Population Dynamics of the Alfalfa Blotch Leafminer, Agromyza frontella, and its Influence on Alfalfa Yield in Massachusetts1

Abstract

The population dynamics of the alfalfa blotch leafminer (ABL), Agromyza frontella (Rondani) (Agromyzidae: Diptera), were studied on Saranac AR alfalfa in western Massachusetts, 1976–1978. Three complete ABL larval generations and a partial fourth occurred in early June, mid-July, mid-August, and late September, respectively. Weekly sampling adequately tracked ABL population trends but was insufficient for detecting first presence of adult and early mine life stages. An average accumulation of 476 thermal heat units, base 6.8°C, was necessary for the completion of one generation. Evidence was obtained that postponement of the first two harvests until after egg deposition by the second and third ABL generation, respectively, reduced densities of later ABL life stages, although this practice may result in a substandard crop quality. The effectiveness of early harvest as a cultural control depends on timing it before development of 3rd instars. Leaflets infested with blotch mines contained 13.8% less crude protein than uninfested leaflets.