Adult Lymantria dispar japonica (females only) and Lymantria mathura (both sexes) flew to commercial lighting during the night in Takizawa Village, Iwate Prefecture, Japan, in large numbers during the first week of August 2008. Males of L. d. japonica were conspicuously absent while 93.8% of responding females were mated and subsequently laid an average of 419.2 eggs each post-flight. For L. mathura females, only 33.9% were mated during the first half of the night and each mated female carried on average 717.4 eggs (max. 1065). Egg counts were facilitated by a newly developed egg mass matrix digestion process and an approximation of egg count based on egg mass dry weight was calculated. These two moth species have dissimilar behavioral strategies—L. d. japonica is diurnal, mated during the afternoon pre-flight and females flew to the lighting after mating. In contrast, L. mathura is nocturnal and both sexes responded to the lighting; females generally arrived as virgins (unfertilized) to then mate with males as the night progressed. Lack of a moth flight in 2009 illustrated dramatic interannual population fluctuation prevalent among many lymantriine moths. An illustration of a 2013 mass flight in Ono City, Fukui Prefecture, suggests a more frequent occurrence of such flights in both time and space.