Geographical variation among three "strains" of Callosobruchus maculatus F. (Coleoptera : Bruchidae), a cosmopolitan pest of stored legume seeds, has been studied. The fecundity, fertility and adult productivity of the strains were measured on seeds of the primary host, the cowpea. The number of eggs laid by females of each strain depended on the number of cowpeas to which she had access. Females of the Nigerian strain laid more eggs on a small number of cowpeas than females of the strains from Yemen and Brazil. The Yemen strain produced fewer adults from individual seeds than the other two strains when similar numbers of hatched eggs were present. Experiments were performed to examine this difference. The results indicated that Yemen individuals were heavier, on average, and consumed a larger quantity of cowpea during development than individuals of the Nigerian or Brazilian strains. All three strains crossed to produce viable and fertile progeny. Hybrids of crosses between the Yemen strain and either of the two smaller strains were of intermediate size and were produced in numbers which were intermediate to those of the two strains involved. The response of the three strains to a cowpea variety (TVu 2027) known to exhibit resistance to bruchid attack was measured. The Brazilian strain suffered much higher mortality on TVu 2027 than the other two strains. The Yemen strain suffered slightly higher mortality than the Nigerian strain, but the mean development period of the Yemen strain was shorter. Progeny of inter-strain crosses suffered an intermediate level of mortality on TVu 2027. The proportion of Yemen and Nigerian individuals which could survive on TVu 2027 increased when successive generations were bred on the resistant variety. The consequences of geographical variation in this beetle pest, for the success of new cowpea lines which incorporate the resistant properties of TVu 2027, are discussed.

The genus Aphelenchoides consists of over 140 nominal species, some of which are pathogenic to plants. Nematologists in statutory and other diagnostic laboratories need to be able to distinguish pathogens with confidence if financial losses and unnecessary chemical control are to be avoided, but no complete key to this genus has been available to date. A revised list of species was produced, deleting descriptions considered too poor for subsequent recognition. Key diagnostic characters were identified and promising features for future diagnostic use were investigated. A polytomous key (polykey) was developed and tested on 14 populations. The findings contributed to a further reduction in the number of valid species from a diagnostician's point of view, and also led to revisions of the polykey itself as key characters were tested for their distinguishing power. The primary key characters were identified as the length of the post-vulval sac (pvs) as a percentage of the distance between the vulva and the anus, the shape of the tail terminus and tail, body length, and the ratios `a' (body length divided by greatest body width) and `c' (body length divided by tail length). Promising key characters for the future are c' (tail length divided by body width at the anus or cloaca), the distance from the vulva to the anus, vulval body width, pvs length as a measure of the vulval body width, head width and all measurements associated with the median bulb. The polykey provides the essential foundation for the development of new diagnostic tools, including new keys for diagnostic use; preliminary work with molecular techniques showed promise for inclusion in diagnostic protocols, but further study will be required to develop reliable methodology for each of the economically important species. Nominators of new species should use the recording form to provide details of morphological and morphometric characters and compare resulting polycodes with others in the polykey and associated databases before publication. Likewise, the key should continue to be tested for its validity.

Studies were carried out to assess the potential of the plant pathogenic fungus <i>Ascochyta caulina</i> as a mycoherbicide against <i>Chenopodium album</i> L., a worldwide weed in arable crops. <i>A. caulina</i> is associated with necrosis of leaves and stems of plants belonging to genera of <i>Chenopodium </i>L. and <i>Atriplex</i> L. The research of this thesis was focused on recognition of factors that may enhance pathogenicity and determine the impact of the pathogen not only on its target host (growing both alone and in competition), but also on a range of temperate crop species. These studies showed that formulation of <i>A. caulina</i> spores with nutrients, especially V-8, and surfactants resulted in faster and greater <i>in vitro</i> germination. During an investigation of <i>in vitro</i> pathogen growth, it was observed that <i>A. caulina</i> grows most prolifically in the nutrient rich liquid medium, V-8. Simple nitrogen and carbon sources also induce a positive effect on the pathogen's growth. Nutrients also promoted the pathogen's enzyme activity of a-amylase and cellulase. The <i>in vitro</i> and <i>in planta</i> enzyme activity varied significantly between <i>A. caulina</i> isolates and plant growth stage affected the a-amylase activity but not cellulase. Observations of <i>A. caulina</i> on detached leaves and whole plants of <i>C. album </i>and several crop species, showed that the surface deposits on <i>C. album </i>leaves are part of a defensive mechanism, and that the pathogen can develop pycnidia on the dead leaves of the non-host plants. <i>A. caulina</i> caused no infection symptoms on any of the crop species. However, under some circumstances it reduced the dry biomass accumulation of some of the non-target plants. Results of the three-way interaction between <i>Beta vulgaris, C. album</i> and the potential mycoherbicide <i>A. caulina</i> showed that the pathogen can reduce the competitive effects of the weed over <i>B. vulgaris </i>by suppressing the growth of the former.

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