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The energy turnover of weevil populations ofRhynchaenus fagi L.,Strophosomus (Schönherr) andOtiorrhynchus singularis L. (Curculionidae) was investigated in the following stages: a) Observation of habits and life-cycle; b) investigation of abundance-dynamics; c) establishing the energy budget of average individuals; d) calculation of the energy turnover of the populations. In the case ofRhynchaenus fagi the investigations were carried out with all developmental and maturity stages, in the cases ofStrophosomus sp. andO. singularis only with the imagimes. 1. The larva ofR. fagi mines the beechleaf. The development takes 5\2-6 weeks from oviposition (beginning of May) to the emergence of the beetle. The imago lives for about 12 months. There is a first eating period lasting until the end of July, followed by about 9 months of almost no food consumption; after hibernation there is the second eating period (May to June) and reproduction (beginning of May). Strophosomus sp. emerges on the ground in September and eats in the top shoots of the trees till the leaves fall; it feeds on fallen foliage on the ground during the winter and high in the trees again from May to July.Strophosomus sp. reproduces mainly parthenogenetically and dies no later than the end of August. O. singularis emerges mainly in May and June. After an intensive feeding period these females also deposit mostly unfertilized eggs. The beetles live for several years, imagines from different years living side by side. 2. Beginning in JulyR. fagi migrates increasingly to nearby spruce forests which are its favourite winter quarters in the Solling. Microclimate and thigmotaxis between the needles protectR. fagi from higher mortality. 3. Abundance and mortality of eggs, larvae, pupae and young beetles ofR. fagi were determined by counting the larvae and pupae on a large number of leaves and by marking leaves with a numbered label. For the beetles a linear progression of mortality was assumed from emergence to reproduction. In the case ofStrophosomus sp. the numbers of beetles emerging and the winter mortality were determined by means of eclector catches. Shaking samples yielded data concerning the mortality after hibernation. WithO. singularis the numbers of beetles emerging was established by means of eclectors. 4. In the case ofR. fagi the production of an average individual is 2.654 cal till the end of the pupal stage, 6.353 cal until death. 1.103 cal are taken up for cocoon and exuviae, 0.809 cal for egg production. From emergence to the end of July of the following yearStrophosomus sp. produces body substance of a calorific value of 28.630 cal, of which 9.690 cal are taken up for eggs. 5. A new calorimetric method was applied for the first time in determining the respiration ofR. fagi during its leaf-mining stages. The results agreed with the calculation of energy losses based on manometric measurement of O-consumption. With all weevil imagines manometric measurement of O-consumption was carried out under near-natural conditions. The imagines showed seasonal changes of the respiration level and theQ , which were usually consistent with their life cycle. 6. The assimilation of an average individual (A=P+R) was calculated from production (P) and respiration (R); in the case ofR. fagi for all developmental and maturity stages, in the case ofStrophosomus sp. for the imagines at different times of the year. The horizontal course of the assimilation curve ofR. fagi during pupal stage and hibernation indicates that the measuring technique is highly accurate. The energy loss through respiration increases at exactly the same rate as the energy potential of the animal decreases. An average individual ofR. fagi assimilates 4.509 cal from its emergence until the end of the pupal stage, 9.042 cal by the end of the first eating period (end of July), and a total of 17.031 cal by the end of the reproduction and second feeding period (June of the following year). An average individual ofStrophosomus sp. (imago) assimilates 24.000 cal by the end of the first eating period (end of November), 32.006 cal by the end of hibernation (late April), and 78.330 cal before it dies. 7. Investigations of consumption (C) and defecation (FU) and the calculation of assimilation (A) asA=C-FU usually confirmed the results ofA=P+R. The ecological efficiency (P/C %) ranges from 14-20% for larvae ofR. fagi, from 7.0-8.5% for all weevil imagines. 8. With an energy turnover of about 21.9\sx10 kcal/ha/yr in a 120-year-old beech wood (B1a) and about 42.1\sx10 kcal/ha/yr in a 60-year-old beech wood (B4), the population ofR. fagi belongs to the ecologically dominant phytophagous insects. The larvae assimilated 62% of this amount, the imagines 31% before hibernation and 7% in spring. TheStrophosomus-population (imagines) assimilated about 4.4×10 kcal/ha/yr in B1a and about 4.3×10 kcal/ha/yr in B4. 9. As far as the production of imagines is concerned (2.1\sx10 in B1a and 4.0\sx10 kcal/ha/yr in B4)R. fagi takes first place among phytophagous insects \3-at least in young beech woods. The production of imagines inStrophosomus sp. amounted to about 1.3×10 in B1a and about 1.2×10 kcal/ha/yr in B4, inO. singularis to about 0.9×10 in B1a and about 1.3×10 kcal/ha/yr in B4. 10. Assuming an averageA/C-efficiency of 20\2-25% the populations ofR. fagi (all stages) and ofStrophosomus sp. (imagines) consumed 103-128\sx10 kcal/ha/yr in B1a and 183-229\sx10 kcal/ha/yr in B4. This means 14\2-17% and 24\2-31% (in B1a and B4 respectively) of the total leaf substance consumed by phytophagous insects, for which Funke (1972a) arrived at a figure of 7.5\sx10 kcal/ha/yr.