Enhanced frugivory on invasive <i>Silene latifolia</i> in its native range due to increased oviposition

<b>1</b>. Why some species become invasive is a timely question in ecology. Most hypotheses assume that changed ecological interactions, especially enemy release, allow species to become invasive. This paper investigates how the release from a specialist frugivore may lead to relaxed selection on avoidance and defence strategies in an invasive plant. <br> <b>2</b>. <i>Silene latifolia</i> was introduced from Europe to North America where its main enemies are absent. Reintroduced North American plants suffer greater damage than European plants by their specialist frugivore <i>Hadena bicruris</i>, which lays eggs in flowers. However, the mechanism is unclear: increased oviposition preference, decreased defence leading to higher egg and larval survival, or a closer phenological match. <br> <b>3</b>. We exposed plants from European and North American populations to this enemy by allowing natural oviposition and also by manually adding eggs. We recorded reproductive traits potentially associated with interactions with the seed predator and monitored the different stages of fruit attack (oviposition, larval growth and survival) to study the relative roles of increased attractiveness versus decreased defence. <br> <b>4</b>. North American plants received significantly more eggs (40.2 ± 2.1 eggs per plant) than European plants (28.4 ± 3.0 eggs per plant) and natural fruit predation was twice as high on North American than on European plants. Oviposition preference rates for North American over European plants persisted also after variation in flower size and phenology were accounted for. <br> <b>5</b>. European and North American plants did not differ significantly in larval survival, larval mass at emergence or in the rate of defensive fruit abscission, indicating that greater susceptibility of plants from the introduced range to the seed predator is largely due to an increased preference for oviposition rather than to changes in palatability or defence after oviposition. <br> <b>6</b>. <i>Synthesis</i>. Our results are consistent with the idea that introduced plants may evolve after introduction, if evolutionary constraints such as those imposed by plant–frugivore interactions, are broken down in the invasive range. For <i>S. latifolia</i>, release from its native predator <i>H. bicruris</i> may have allowed evolution of increased attractiveness to pollinators without the burden of attracting the frugivore.