Pest insect control in organically-produced crops of field vegetables.
In the UK, the demand for organic vegetable and salad crops is increasing, mainly as a result of the requirements of the multiple retailers. However, approximately 85% of the organic fruit and vegetable produce sold in the UK is imported. A major constraint to growing field vegetable crops, and particularly organically-produced crops, is the reduction in crop yield and quality caused by pest insects. This paper will consider the control techniques currently available to organic growers and other techniques that may become available in the future. Growing plant varieties with complete or even partial resistance to pest insects can be an effective way of reducing crop damage. There are already varieties of carrot, with resistance to carrot fly, and lettuce, with resistance to certain pest aphid species, which are available commercially. Cultural techniques to exclude, deter or avoid pest insects are also being used by some organic growers. Although isolating new crops from sources of infestation can be a highly effective control strategy, many organic growers cannot use it, as the land converted for organic production is still limited. Various crop covers can be used to prevent pest insects from damaging field crops, but to be effective such covers have to be in place before the pests enter the crop. Several researchers have tried to develop techniques to prevent pest insects from finding their host-plants. No technique involving semiochemicals has been sufficiently successful to be used in field vegetable production in the UK. Other studies have shown that the numbers of pest insects found on crop plants are reduced considerably when the crop is allowed to become weedy, is intercropped with another plant species, or is undersown with a living mulch. Hence, work is now needed to select background plant species that will both reduce pest insect numbers and cause the least reduction in yield to the harvested crop plants. There is also a need to obtain a better understanding of "companion planting", a practice used frequently by organic growers. To date, microbial control is the only biological technique that has been used successfully in field vegetable crops in the UK. However, only the toxicant produced by one microbial agent, the bacterium Bacillus thuringiensis, has so far been registered for use. The use of bacteria, fungi and viruses to control pests of field vegetable crops certainly has possibilities. However, in many cases there are still problems to be overcome to select pathogens that are compatible with, or can still be effective in, the wide fluctuations in temperature, humidity and soil moisture that occur under field conditions. Attempts are now being made to use entomopathogenic nematodes and predatory arthropods to control one major pest insect, the cabbage root fly. Techniques developed to improve the timing of application of various crop protection procedures in systems of conventional vegetable production apply equally well to organic production, despite the choice of control options being more limited. In particular, models to forecast the timing of pest insect attacks could be used to great effect, to indicate the best times to plant, protect and harvest a specific crop to minimise pest insect damage.