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Movements of the Vectors of Virus Diseases of Cacao in Ghana. I.—Canopy Movement in and between Trees

Published online by Cambridge University Press:  10 July 2009

P. B. Cornwell
P. B. Cornwell
Affiliation:
West African Cocoa Research Institute, Tafo, Ghana.
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Extract

An examination was made of the movements of Pscudococcus njalensis Laing, the dominant vector of swollen-shoot disease, on cacao in Ghana. The mobile population is composed almost entirely of first-instar nymphs (92%). Movement is initiated at about 23·5°C. and activity becomes more pronounced at higher temperatures. Movement is maximal during mid-afternoon when many hundreds of insects become mobile on heavily infested trees. The density of mobile mealybugs increases from the base of the trunk and reaches a maximum at a few feet below the top of the canopy. Under experimental conditions, nymphs walked at least 28 ft. in search of favourable feeding sites and their dispersion increased proportionately with the number of canopy bridges. On cacao, adults are occasionally carried by the ant, Crematogaster striatula, Emery.

Using insects labelled with radioactive phosphorus, the assumption was confirmed that P. njalensis is capable of walking from tree to tree via the canopies of farmers' cacao. In a plantation of 8-year-old Amelonado cacao, 40 per cent, of the branches were in contact at 4-ft. spacing and about 20 per cent, at spacings between 5 and 7½ ft. No branches were in contact where the trees were spaced more than 12 ft. apart. At the closest spacing, the ratio between the number of mobile mealybugs that reached adjacent contact trees and those that did not was about 9:100, this ratio being reduced to 1 or 2:100 amongst trees growing 5 to 7½ ft. apart.

The significance of the movement of mealybugs in the canopy in relation to virus spread is emphasised. Methods of preventing vector dispersal by pruning, wide spacing and interplanting with a secondary tree crop are discussed. The importance of a closed canopy in preventing attack on the trees by Miridae is stressed.

Type
Research Paper
Information
Bulletin of Entomological Research , Volume 49 , Issue 3 , September 1958 , pp. 613 - 630
Copyright
Copyright © Cambridge University Press 1958

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