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 term "early defoliator" combines a multitude of species (primarily butterflies) whose caterpillars feed on oak leaves in the spring. The most common representatives are the European oak leafroller, the mottled umber moth and the winter moth. In addition, there are also a myriad of other species that feed on oak leaves that have not been completely observed. These species display a dissimilar biology. A wide range of leafroller species as well as the groups of the quakers and leaf-miner moths belong to the early defoliators.
European oak leafroller
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.
|Moth||wingspan approx. 20 mm||forewing green, hindwing bright grey with a white edge, head is yellowish||emerges at the end of May (Central Europe) |
Main time of flight is in June
|Pupa||up till 10 mm long||early on green, later brown to dark brown|
|Caterpillar||up to 20 mm long||in the beginning ocher-grey, from the 3rd larval stage (after approx. 7 days) green with black dots and a black-brown head (Fig. 1)||from caterpillar development to pupation takes 3-4 weeks|
|Eggs||0,7 to 0,8 mm||flat like a watch crystal|
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.
|Mottled umber moth||Moth||Wingspan ♂ up to 40 mm, ♀ wingless and up to 14 mm long||Forewings ♂ grey- yellow to bright grey undertones, ♀ yellow to whitish with conspicuously dark specks||Flight season from the end of September till December, just after the first frost|
|Caterpillar||Up to 35 mm long||Compare Fig 2.|
Lepiforum – Mottled umber moth
|Development from the end of April till the end of May/ beginning of June|
|Winter moth||Moth||Wingspan ♂ up to 30 mm, ♀ up to 8 mm long with wing stubs||♂ Wings yellow brown with undulating crossbands., ♀ brown-grey colored||Flight season from the end of September till December, just after the first frost|
|Caterpillar||Up to 35 mm long||Lepiforum – Winter moth||Development from the end of April till the end of May / beginning of June|
|Northern winter moth||Moth||Wingspan ♂ up to 36 mm, ♀ up to 8 mm long with wing stubs||♂ Coloration slightly brighter than the winter moth, ♀ brown-grey coloration||Flight season from the end of September till December, just after the first frost|
|Caterpillar||Up to 35 mm long||Lepiforum – Northern winter moth||Development from the end of April till the end of May / beginning of June|
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 and oak processionary belong to the late defoliators. They begin feeding after bud break and continue till the end of June so that the Lammas growth of the oaks is also affected.
The gipsy moth (Lymantria dispar) also displays a strong gender dimorphism.
|Moth||Wingspan ♀ 50-80 mm, ♂ 35-50 mm||Basic color ♀ brownish-white, ♂ darker, both with dark, toothed cross stripes||Swarms from the beginning of July till September (weather dependent)|
|Caterpillar||♀ till 75 mm long, ♂ till 50 mm long||Lepiforum – Gypsy Moth||Emerges as soon as the average temperature is over +10 °C, in Bavaria usually in April|
|Eggs||Spherical, yellow-brown||Develop immediately after oviposition and the caterpillars overwinter in the eggshell|
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.
|Moth||Wingspan 24-34 mm||Wings grey to grey-brown||Swarms from July to September|
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.
Two spotted oak borer
The two spotted oak borer (Agrilus biguttatus) lives foremost in warm oak forests. It belongs to the family of the jewel beetles.
|Beetle||8-13 mm long||Metallic gold-green, green or bluish with a white hairy spot on each wing cover||May till August|
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.
Oak powdery mildew
The oak powdery mildew (Erysiphe alphitoides) belongs to the family of the true powdery mildew fungus. It lives as a parasite on oaks. The majority of the mycelium grows on the surface of the leaf. Only specially produced "sucking" hypha (houstoria) penetrate into the leaf in order to take up nutrients. The fungus infests primarily young leaves that are less than three weeks old. Temperatures between 20 and
25 °C and high humidity are beneficial for the oak powdery mildew. The fungus first appeared in Europe in 1907.
European oak bark beetle
Besides the native oak trees, the European oak bark beetle (Scolytus intricatus) also infests red oak, chestnut, beech, willow and poplar.
|Beetle||3-3,5 mm long||Cylindrical, dark||May to June, occasionally a second generation also in September|
The females create a horizontal orientated gallery that cuts through the sapwood. From here, very long and dense larval galleries branch off.
