Biological plant protection in the tropical greenhouse Witzenhausen

Even or especially under the protected conditions of a greenhouse, the plants are exposed to a not inconsiderable disease and pest pressure.

In general, animal pests in particular often cause gardeners* great concern. The problems with whiteflies (Trialeurodes vaporariorum, Bemisia spp., Aleurothrixus floccosus), spider mites (Tetranychus urticae et al.) and thrips (Frankliniella spp, Scirtothrips spp.) as well as aphids (Aphidiae spp.), mealybugs and mealybugs (Pseudococcus longispinus, Planococcus citri, and others), cover scale insects (Aodiniella spp.), cup scale insects (Coccus hesperidum, Saissetia spp.), but also mice or rats and birds vary in severity. For years, insecticides and acaricides had to be used regularly and increasingly to control the pests. On average, 25 sprays per year were eventually necessary in the mid-1980s. Overall, pest pressure forced ever higher concentrations and different combinations of the pesticides used.

The increasing plant protection problems led to the first trials with biological methods in 1981. For the first time, the ichneumon wasp Encarsia formosa was used against the whitefly and the predatory mite Phytoseiulus persimilis against the spider mite. Unfortunately, control was only successful at the beginning. Overall, chemical intervention was necessary again and again due to incorrect handling, but also due to a lack of beneficial insects or temperatures that were too low or too high. However, the number of chemical interventions could be significantly reduced and at the same time a switch to mild-acting agents was made. The vicious circle between the use of beneficial insects and their decimation by the use of chemical agents against pests that could not be controlled biologically was difficult to break. In the spring of 1986, a sharp increase in pest pressure necessitated a large-scale control operation by fumigation with Dedevap or Bladafum II. Nevertheless, the possibilities of biological plant protection were pursued with great perseverance and enthusiasm.

At the end of the 1980s and the beginning of the 1990s, several companies producing beneficial insects were founded in Holland and Germany with an extensive supply program and, accordingly, a wider range of beneficial insect species. At that time, the Tropical Greenhouse had sufficient experience and the personnel capacity to begin the consistent conversion to biological plant protection.
In the Tropical Lowland/Palm House Department, the conversion began in 1990. The most important measures resulting from the conversion concept were the establishment of regular, intensive crop monitoring and a widespread avoidance of chemical plant protection preparations. After the biological control methods had proved successful in the Palm House and the responsibility for stock controls, ordering of beneficial insects and releases had been transferred to a horticulturist, the Tropical Highlands/Coffee House (1992), Tropical Lowlands/Cocoa House (1994) and the Subtropics/Orange and Field Crops Departments (1995) could also be converted.

The costs for the use of beneficial insects as well as the time required for crop inspections and the application of beneficial insects are higher than for the use of chemical agents. The fact that the use of beneficial insects in the greenhouse for tropical crops is nevertheless justified is due to the many positive effects. These include, above all:

• No exposure of employees and visitors to chemical treatment agents - the greenhouse can be entered at any time.
• No damage to plants by chemical agents in the form of spray stains, burns, necroses or reduced growth
• No damage to soil, water, air by residues of pesticides
• A mostly uncomplicated application of the beneficial insects
• and very important
• No development of resistance in pests!

Pests that do not play a role in commercial horticulture under glass and against which there are therefore no acquirable beneficial insects remain problematic. Aleurothrixus floccosus, a whitefly on citrus, coffee and fruit plants, which has been causing extreme problems from time to time for several years, should be mentioned at this point. Currently, the most important "problem pest" on citrus plants in the orangery is the woolly pouch scale aphid Icerya purchais. At the moment, it can only be controlled by mechanical measures such as pruning the infested plant parts or by laborious "washing off" of infested plant parts. Thrips, soft-skin mites, spider mites, whiteflies and aphids cause recurring problems in "waves".

The exchange of experience within the Association of Botanic Gardens (Verband Botanischer Gärten e.V.) is helpful for all questions concerning biological plant protection. The colleagues of the tropical greenhouse were among the co-founders of the working group in Dresden in 1998 and the institution was one of the first gardens to carry out biological plant protection. Once a year, the employees of the working group meet for a two-day exchange of experiences.

The teaching and research facility "Tropical Greenhouse" is committed to the mission statement of the Department of Organic Agricultural Sciences and considers the standards of organic agriculture to be feasible also in a special plant collection. Therefore, in the future, the activities should not be limited to biological plant protection and the release of beneficial insects, but the plant and its health maintenance should be the focus. The conversion of fertilization measures is therefore a topic that began with the experimental use of compost, manure and Bio-Trisol for the use of organic fertilizers, continued with the use of manna grain and with which the gardeners* currently continue to deal.

Despite, or perhaps because of, the successful conversion of the tropical crop greenhouse to organic pest management, the issue will continue to be addressed in the future for operational, economic and scientific reasons, despite the fact that the greenhouse operation is forced to rationalize due to staff shortages and has limited financial resources.