TRANSTROPHIC INTERACTIONS IN CASSAVA

Cassava plants exude phloem saps at the base of the petioles of the youngest leaves. The effect of these nutrient-rich droplets on the interaction between the predatory mite Typhlodromalus limonicus and the herbivorous mite Mononychellus tanajoa was investigated in a semi-field setting. These two organisms were chosen as a model system due to their strong association with cassava. The hypothesis that exudate production can be considered in terms of extrinsic defense was tested experimentally. Because non-producing clones could not be found exudate production was mimicked (or not) by applying honey droplets on the petioles of plants from which exuding plant parts were aborted. The experiments indicated that: (1) The presence of exudate on otherwise clean plants does not prevent extinction of the predator population, but the rate of population decrease is consistently lower than when no exudate is present. (2) When a second food source enables the predators to reproduce, higher population densities are always attained when the sugar source is also present. (3) Higher predator numbers invariably coincide with lower herbivore abundance. (4) Lower prey abundance does not lead to a reduction in egg production when honey is present. (5) Presence of honey leads to enhanced juvenile/adult survival or reduced emigration and thus to a higher number of female predators. In interpreting the results careful attention was paid to the effect of cassava mildew because spores of this fungus were shown to be an adequate food alternative for the predator under study. The presence of this mildew (Oidium manihoti) hampered straightforward interpretation of some experiments but left the main conclusions unaltered.