Impact of predation by coccinellid larvae on colonies of the mealybug Pheizacoccus ntanìhotì in crop lands

A field study of predation by coccinellid larvae (Col. Coccinellidae) on populations of cassava mealybugs Phenacoccus nzanilzoti Matile-Ferrero (Hom. Pseudococcidae) was made at two localities in the Congo, with an indigenous species Exochomusflaviveiitris Mader and an exotic one, Hyperaspis raynevali Mulsant. H. rayizevali larvae were released on cassava plants on 7 August on 17f4 mealybugs/plant and on the 5 September on 89 f 25 mealybugslplant. The releases of E.flaviventris larvae occurred on 17 August with 192~7 mealybugs/plant and on 6 October with 72433 mealybugs/plant. 4weeks after the first introducton of H. raynevali, the numbers of the pest multiplied by 5 on the treated plants and by 12 on the controls. In parallel, E.flaviventris larvae maintained the P. nzaiiihoti populations below the level of 20 mealybugs/plant for 3 weeks, whereas on control plants their number increased by 3.3. It was not possible, however, to obtain reliable data for the second release, because it coincided with a decline in mealybug populations attributable to natural regulatory phenomena (local E. flaviveatris, rain, physiological state of the host-plant). The impact of E. flaviventris and H. raynevali was influenced by the density and age structure of the prey, but the predatory potential of the two coccinellids could not be compared.