Impact of Forest Management on Insect Abundance and Damage in a Lowland Tropical Forest in Southern Cameroon

Abstract

The rate of deforestation in West Africa was around 2% (12 000 km 2 ) per annum during the 1980s, and recent evidence from Cameroon suggests that the rate of deforestation there has increased during the 1990s. In contrast, only 360 km 2 of plantation forests were established annually during the mid-1980s in West African countries. Recent research in the Mbalmayo Forest Reserve in southern Cameroon has examined contrasting silvicultural techniques for regenerating degraded forest with West African tree species such as Terminalia ivorensis. These techniques included complete clearance, where all existing trees were felled and these and all other vegetation removed from the plots; and partial clearance methods (partial manual clearance, partial mechanical clearance and line planting), where tree-felling resulted in a reduction of 50% of the canopy cover. This paper describes a study of the impact of these techniques on arthropod abundance and the damage caused to T. ivorensis by insect pests within the Mbalmayo Forest Reserve. The amount of damage caused by leaf-mining insects, the abundance of gall-forming mites, and the number of trees affected by shoot borers were not affected by silvicultural practice. The amount of damage caused by leaf-chewing insects was greater in the line planting plots than the complete clearance plots, but the leaf area removed by insects rarely exceeded 6%. It is concluded that the degree of forest clearance before establishing plantations of T. ivorensis. does not markedly affect the amount of damage caused by arthropod pests at the spatial scale at which this study was carried out. Arthropod abundance was assessed by insecticide knockdown fogging in two study sites within the Mbalmayo Forest Reserve. A mean total of 196 arthropods m -2 was recorded, ants being the most common arthropod group (63%), followed by adult Diptera (10%) and then Hymenoptera other than ants (4%), Homoptera (4%), Thysanoptera (4%), adult Coleoptera (3%) and other groups (13%). Ants, Diptera, Araneae, Thysanoptera, Homoptera, Coleoptera and Orthoptera were significantly more abundant in the partial manual clearance plot than the complete clearance plot in one or both of the study sites. Because several of these groups are composed mainly of predatory and parasitic species, it is possible that, despite the pest damage results summarized above, the long-term likelihood of pest outbreaks occurring on T. ivorensis is lower in partial clearance than complete clearance plantations. However, our current lack of knowledge of the species composition of different arthropod groups limits the relevance of these findings.