Factors influencing the population density of the weevil Microplontus millefolii Schze. (Coleoptera, Curculionidae: Ceutorhynchinae) in the environs of St. Petersburg

Abstract

Environmental factors determining the population density of the weevil Microplontus millefolii, a stem miner of the common tansy, Tanacetum vulgare, were investigated in the suburbs and environs of St. Petersburg, Russia. Larvae or empty mines of M. millefolii were found in 39 out of 43 inspected sites; in total, 620 out of 2283 T. vulgare stems were infested. The percentage of infested stems per site ranged from 0 up to almost 90%, the density of infestation (the mean number of larvae and empty mines per stem) ranged up to 2.3 with the mean of 0.51 for all inspected sites. The surrounding vegetation was the single environmental factor significantly correlated with the percentage of infested stems (30 and 20% in sites with domination of grasses and of forbs, correspondingly). The mechanisms of this dependence are not clear. Probably, the domination of grasses (that is a relatively advanced stage of the vegetation succession) is correlated with the longer period of time for accumulation of phytophagous insects associated with tansy. The analysis of data for individual stems (within-site distribution) showed that the mean number of the weevil larvae almost linearly increased with the diameter of the stem. The dependence on the host plant population density was also strong but not linear: both in dense patches (percent cover of more than 50%) and in sparse plants (percent cover of 1% and less) the density of infestation was higher than at a medium abundance of the host plant. In addition, in the patches with only a short time shadow from trees or tall herbs the density of infestation was higher than in those where the shadow stayed for longer or was absent at all. The mean density of T. vulgare infestation by M. millefolii was independent of the type of habitat (meadows, old fields, roadsides, ruderal sites), the size of the site, the average cover of T. vulgare, the total cover of green plants, the degree of anthropogenic disturbance, soil composition, and humidity. This ability to infest T. vulgare in very different habitats allows considering M. millefolii as a potentially effective agent for biological control of the common tansy in the invasion areas.