Plants and predators, partners in crop protection

Of the various ways in which the agricultural research community can help farmers to ensure that the food grown for the consumer's plate is not consumed by insect pests before it leaves the field, biological control is one of the most attractive. Success will be achieved not by promoting a single methodology, but by understanding how all the factors involved interact with earth other Small holder farmers, particularly in sub Saharan Africa, have sought to reduce the impact of insect pests' advantage by developing complex farming systems. Dr Tsedeke Abate, from the Institute of Agricultural Research, Ethiopia, speaking at the CTA seminar Insect pest controller small holders: Integrating biological control and host plant resistance, pointed out that Ethiopia's staple cereal crop, tef, is remarkable free of pests, possibly because farmers plant a wide range of varieties which they rotate regularly. The agricultural research community, however, when seeking for nonchemical techniques of plant protection, has tended to look for single, and simplified, solutions: plant breeders have sought to develop plants which have an insult resistance to one specific pest; while entomologists have tended to concentrate on classical biological control which again targets one pest species. Furthermore, there has been little cross fertilization between the two scientific discipliunes. The two approaches - complexity versus simplicity - often conflict, with the result that the messenger - the extension service is unable to deliver the message because it is caught in a noman's land between opposing lines of thought. A first step towards making better use of natural enemies to control pests could be to develop procedures for closer collaboration between plant breeders, entomologists and farmers. p['here are examples of combining host plant resistance and biological control to tackle different pest problems within one crop, for example cotton, but little account has been taken of the opportunities to combine the two disciplines to control the same pest species. Taken singly, neither approach has produced satisfactory or sustainable pest control: for example, a high level of resistance to insect pests, often the goal of a plant breeding programme, may do more harm than good. Insects, particularly aphids and white flies, which have a short life cycle, have contintlallv demonstrated an ability to overcome resistance by developing highly virulent biotypes that destroy not only the producer's current crop but also years of plant breeding effort and investment. Furthermore, a high level of resistance, such as that which is sometimes achievable with transgenic plants, may reduce the pest population to a level at which a natural enemy cannot maintain itself. This may not matter in the short term, but in the longer term it leaves the field clear for new populations of pests to emerge unthreatened by parasites and predators. Effective, non-chemical control of pests depends upon understanding the interactions not only between plant and pest but also between plant, pest and natural enemy. These interactions are important because plant protection strategies which are developed from an understanding of the interaction between plant and pest alone, may, when examined in a broader context, be found to be counter-productive. Plants and predators Interactions may be governed by physical, chemical or semiochemical factors. (Semiochemicals are chemicals such as attractants, repellents and deterrents which are involved in the interaction between organisms). For example the physical environment of a hairyleaved plant, may have a greater impact on the natural enemy of the pest than on the pest itself. Tomato plants have been bred for sticky hairs on the stems, to which tiny pest insect mites become glued and then die. But red spider mites, which have become an increasing problem in southern Africa in recent years, sit on the leaves to teed whereas their natural enemies, predatory mites, must seek their prey by walking (since they cannot fly) through the perilous tomato stem jungle of sticky hairs. The advantage is clearly with the pest rather than the predator. Where a similar problem was identified with cucumber plants in the Dutch horticulture industry, breeders were able to develop seminary varieties with such outstanding success that a national postage stamp was issued to commemorate the achievement. Interactions governed by chemicals can also have both positive and negative effects on natural enemies. Plant nutrients and resistance factors can influence growth rate, mortality and size of herbivorous insects at both the individual and population levels and these in turn can influence attack by natural enemies. From the point of view of plant protection, a low resistance variety, combined with some natural enemy action, may achieve results which are as effective as those of a highly resistant variety without natural enemy action. Furthermore, lower resistance varieties will be easier to achieve and sustain. Semiochemically mediated interactions are possible between herbivores and natural enemies. Damage by herbivores tends to increase the emission of plant volatiles, and natural enemies can exploit this information in order to detect their food source. Even the uninfested leaves of a plant under attack emit these special volatiles, which are specific to herbivore damage and are not the same, for example, as those volatiles emitted when a plant suffers mechanical damage. Some varieties seem to be more effective than others at producing this 'SOS signal' to natural enemies and plant breeders could select for this cheap and sustainable form of pest control. Farmers are already practicing some IPM strategies and yet much more needs to be done because insect pests continue to take more than farmers can afford to relinquish. Farmers need information to identify and understand the role of natural enemies of insect pests. Where this has been given, at in the Farmers Field Schools in Ghana for example, results have been encouraging. An integrated approach, which combines the efforts of farmers, plant breeders and entomologists, could help to ensure that more of the food grown for the consumer actually depends up on the plateOf the various ways in which the agricultural research community can help farmers to ensure that the food grown for the consumer's plate is not consumed by insect pests before it leaves the field, biological control is one of the most attractive...