The Role of Host Plant Resistance in Insect Pest Mis-management

Pest management has, of course, its reciprocal -pest mz's-management!Most textbooks on pest management indeed include a discussion which describes the components of pest mis-management as overdosing with insecticides, the destruction of biological control, the abandonment of cultural controls and the introduction over large areas of highyielding but pest-susceptible crop varieties.Overdosing is usually regarded as the prerogative of the insecticide applier but, as we who work on other aspects of pest control polish our halos, we would do well to reflect on whether we are totally blameless of that same sin.I propose to explore this idea in relation to the area of pest management I know best, host plant resistance to insects.I would, however, invite specialists in other areas, such as biological and cultural control, to examine their specializations from the same viewpoint.My starting point is that if a control technique is already adequate on its own, there may be advantages in making it less effective if it can be used in combination with other methods.With plant resistance, for example, any yield penalty arising from the diversion by the plant of resources to the resistance mechanism is likely to be positively correlated with the degree of resistance.'Energy or other resources which the plant diverts for defence cannot be used for growth or reproduction' (Hodkinson & Hughes, 1982).Others have argued that the cost of resistance to the plant may not be significant in relation to the overall economy of the plant or that the materials are only stored temporarily and recycled.However, it would seem remarkably easy to find apparent quantifications of the cost of resistance.For the purposes of writing this editorial I have just examined data on 31 pigeon pea varieties screened at ICRISAT (International Crops Research Institute for the Semi Arid Tropics); these data just happened to be on my desk.A regression of yield under insecticide protection against estimated resistance to insect pod damage has a strong negative slope (P = <0.001),with a 31% yield loss predicted for 90% resistance to insects.It therefore seems likely that partial plant resistance has the benefit over a high level of resistance of greater compatibility with yield aspirations.A second advantage of only partial plant resistance is that the selection pressure on a pest, which could lead to an adapted biotype increasing in frequency, will be reduced compared with a high level of plant resistance, especially as the latter will often be based on a single mechanism.For example, work with which I was associated at IITA (International Institute for Tropical Agriculture) in Nigeria identified some cowpea varieties with such high levels of antibiosis to cowpea aphid that it was impossible to establish aphids on the plants.We further discovered that the resistance was based on a grafttransmissible allelochemical which has since been shown to be governed by a single gene.It is perhaps symptomatic of such resistance that we immediately found one sample of cowpea aphids from the IITA farm which was not affected by the resistance.