Biological plant protection in the tropical greenhouse in Witzenhausen

Even or especially under the protected conditions of a greenhouse, plants are exposed to considerable disease and pest pressure.

In general, animal pests are often a major concern for gardeners. The problems with whiteflies (Trialeurodes vaporariorum, Bemisia spp., Aleurothrixus floccosus), spider mites (Tetranychus urticae and others) and thrips (Frankliniella spp, Scirtothrips spp.), aphids (Aphidiae spp.), mealybugs and mealybugs (Pseudococcus longispinus, Planococcus citri, etc.), scale insects (Aodiniella spp.), cup scale insects (Coccus hesperidum, Saissetia spp.) as well as mice, rats and birds. For years, insecticides and acaricides had to be used regularly and increasingly to control the pests. On average, 25 sprays per year were necessary in the mid-1980s. Overall, the pest pressure forced ever higher concentrations and different combinations of pesticides to be 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, the control was only promising at the beginning. Overall, chemical intervention was repeatedly necessary 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 was significantly reduced and at the same time a switch was made to mildly effective agents. The vicious circle between the use of beneficial insects and their decimation through the use of chemical agents against non-biologically controllable pests was difficult to break. In the spring of 1986, a strong increase in pest pressure made it necessary to carry out a large-scale control campaign 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 product range and a correspondingly wider spectrum of beneficial insect species. At this time, the Tropical Greenhouse also had sufficient experience and the personnel capacity to begin the consistent conversion to biological plant protection.
In the Tropical Lowlands/Palm House Department, the conversion began in 1990. The most important measures resulting from the conversion concept were the establishment of regular, intensive population controls and the extensive avoidance of chemical plant protection preparations. Once the biological control methods had proved successful in the Palm House and responsibility for stock control, beneficial insect orders and releases had been transferred to a gardener, the Tropical Highlands/Coffee House (1992), Tropical Lowlands/Cocoa House (1994) and Subtropical/Orange and Field Crops Departments (1995) were also converted.

The costs for the use of beneficial insects and the time required for stock checks and the application of beneficial insects are higher than for the use of chemical agents. The fact that the use of beneficial insects in greenhouses 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 the plants from chemical agents in the form of spray stains, burns, necrosis or reduced growth
• No damage to soil, water or air due to pesticide residues
• Usually uncomplicated application of beneficial insects
• and very importantly
• 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 available beneficial insects remain problematic. One example is Aleurothrixus floccosus, a whitefly on citrus, coffee and fruit plants, which has been causing extreme problems from time to time for several years. Currently, the mealybug Icerya purchais is the most important "problem pest" on citrus plants in the orangery. At the moment, it can only be controlled by mechanical measures such as pruning the infested plant parts or by laboriously "washing off" infested plant parts. Thrips, soft-skinned and spider mites, whiteflies and aphids cause recurring problems in "waves".

The exchange of experience within the Verband Botanischer Gärten e.V. (Association of Botanical Gardens) is helpful for all questions relating to biological plant protection. The colleagues at the Tropical Greenhouse were among the co-founders of the working group in Dresden in 1998 and the facility was one of the very first gardens to carry out biological plant protection. The members of the working group meet once a year for a two-day exchange of experiences.

The "Tropical Greenhouse" teaching and research facility is committed to the mission statement of the Department of Organic Agricultural Sciences and believes that the standards of organic farming can also be implemented in a special plant collection. For this reason, future activities should not be limited to biological plant protection and the release of beneficial organisms, but should focus on the plant and its health. The conversion of fertilization measures is therefore an issue 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 which the gardeners are currently continuing to address.

Despite or perhaps because of the successful conversion of the greenhouse for tropical crops to organic plant protection, the topic will continue to be addressed in the future for operational, economic and scientific reasons, although the greenhouse operation is forced to take rationalization steps due to a lack of personnel and only has limited financial resources.