The population dynamics of the brown cocoa mirid, Sahlbergella singularis haglund in Ibadan, Nigeria

摘要

The brown cocoa mirid, Sahlbergella singularis Haglund has been adjudged the most economically important insect pest of cocoa across West African the sub-region. The pest is capable of causing yield loss of about 30% in the first cocoa growing season and up to 70% yield loss in not less than two cocoa seasons if not controlled. This paper presents the population dynamics of S. singularis in Ibadan, Nigeria and the interactions between field populations of mirids and density dependent (natural enemies) and density independent (weather parameters) factors. One hundred mature cocoa trees were randomly sampled from base to 1.5m Girth at Breast Height (GBH) fortnightly on a two hectare plantation for adult S. singularis and its natural enemies (Camponotus species, Crematogaster brevispinosa, Oncophylla longinoda, Acantholepis capensis and Palothyreus tarsatus) using standard procedures. Mirid population and natural enemy abundance were correlated to monthly weather parameters (Temperature, Relative humidity and Rainfall). The population dynamics followed a similar trend over a period of three years of study. The population of the pest rapidly built-up in August of each month and this coincided with the period of the main cropping season of cocoa, as cocoa trees were in massive pod production. Weather parameters such as temperature and relative humidity played a major role in the fluctuations of mirid population. Rainfall did not seem to have any impact on the pest dynamics. Peak relative humidity data of 88.2%, 84% and 83.5% were recorded in August 2004, 2005 and 2006, respectively and the temperature readings declined around this period, inadvertently favouring the multiplication of mirid numbers in the field. The roles of density-dependent factors, that is, natural enemies, were observed in the field and reported. Ants of various genera were observed to exert different degrees of predation in the field, however, there were no parasitoids encountered both in the field and laboratory throughout the period of this study. Knowledge of the seasonal abundance of the pest is important to generate enough background information needed for effective control of mirid in Nigeria. Key words: Population, Dynamics, Mirid, Weather, Natural-enemies