, spread rapidly, entered Switzerland in 1988 and was reared from parasitized chestnuts, acorns and soft fruits in 1990/1991 in Canton Ticino. Later work in Switzerland showed C. amoena still bred in parasitized apples in addition to chestnuts and acorns. We predicted and verified it moved north into cider apple orchards on the Swiss/German border and south into northern Italy, also from Vienna into southern Austria and in France from Jura, Ain, Rhone and Is P re into the Ardeche region. Pest larvae exit chestnuts in autumn; C. amoena may use them into July, then breeds in apples. We investigated overwintering in chestnuts in March 2004 in southern Switzerland, northern Italy and the Ardeche and in acorns in southern Switzerland. Parasitized chestnuts were empty; C. amoena larvae were in acorns along with Cydia splendana caterpillars. Europe?s hot summer in 2003 delayed chestnut drop but adds evidence this represents a vacant niche for C. amoena. Breeding in parasitized acorns and apples may account for C. amoena?s spread in Germany and throughout Europe.