How Quality of Host Plant Affects a Predator‐Prey Interaction in Biological Control

Abstract

In cases of successful biological control the pest population is maintained at low densities by a natural enemy, but the mechanisms by which the predator or parasitoid manages to control the pest are often unknown. This study was undertaken to determine the response of a predator to different population growth rates of its prey within a biological control system. Population growth rate of the phytophagous mite Panonychus ulmi was varied by altering the nutritional status of the host plant (apple trees [Pyrus malus]). NPK fertilizer was added to the soil, resulting in higher nitrogen, lower phosphorus, and lower potassium levels in the foliage of the trees. There appeared to be nonlinear relationship between nitrogen and P. ulmi densities, with density increasing and then decreasing as nitrogen concentration was increased. Two predaceous mites, Typhlodromus pyri and Zetzellia mali, were implicated in the control of P. ulmi, but the effectiveness of both was reduced at high nitrogen levels. The predators therefore did not fully compensate for increases in the population growth rate of their prey, although P. ulmi densities were maintained well below the economic threshold in all cases. However, the probability of persistence of the prey populations was not significantly affected by the nutrient regimes, suggesting that the fundamental dynamics of the predator—prey interaction were not altered by the range of population growth rates in this study.