Explaining spatio-temporal patterns of impact to Dalmatian toadflax by the stem-mining weevil Mecinus janthiniformis: Effects of host quality and attack intensity on weevil population growth

Insect herbivores released as classical biological control agents of weeds are notorious for large fluctuations in abundance in space and time, but rarely do we understand which factors affect variable population growth across a landscape.  Impact of the stem-mining weevil Mecinus janthiniformis Toševski & Caldara sp.n. (Coleoptera: Curculionidae), released as a biological control agent of the noxious weed Dalmatian toadflax (Linaria dalmatica (L.) P. Mill), varies greatly across northwestern NA and sometimes infestations rebound after weevil release, possibly due to negative dependence operating in the weevil population.  In a factorial experiment we examined how host quality (soil nitrogen levels) and weevil attack intensity (0, 2, 4, and 8 of mating weevil pairs) affect the population growth potential of M. janthiniformis.  We hypothesized that weevil population growth will decline with reductions in resource quality (smaller host size) and as attack intensity increases.  In general, performance of M. janthiniformis increased with host quality and 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 host quality. The results of this study suggest that diminishing resource quality (host size) due increases in weevil abundance invokes negative density-dependence that reduces weevil population growth and may permit infestations to rebound.  Our results provide insight into one of several mechanisms that may explain the spatially variable impact of M. janthiniformis on Dalmatian toadflax in the northwestern U.S.