The European oak leafroller feeds immediately after bud break. It begins at the top of the crowns and wanders downward. Not only the number of caterpillars but also the speed at which budding occurs has a high influence on the intensity of the damage. Buds that break early will have already produced enough leaf material when the caterpillars emerge and only slight damage will occur. Oaks that break their buds later will have less leafy material available and complete defoliation will occur more frequently. The winter moths also feed on freshly broken buds in May, however, from the bottom of the crown to the top.
In healthy stands, a single complete defoliation through early defoliators only usually results in a decrease in growth. The leaves will be regenerated through replacement sprouts and Lammas growth. Unfortunately, damaged oaks can only replace leaves slowly and incompletely. The oaks will die only when strong defoliation repeatedly takes place or in combination with other damaging factors.
Defoliation from the gipsy moth and the oak processionary produce even more damage. They can, depending on the climate, persist till the end of June and affect the Lammas growth of the oaks. Then a decrease in growth will occur with a defoliation of 25 percent. The decrease in timber production is not so problematic, but much rather the decrease in the vitality of the oak stand. Through a complete defoliation from the gipsy moth only a few single oaks will die, but a second defoliation from the moth will result in the death of 25 percent of the trees.
It becomes especially dangerous for oak stands when mass propagations from early and late defoliators occur simultaneously. An enduring defoliation of the oaks is the result (Fig. 7). Consequently, the mortality rates also increase (Fig. 8). If more than seventy percent of the crown has been consistently defoliated over the entire growing season then entire stands can die off (losses of up to ninety percent. within three years).
The situation is similar when the powdery oak mildew is involved. If an oak is forced to re-sprout (e.g. from early defoliators) and favorable conditions for the oak powdery mildew exist, then the leaf fungus can massively infest an oak forest. The sprouts from trees infested from oak mildew will die off. Oaks in Lower Franconia that have been damaged in this way could not re-sprout and are easily recognizable.
Through the decrease in oak vitality there is an improvement in the habitat conditions for the two spotted oak borer (i.e. an increase in crown transparency and a weakening of host trees). The secondary pest infests most notably weakened oaks. The larvae devour horizontal galleries in the inner bark that interrupts the flow of sap. The affected part of the crown or the entire tree dies off. Vital trees can ward off the infestation through sap that can be seen "weeping" from the bark at sites of oviposition.
Forest health measures
Fundamental for forest protection is the ability to monitor for the pest and to derive a prognosis for damage.
European oak leafroller: The prognosis is only possible through the observation of defoliation and caterpillar emergence. To check for the emergence of caterpillars, samples from oak twigs are taken in January / February where the number of caterpillars per 100 buds are tallied. Surveys for defoliation takes place at the beginning of June before the Lammas growth on long-term observation plots.
Winter moths: Between September and the beginning of January the scientists of the Bavarian State Institute of Forestry (LWF) carry out a representative sample (dependent on the size of the area) from oak and beech using glue bands over the circumference of the trees at breast height. The glue bands are checked weekly between October and December (depending on the length of the swarm). The thresholds for issuing a warning are species specific:
- Mottled umber moth: 0,4 females per centimeter glue band
- Winter moth: 1 female per centimeter glue band
The prognosis with pheromone traps is not possible with every type of winter moth species in the group.
Gipsy moth: Pheromone traps are placed in selected stands that are suspect to infestation during the flight season. As a result, the number of males caught will be determined. The threshold for issuing a warning is when there are more than 1,500 moths caught per trap during the entire flight season. If this threshold is surpassed, then the number clutches will be counted in order to determine population density and to predict the amount of defoliation.
Oak processionary: The search for clutches begins with samples of ten, one meter long oak twigs per tree that are taken from the top part of crowns in stands that are susceptible for infestation. The critical density for medium to complete defoliation lies by one clutch per twig.
Two spotted oak borer: In the late summer the symptoms of infestation are readily seen while walking through the woods. Stems with weeping sap should be observed continually. Further signs are small holes from woodpeckers underneath the live crown or newly developed dead branches. Infested trees should be felled in winter. Stems and branches down to a diameter of ten centimeters and bark should be removed from the forest by April. Snags that have been sanding for more than one growing season do not pose a risk and can be left in the forest.
Abatement for caterpillars in Bavaria with authorized insecticides applied by aircraft can only be carried out when a defoliation is predicted that can endanger the entire stand. A prerequisite is that a comparably effective treatment doesn’t exist or that the aerial application produces fewer impacts for human health and the environment in comparison to an application from the ground. The damage estimate will consider if a treatment is necessary in order to save the stand. Prior damage in the stand and the combination of damage from other pests also has to be taken into consideration. The experts of the LWF prepare this prediction.