The role of nectar plants in severe outbreaks of armyworm Mythimna separata (Lepidoptera: Noctuidae) in China.

The period from March to mid April, when oriental armyworm Mythimna separata (Walker) moths migrate from South China to Central China over several nights, is an important window of time in annual armyworm population increase. The presence of nectar sources along the pathway of the migratory population is a prerequisite for moths to reach target habitats and lay eggs. Using flowering periods and geographic distributions, the major spring nectar plants suitable for M. separata moths were identified from among 102 species/varieties of apicultural nectar plants. The nectar plants proposed as important to M. separata include milk vetch Astragalus sinicus L., rape Brassica napa L. and six other species. Spearman's rank correlation analyses were conducted between the annual population size of M. separata and the acreages of milk vetch and rape in the daily stopover areas for migrating populations of M. separata during 1950-1979. The Spearman's coefficient between milk vetch and M. separata was 0.6259 and the correlation was highly significant (P<0.001). Further regression analysis with data from 1950-1979 and from 1980-1992 also revealed a close relationship between annual acreage of damaged crops/wheat and acreage of milk vetch. These results strongly suggest that the unprecedented enlargement in the geographic distribution of milk vetch from Central China into South China was the key factor in the frequent severe oriental armyworm outbreaks that occurred during 1966-1977. This is thought to be the first report in the world that reveals the key role of nectar sources in long distance, regional scale, migration of moths. The argument for the key role of milk vetch is supported by the simultaneous decline in the level of damage inflicted by M. separata and the acreage of milk vetch after 1980.