Response of beech (Fagus sylvatica) to elevated CO₂ and N: Influence on larval performance of the gypsy moth Lymantria dispar (Lep., Lymantriidae)

Two-year-old beech seedlings were kept from germination to bioassays with Lymantria dispar under the following conditions: ambient CO₂/low N, elevated CO₂/low N, ambient CO₂/elevated N, and elevated CO₂/elevated N. The effect of these growing conditions of the trees on the performance of the defoliator L. dispar was studied 2 years after initiating the tree cultivation. The developmental success of third-instar larvae of L. dispar was characterized by the weight gained, percentage of weight gain, relative growth rate (RGR), relative consumption rate (RCR), and efficiency of conversion of ingested food into body substance (ECI). Contrary to our expectations, additional N-fertilization did not increase and elevated CO₂ did not delay larval growth rate. However, the environmental treatments of the beech seedlings were found to affect the larval performance. Larvae consumed significantly higher amounts of foliage (RCR) on beech trees under controlled conditions (ambient CO₂ and low N) compared to those under elevated CO₂ and enhanced N. The opposite was true for ECI. The lowest efficacy to convert consumed food to body substance was observed under control conditions and the highest when the larvae were kept on beech trees grown under elevated CO₂ and additional N-fertilization. These opposite effects resulted in the weight gain-based parameters (absolute growth, percentage of growth, and RGR) of the gypsy moth larvae remaining unaffected. The results indicate that the gypsy moth larvae are able to change their ECI and RCR to obtain a specific growth rate. This is discussed as an adaptation to specific food qualities.