8. Thesis structure
1.12 Mechanisms of bruchid resistance
Understanding the mechanisms underlying bruchid resistance is essential for developing appropriate breeding strategies. The most widely accepted classification of mechanisms of resistance is that proposed by Painter (1951). He categorised insect resistance into non-preference, antibiosis and tolerance. Non-preference resistance is where features of a plant or grain prevent the insect pest from using it for oviposition (egg- laying), feeding and shelter or a combination of the three. Panda (1979) described two types of non-preference: one observed in the presence of the host and the other observed in the resistant plant or variety even in the absence of the preferred host. Factors that condition preference or non-preference of the host have been covered in detail (Painter,
1951). Genetic variation for weevil non-preference has been reported among maize cultivars (Kang et al., 1995; Derera et al., 2001b).
Antibiosis is the mechanism by which a colonised host is resistant, because it has an adverse effect on an insect’s development, reproduction and survival (Dent, 2000).
When antibiosis is operating, the host will injure, reduce the reproduction potential, slow the rate of development or kill the insect pest or indirectly affect the insect by increasing its exposure to its natural enemies (Painter, 1951). Allelochemicals and primary metabolites (phyto-toxins) are generally associated with antibiosis. Studies have shown that hydoxycinnamic acids (phenolics) are important in grain resistance to storage pests (Classen et al., 1990; Arnasson et al., 1992; Sen et al., 1994). In maize, resistant hybrids have high levels of phenolic acids, which cause adverse effects in weevil feeding and survival (Sen et al., 1994). In screening genotypes for bruchid resistance, biochemical analysis should be considered.
Tolerance is the ability of the host to rapidly recover, repair or withstand infestation.
However, tolerance is viewed by others (Beck, 1965) to denote a mere biological relationship, while antibiosis and non-preference are chemical and physical resistance devices. Consequently, Horber (1989) concluded that host plant tolerance was inapplicable to storage pests, because damage inflicted on stored produce is irreversible. It is therefore antibiosis and non-preference mechanisms of resistance that are of relevance in this study. However, the three mechanisms of resistance, wherever applicable, will influence the population dynamics of insects either under laboratory or field conditions by their action on the life history parameters: initial colony size, fecundity of adults, developmental period and mortality of larvae or adults (Dent, 2000).
Several factors, including the physical and chemical, have been used to explain seed resistance to storage pests. It has been demonstrated that physical factors such as seed coat hardness and seed coat roughness confer resistance to bruchids (Giga, 2002). A hard seed coat may prevent larvae from successfully penetrating the seed, while a rough seed coat provides difficulties for Z. subfasciatus in particular, because it glues its eggs on the seed testa; hence rough seeds are less preferred for oviposition (Nwanze and Horber, 1976; Messina and Renwick, 1985). Lale and Kolo (1998)
suggested that the presence of biochemical factors in the seed coat, irrespective of coat texture, may cause reduced oviposition and the poor survival of bruchid eggs on some resistant cowpea varieties. Tannins in the seed coat (Deshpande, 1992) and trypsin inhibitors (Savelkoul et al., 1992) have been implicated in the resistance of seed to bean weevils. Kemal and Smith (2000) found that the proportion of larvae entering faba bean varieties, and hence capable of completing their development and emerging as adults, was greater in decorticated seeds than in whole seeds. They concluded that the resistance of faba bean varieties could be due to the properties of seed coats or biochemical antibiosis as development was very successful on seeds without a seed coat. However, contrary to these findings Edde and Amatobi (2003) showed that cowpea seeds with intact seed coats were preferred to decorticated seeds for oviposition and therefore it was concluded that the seed coat may not be a useful aspect to consider when breeding for bruchid resistance in the cowpea.
Cardona et al. (1989) further observed that even though the testa may occasionally act as a physical barrier, factors responsible for resistance were chemical in nature and they were present in the cotyledon. It is therefore clear that resistance to post-harvest insect attack is a function of interrelated component factors of antibiosis and non-preference.
Other chemical factors within the seed, such as phytohemagglutinin (PHA), have also been reported to confer resistance in dry beans (Ishimoto and Kitamura, 1989). In all these cases, the growth and development of larvae feeding is inhibited or retarded (Gatehouse et al., 1979; Baker et al., 1989), indicating the importance of antibiosis as a mode of resistance. Varietal differences in the degree of field infestations of some wild bean accessions have been demonstrated (Cardona and Kornegay, 1989; CIAT, 1996;
Schoonhoven et al., 1983). Resistance was expressed as reduced oviposition, prolonged larval development and reduced progeny weight. Semple (1987) indicated that crop varieties can display variable resistance depending on the crop’s geographical origin and agronomic cultural practices. This suggests that different production environments may result in differing levels of resistance and affect crop varieties’
susceptibility to storage pests.
There is a sufficient body of literature explaining the bean’s resistance mechanism to bruchid infestation under controlled laboratory conditions (Hartweck et al., 1991; Fory et al., 1996; Guzmán et al., 1996). The presence of arcelin, seed hardness, seed coat
thickness, tannins, lectins and trypsin inhibitors have been used to explain antibiosis or non-preference resistance mechanisms to bruchid infestation. Very little is known about or documented on the relationships of qualitative plant traits such as growth habit, pod colour and flower colour to understand field resistance mechanisms. The behaviour of the two bruchid species in the discovery and identification of their host plant for oviposition and factors that dictate host searching are not clear. It is argued that plant resistance is not dependent on a single mechanism, but that there are overlaps between the morphological and biochemical bases of resistance. In this study, the influence of qualitative traits such as flower colour, plant height, growth habit and pod colour on bruchid resistance or susceptibility in storage was investigated. Earlier studies suggested that pod colour had an influence on varietal preference for A. obtectus oviposition (Johnson, 1981; Labeyrie, 1981).
The relationship between bruchids and their host plant and, therefore, the mechanism of adaptation of these pests to their food base is complex (Taylor, 1981). Understanding the factors that influence a bean attack by bruchids in the field may assist breeders to develop appropriate intervention strategies (Teshome et al., 2001). Zabrotes subfasciatus cannot perforate pods to lay its eggs nor can it lay eggs on pods as is the case with A. obtectus or C. maculatus (Ouedraogo and Huignard, 1981). This observation might have led to the understanding that A.obtectus is both a field and storage pest and that Z. subfasciatus can only attack beans once they are stored (Deborah et al., 2003).