Forest protection with sessile oak

Oak forests are characterized by high levels of biodiversity and, at the same time, are threatened by mass propagation of insect pests. The so called "oak defoliator complex" is made up of the European oak leafroller, various winter moths, the gipsy moth and the oak processionary. If early and late defoliators emerge together, then the vitality of the oak forests can be reduced to such a point that afflicted stands will die.

The European oak leafroller (Tortrix viridana) belongs to the family of the leafrollers (Tortricidae). The caterpillar is polyphagous and feeds on many kinds of deciduous trees and shrubs, but it prefers oak.

Females lay around 50 eggs, always two at a time and on the outer tips of twigs nearby buds. Decisive for the development of the caterpillar is the timing between the emergence of the caterpillar and bud break. If bud break doesn’t occur within four days after emergence, then the caterpillars will starve.

After development, the larvae usually pupate in a spun and rolled up leaf. After a complete defoliation the pupae can be found in cracks in tree bark or in leaf rolls in the shrub and herbaceous layer.

The distribution of the European oak leafroller is identical with that of European oak species. It spans from the British Iles to Spain, from Italy to the Crimean islands and the Caucasus and in the north to the Baltic Sea. In some regions, the European oak leaf roller appears in chronically high densities (permanent areas). Additionally, there exist latent and gradient areas with respectively low and varying densities. Mass propagations can also be found in peripheral areas.

Mass propagations of the European oak leafroller do not show any regular fluctuations since the temporal correlation between the emergence of the caterpillar and bud break is dependent on the weather and thus, coincidental. After four to five years with optimal conditions for propagation the population collapses due to disease.

The mottled umber moth (Erannis defoliaria), the winter moth (Operophtera brumata) and the northern winter moth (Operophtera fagata) also belong to the early defoliators. The three species are found on various deciduous trees, however, the mottled umber moth prefers oaks, the winter moth prefers European hornbeam and the northern winter moth prefers European beech.

The winter moth group displays strong sexual dimorphism. The males are good fliers while the females are entirely incapable of flight. The latter therefore climb up on the stem in order to lay their eggs on buds or cracks in the bark in the canopy.

The maturation of the caterpillars is complete as soon as the leaves of the oaks are fully developed. By disturbances the caterpillars rappel down on a strand of silk and climb up again later. They pupate in June in the topsoil.

The distributional range of the mottled umber moth spans over north and central Europe till northern Italy and from the Caucasus to the Baltic’s. The moth was also accidentally introduced into North America. The winter moth is prevalent in Europe, Transcaucasia and east Asia. The northern winter moth is native to Europe.

The gipsy moth (Lymantria dispar) also displays a strong gender dimorphism.

There is a variety of color variation with the gypsy moth. In some cases, large differences in the size of the stages occur over a small area. The reasons for this are in part genetic variation and environmental influences (moisture, quality of nutrition etc.).

The female lays the eggs in a clutch of 100 to 1,000 eggs and covers them with a yellow-brown anuswool. The clutches are often found on the lower parts of the stems. With high population densities they can also be found on the entire stem and on branches in the crown.

After hatching, the caterpillars produce hairs in their first days and remain at the site of the clutch and move only to feed in the tree canopy. They feed till the third caterpillar stage during the day. After the third or fourth caterpillar stage they hide in the cracks of bark during the day and feed at night. In order to avoid shortages in food they spin a thread of silk and let themselves be transported by the wind. In this way they can reach new sources of food. Older caterpillars feed very wastefully. Amid mass propagations the uneaten remnants of fresh leaves can cover the forest floor. During their development gipsy moth caterpillars eat one square meter of leaves and let much more fall to the ground. The caterpillar development lasts, depending on the supply of food, six to twelve weeks, but usually till the end of June.

The broad diet of the gipsy moth covers about 400 plants. It feed primarily on deciduous trees but also doesn’t refuse conifers. Moths of different origins prefer a different variety of host plants (in Bavaria especially the oaks). The moth also exploits unusual sources of food. The "Bavarian" gipsy moth will die after ingesting black locust due to the metabolically poisonous contents of the tree, however, some gipsy moth populations in Austria and Hungary can develop normally on this tree species. During mass propagations the gipsy moth feeds on all available vegetative substances, even rice and grain.

The distribution of the gipsy moth is lies from England to Japan. Its southern border is the Mediterranean Sea and the northern border is a line from the middle of Sweden to Moscow. In 1869 the gipsy moth was imported to America for the commercialized production of silk. In the 20th century the moth later spread over large areas in the north eastern part of the United States where it is a much-feared pest.

The mass propagations in Europe have occurred with increasingly shorter intervals from west to east. The intervals are, on average, every twelve years in France to seven years in the Balkans. From 1992 to 1994 there was a wide spread mass propagation in Bavaria with a need for abatement measures on 23,000 hectares (2005-2007 on 7,000 ha, 2010-2011 on 3,000 ha).

On average, mass propagations last four years without any mitigation. The population crashes mainly as a result of food shortages. Antagonism through various parasites and pathogens also significantly contribute to such population declines. This occurs only in the third or fourth year of the outbreak and too late to prevent high rates of oak mortality.

The warm loving oak processionary (Thaumetopoea processionea) is originally an insect of open lands. It initially existed on single oak trees in parks, tree-lines streets, parking lots, forest edges and thickets. Since the end of the 1990’s the moth also infests forest stands over a wide area.

The female will lay between 100 to 200 eggs in a small rectangular plaque (systematic rows of eggs in a single layer) on thin twigs in the crowns of oaks. The eggs are covered with a grey, putty-like substance. The caterpillars emerge in the spring during bud break and are oligophagous. They feed only on oak species and have a preference for English and sessile oaks.

A characteristic (and namesake) of the caterpillars is the way in which they move, a "procession". At first they form thin lines, then later wide bands as they head from the spun nests to the crowns of the oaks to feed. After the third larval stage 0,1mm long urticating hairs, which are produced on warts, can lead to strong allergic reactions in humans. The caterpillars start to pupate in the middle of June in tightly spun webs on oak stems or on strong branches in the canopy.

The oak processionary is most prevalent in central and southern Europe, from Spain till the Balkans and from southern Scandinavia to Turkey.

The two spotted oak borer (Agrilus biguttatus) lives foremost in warm oak forests. It belongs to the family of the jewel beetles.

The larvae feed under the bark in the bast from oak stems and larger branches. At the same time they produce a zigzag formed gallery that can encompass the entire stem. They overwinter up to two times in the bast under the bark before they pupate in the outer bark. The beetles emerge in May. They feed on oak leaves and lay their eggs on the bark of the main stems of oaks.

The two spotted oak borer is distributed from Spain to Russia and from North Africa to Scandinavia.