Influence of varying soil nutrient conditions on the population dynamics of Mecinus janthiniformis, a stem-mining weevil of Dalmatian toadflax (Linaria dalmatica)

Differing soil nitrogen contents across the northwestern U.S. may be a factor influencing the performance of the stem-mining weevil Mecinus janthiniformis Toševski & Caldara sp.n. (Coleoptera, Curculionidae), a biological control agent for the noxious weed Dalmatian toadflax (Linaria dalmatica (L.) P. Mill).  We manipulated soil nitrogen levels and weevil attack intensity in a factorial experiment to assess their individual and combined effects on the population dynamics of M. janthiniformis.  Greenhouse propagated L. dalmatica plants were planted in late spring 2011 in a common garden, receiving three levels of N fertilization and four densities (0, 2, 4, and 8) of mating weevil pairs.  In the fall, stems from each plant were dissected to measure weevil performance (female replacement, individual weight, and proportion of individuals completing development).  In general, performance of M. janthiniformis increased with soil N, but decreased with the density of mating pairs.  Negative effects on weevil performance were strongest at the highest weevil pair density (8 per plant) and lowest soil N treatment.  The results of this study suggest that weevil population dynamics are subject to negative density-dependence, but the strength of this effect may be influenced by an interaction between local soil N availability and weevil attack intensity.  Our results provide insight into one of several factors that may explain the spatially variable impact of M. janthiniformis on Dalmatian toadflax in the northwestern U.S.