Plant‐mediated effects of host plant density on a specialist herbivore of Solanum carolinense

Host plant density can affect insect herbivore oviposition behaviour, which can, in turn, affect both plant and herbivore populations. Because clear generalisations about density effects on oviposition remain elusive, a better understanding of underlying mechanisms is needed. One such mechanism is plant‐mediated effects (i.e. changes in plant traits with density), which are often suggested but rarely isolated experimentally. In an experiment directly manipulating host plant density ( Solanum carolinense ) in the field, oviposition by a specialist herbivore ( Leptinotarsa juncta ) declined as plant density increased. A greenhouse cage experiment isolated the effects of plant‐mediated traits by removing neighbour plants before introducing L. juncta females. Oviposition declined as host plant density increased, supporting the importance of plant‐mediated effects in this system. To determine whether food quality contributed to plant‐mediated effects, larval growth rates were measured on leaves from both field‐ and greenhouse‐grown plants. In both the field and the greenhouse experiments, larval growth rate was not significantly affected by plant density. It is concluded that plant‐mediated effects contribute to, but do not completely explain, the strong influence of density on oviposition in the field experiment. These results suggest that considering plant‐mediated effects may help to explain variability in herbivore responses to plant density.