Physiologischer Zustand der Wirtspflanze und Massenvermehrung von Forstschädlingen

Zusammenfassung1 Die Nahrung, d. h. ihre Menge und Qualität, ist der Hauptfaktor des Massenwechsels sowohl der stammbewohnenden als auch der laub‐ bzw. nadelfressenden Waldschädlinge. 2 Die Natur der Resistenz von Waldbeständen gegen die Schädlinge hängt vom Gehalt an Schutzstoffen ab, die besonders für die Junglarven und ‐raupen toxisch sind. Daher nimmt die zahlenmäßige Stärke der Schädlinge in gesunden Beständen selbst bei einer hohen Eierproduktivität und einer unterdrückten Tätigkeit der natürlichen Feinde nicht zu. 3 Der Beginn einer Massenvermehrung sowohl der stammbewohnenden als auch der Blattinsekten wird nur im Falle einer physiologischen Schwächung der Futterpflanzen möglich, wenngleich der Grad und Ort einer solchen Schwächung verschieden sind. 4 Die Witterung, der Boden und andere ökologische Bedingungen üben einen beträchtlichen Einfluß auf die Schwankung der Stärke der Schädlinge aus, jedoch hauptsächlich nicht direkt, sondern indirekt über den Zustand der Futterpflanzen. Der wichtigste Faktor wird in jedem Fall derjenige sein, der vom Optimum am weitesten entfernt ist.Summary Food ‐ its quality and quantity ‐ is the main factor of population dynamics of both secondary and primary forest pests. The complete development of foliage devouring insects though possible on healthy plants proceeds with a very high mortality of the youngest larvae. Mass outbreaks of these pests become possible only in the case of physiological inferiority of the plants owing to a decrease of their protective properties. This fact is proved by: 1. the absence of the hearths in natural forests of basic types without cuttings; 2. geographical distribution of the bulk of the hearths on the border between the forest and the steppe where the growth of trees is slackened by moisture deficit; 3. ecological restriction of insect pests to the sites with deteriorated conditions of growth; 4. mass outbreaks appearing under the effect if the climatic factors which unfavourably affect the condition of the plants; 5. effect of various anthropogenous factors weakening the plants; 6. experimental data on better survival of the caterpillars reared on the detached twigs or on the underdeveloped plants compared to the caterpillars reared on the plants growing under optimal conditions. The nature of the resistance of trees to the foliage devouring insects is determined by both internal and external protective factors. e. g. for pines (and also lemon trees) ‐ by the content of the oils causing a toxic effect upon young stages of the larvae. A similar rôle is played by other substances such as phytoncides. Their content fluctuates in relation to the site conditions, the weather and the effect of anthropogenous factors which results in the spot character of the hearths arising. Apart from this, the density of the integument of plants is also very important for the young larvae. Weather, soil and other conditions affect pests both directly and, mainly, indirectly through the plants, the most important being the factor most distant from its optimum. Favorable conditions for the pest in the weakened stands increase its aggressiveness and the possibility of the formation of hearths in the healthier stands. Measures improving the growth of plants (melioration, fertilization etc.) increase the resistance of plants and thus decrease the survival of the foliage devouring and sucking insects. The rise of hearth requires apart from the physiological inferiority of the trees the presence of the pest in the stand in a condition fit to the mass outbreak. This process takes place during a depression when the fittest and healthiest individual of the pest survive.