Population dynamics of cereal leaf beetles, Oulema melanopus L. and O. lichenis Voet (Coleoptéra: Chrysomelidae), on wheat fields in southern Belgium.

In order to determine the pest status of cereal leaf beetles in Southern Belgium, the population densities of Oulema melanopus (L., 1758) and O. lichenis (Voet, 1806) were studied at Ciney and Louvain-la-Neuve during 1993 and 1994. During the two-year study period Oulema spp. populations had never reached the economic threshold of 2.5 larvae per tiller. The levels of the Oulema populations were significantly different between the 2 sites in 1994; Louvain-la-Neuve supporting greater densities than Ciney. Phenological differences between the two locations were postulated to be the result of two phenomena : (1) the early attainment of the physiological time of development, expressed in degree days, necessary for immature stage development, in Louvain-la-Neuve, the warmest location, and (2) the early attainment of the cold sum necessary to end the adult diapause in Ciney, the coldest site, however this second argument stays as a hypothesis that must be tested.