PLANT-MEDIATED INTERACTIONS BETWEEN 2 SPATIALLY SEPARATED INSECTS

The interactions between a root-chewing insect, Phyllopertha horticola, and a leaf miner, Chromatomyia syngenesiae, mediated through the annual forb, Sonchus oleraceus, were tested by a factorial experiment. While there was no effect of either foliar or root herbivory on vegetative plant biomass, a decrease in the number and size of leaves was recorded. Only foliar herbivory caused a reduction in root biomass. Root herbivory increased the pupal weight (a measure of fecundity) of the leaf miners. This was attributed to changes in food quality, as the amount of leaf consumed by the leaf miners was not affected by root herbivory. Total foliar nitrogen was measured as an indicator of food quality and was reduced by leaf miners, but not significantly by root herbivory. Foliar herbivory reduced the growth rate of the chafer larvae. The results are discussed in terms of competition between spatially separated herbivores. The interaction is asymmetric, with the leaf miners benefiting from root herbivory, while the chafer larvae are disadvantaged by foliar feeding. The 'plus-minus' nature of this interaction provides an example of contramensalism.