Dynamic Relations between the Antler Moth and Meadow Vegetation in Northern Sweden

The antler moth (Cerapteryx graminis L.) is a noctuid that occurs over wide areas of Europe and that has recently colonized North America. The larvae feed almost exclusively on grasses in spring and early summer. In northern latitudes and at high altitudes they can reach densities of 100—1500 individuals/m². During the outbreak years this herbivore may have a great impact upon the vegetation. In a long—term investigation (11 yr) we recorded the larval densities, plant standing crop, and number of shoots of the abundant grass and herb species in northern Sweden. We also experimentally increased the density of larvae to study the effect of larvae on the individual size, mass, and nutrient concentration of grasses and herbs. During the study period the larval density was negatively correlated with the mean June temperature. We observed two outbreaks of Cerapteryx, which reduced grass biomass and led to increased population sizes of some herb species during the same and/or subsequent years. The annual herb Rhinanthus minor increased in abundance in the years of the Cerapteryx peaks. Increased numbers of flowering shoots were recorded in the following year for the perennial herbs Gnaphalium sylvaticum, Leontodon autumnalis, Trifolium repens, and Veronica serpyllifolia. The abundance of herbs decreased in the following 3—4 yr when the larval densities of Cerapteryx declined, while the grasses increased. There was a negative correlation between the biomass of grasses and the biomass of herbs harvested in early September for the 11 yr. suggesting that herbs also responded to grass consumption by larvae. The nitrogen concentrations in the aboveground parts of grasses and herbs were highest in the same year or in the year following the outbreak. The experiment with increased larval density (°560 larvae/m²) showed that at this level: (1) grass ramets of four species in late summer were lower in height and biomass while the individuals of four out of five species of herbs had greater dry mass, but not height; (2) the concentrations of nitrogen, phosphorus, and potassium in the aboveground parts of the plants were higher than found at the control level (°60 larvae/m²). Our data thus indicate that the grass—herb dynamics are influenced by Cerapteryx. Other important factors affecting the grass—herb dynamics are voles, summer air temperature, and precipitation.