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Title: Resistance mechanisms in cocoa to the cocoa pod borer Conopomorpha cramerella (Snellen)
Author: Awang, Alias
ISNI:       0000 0004 2717 6962
Awarding Body: University of Reading
Current Institution: University of Reading
Date of Award: 2011
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The underlying resistance mechanisms to cocoa pod borer (CPB) in cocoa were investigated. Experiments performed under field conditions in Malaysia showed that female CPB preferred to lay eggs on pods of clones with a rough pod surface compared to clones with a smooth pod surface. This appeared to be related to a higher density of trichomes on rough pods. The production of volatile compounds by cocoa pod trichomes is thought to stimulate females CPB to lay more eggs. Cocoa pods develop a lignified sclerotic layer and the thickness of this sclerotic layer determines the hardness of the pod, but pod hardness also depends on the composition of sclereid cells in the sclerotic layer. The sclerotic layers are soft in young pods and gradually increase in thickness and hardness as the pod grows. The quality of the pod as a food source affected the life-history traits of the CPB. However, the concentration of nitrogen and polyphenols in the pod walls did not appear to be a factor limiting the growth and development of CPB larvae. Therefore, the importance of other food quality components such as lipids, proteins, amino acids and carbohydrates (including water) warrants further investigation. The variation in CPB attack between clones under natural field conditions appeared to be influenced by plant and insect variables, and modified by environmental conditions. Any changes in the plant variables affect the spatial and temporal distribution of CPB and then, subsequently influence the expression of resistance. Clones with good resistant traits also face severe infestation in the absence of favourable pod size, particularly during low pod production or the presence of non-bearing cocoa trees within rows or blocks. The basis for characterising resistant clones was proposed. A resistance index based on eight variables, low number of CPB eggs recorded on the surface of the pod, low number of entry holes observed on the pod, harder sclerotic layer, smoother pod surface, high larval mortality in the pre-sclerotic layer, low larval penetration through the sclerotic layer, low number of exit holes, and low pod damage index was derived and is proposed as a basis for screening and selecting resistant clones to CPB. The lowest average rank for these variables reflects the highest overall resistance level to CPB. Among the eight variables, the percentage larval penetration appeared to be a prominent key to the damage severity of the cocoa pod.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available