In recent years the oak processionary – a butterfly species native to Germany – has multiplied on a massive scale in some parts of the country, in particular in the northeast and southwest and in parts of North Rhine-Westphalia. Its defensive bristles can damage human health. If contact with the bristles occurs, severe pruritus lasting for several days may result. The skin develops spots or wheals similar in appearance to insect bites in a condition known as caterpillar dermatitis. The bristles can affect the mucous membranes in the respiratory tract of some people, while triggering coughing, bronchitis or asthma in others. It can also cause irritation of the eyes ranging from redness and itchiness to conjunctivitis. In rare cases the bristles can even trigger a cardiovascular reaction.
Human contact to the butterfly is practically unavoidable if the oak processionary is present in trees which are in the vicinity of settlements or in busy public places. In these cases it may be necessary to take action to protect the population. Chemical or biological control measures in oak woods, however, may only be carried out if entire forest stands are at risk of defoliation caused by the caterpillar. It is contested whether or forest stands actually die off because oak trees can revive after defoliation.
The Federal Environment Agency's recommendation is to weigh possible alternative measures to control the oak processionary against resorting to chemical or biological remedies. If infestation is mild and/or if people rarely go to affected areas, there may be other ways to help. Infested forests can be temporarily closed off or supplied with warning signs to advise anyone going for a stroll or forest workers. When a few individual trees are infested in easily accessible areas, for example near child day-care centres, swimming pools and cemeteries, caterpillars and nests should be removed by professionals. Only if these measures are ineffective should chemical and biological products be used. As a general rule, any eradication measure taken to protect tree stands is governed by plant health legislation and thus subject to the established provisions governing the use of the specific product. Any eradication measure taken to protect human health is governed by biocide laws and associated provisions.
For the sake of the environment, any chemical or biological measure should use products that are formulated specifically to combat the pest in question if at all possible. The preferred biological measure is a product formulated on the basis of Bacillus thuringiensis, as it targets butterflies only, keeping any negative effects on other insects to a minimum. Eradication measures should be carried out when there is no wind or rain so as to reduce stress on the environment, as the spread of the agent is then contained to the surrounding area. Restrictions on space and time must also be observed for the protection of other insects. Above all, an eradication measure must always respond only to the situation at hand and at a specific location. For the sake of health protection the Federal Environment Agency believes it is unwarranted to take chemical or biological measures in forest and nature reserve areas or other remote areas.
Plant protection law governs eradication measures in wooded areas. The use of any product is thus subject to the terms of its authorisation as a plant protection agent and other provisions concerning the frequency of application and the risk reduction measures to be implemented. Other considerations include minimum distances to forest edges, the possible presence of moth species which are protected under EU law, and ensuring there is an area of refuge of at least 50% of continuous land area. The application of a product may only occur in intervals of at least two years. Application in nature reserve areas is prohibited for the sake of the protection of endangered and protected species of insects.
Oak wood is used in forestry in Germany, including in protected areas. Prior to authorisation of a chemical or biological eradication measure, the competent authority (usually the plant protection service) should consider imposing a temporary ban on logging oak wood. A halt to logging can at least partly offset the damage caused to tree stands by pest feeding. This applies in particular to special areas of conservation, in which protection of the environment has priority over the commercial goals of forestry. The plant protection services of the federal states (Bundesländer) are in charge of granting the authorisation for chemical measures to eradicate the oak processionary.
Biocide law governs the conditions under which control measures may be taken in heavily infested areas near human settlements (e.g. forest edges), individual trees or heavily frequented places (e.g. parks, schools, kindergartens etc). Oak processionary control measures in these areas should be as target-oriented as possible. As with plant protection agents, biocides must be applied in observance of the distance applicable: between 25 and 100 metres to surface waters. Only equipment that enables the targeted spreading of pesticides may be used. The district administrations are the competent bodies for authorisation of biological pest control measures.