In greenhouses and polythene tunnels, high temperatures often all year round and sub-tropical species of plants bring with them exotic pests and diseases. Further, the increase of both pests and diseases is much quicker than comparable pests or diseases growing outdoors. Also, these greenhouse inhabiting organisms have, over the last half-century, developed resistance to almost all available pesticides.
Biological control of exotic pests requires exotic predators and parasites. And so the health of the major greenhouse crops is in large measure due to two organisms: a South American mite which eats all stages of the glasshouse red spider mite, and a tiny South-East Asian wasp that parasitizes the glasshouse whitefly.
The conditions in a greenhouse have two advantages for biological control. Firstly, the environment is relatively isolated so that the controlling organisms are not likely to disappear. Secondly, the glasshouse environment is relatively stable and allows biological control to be more measured (than in the outdoor situation) with interactions between pests and their predator or parasite being more predictable.
Almost all commercial production of glasshouse crops in the UK now uses biological control. The two commonest biological control organisms are described below in some detail. Much more information is available from commercial companies or from the Internet. A more extensive listing of commercially available biological control species is given in Table 16.1.
Phytoseiulus persimilis (see Figure 16.7)
This is a 1 mm globular, deep orange, predatory tropical mite used in greenhouse production to control glasshouse red spider mite (see p224). It is raised on spider mite-infected beans and then evenly distributed throughout the crop, such as cucumbers, at the rate of about one predator per plant. Some growers who have suffered repeatedly from the pest first introduce the red spider mite throughout the crop at the rate of about five mites per plant a week before predator application, thus maintaining even levels of pest-predator interaction. The predator's short egg-adult development period (7 days), laying potential (50 eggs per life cycle) and appetite (five pest adults eaten per day), explain its extremely efficient action.
Encarsia formosa (see Figure 16.8)
This is a small (2 mm) wasp which lays an egg into the glasshouse whitefly scale (see p210), causing it to turn black and eventually to release another wasp. This parasite is raised commercially on whitefly-infested tobacco plants. It is introduced to the crop, such as tomato, at a rate of about 100 blackened scales per 100 plants. The parasite's introduction to the crop is most successful when the whitefly levels are low (recommended less than one whitefly per 10 plants). Its mobility (about 5 m) and successful parasitism are most effective at temperatures greater than 22°C when its egg-laying ability exceeds that of the whitefly.
The wasp lays most of its 60 or more eggs within a few days of emergence from the black scale. Thus, a series of weekly applications from late February onwards ensures that viable eggs are laid whenever the susceptible whitefly scale stage is present. The appearance of newly infected black scales on leaves is often taken as an indication that parasite introductions can be stopped.
An understanding of each pest's and each biological control organism's life cycle is vital to ensure success in control. A combination of biological methods may be used on some crops, such as chrysanthemums, tomatoes, peppers, aubergines and cucumbers in order to simultaneously control a range of organisms occurring on the crop at the same time (see Table 16.1, and integrated control, p292). Several specialist firms now have contracts to apply biological control organisms to greenhouse units. There are several practical points that confront growers in both the outdoor and the glasshouse situation.
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