Companion planting of insect repellent plants with khaya ivorensis and its impact on growth and hypsipyla shoot borer attack of the host species

Abstract

African mahoganies belonging to the genera Khaya and Entandrophragma suffer severely from damage caused by the shoot borer, Hypsipyla robusta (Lepidotpera: Pyralidae). Attacks result in profuse branching of the stem which significantly reduces the timber value of the tree.  The impact of shoot borer attack on Khaya ivorensis planted in mixtures with two ‘insect-repellent’ companion species [Azadirachta indica and Cedrela odorata] and one ‘neutral’ species [Triplochiton scleroxylon], was evaluated in the field. K. ivorensis was planted at densities of 11%, 25%, and 50% with each of the companion species and compared to a 100% K. ivorensis control plot.  Shoot borer infestation was not observed in the plantation until after 20 months. Damage was recorded on 53% of the host species during the initial assessment with no significant differences in percent of host plants attacked per treatment.  Howver, attack intensity was lower in the mixed species plots than in the control, though the levels of reduction were small. Generally, K. ivorensis mixtures in which Cedrela odorata was the companion species showed higher prospect to  mitigate damage than those with A. indica or T. scleroxlyon. K. ivorensis in C. odorata mixtures recorded lower incidence of attack, and consequently lower incidence of epicormic branching.