Experimental analysis of constitutive and induced defence in a plant–seed–predator system

1. The functional significance of candidate defence mechanisms, including fruit abortion, fruit morphology and seed provisioning of Silene latifolia (Miller) Kraus (Caryophyllaceae), was tested using experimental exposure to its seed predator <i>Hadena bicruris</i> Hufnagel (Noctuidae). This moth lays eggs in <i>S. latifolia</i> female flowers, and each larva consumes three to four fruits to achieve development. Specifically, we sought to explain why fruit walls are thicker in European plants compared with plants in the introduced range where the seed predator is absent, which suggests a defensive role for fruit wall. We also analysed the carbon and nitrogen content of the seeds of exposed and unexposed plants to investigate whether plants respond to moth attack by modifying seed provisioning. <br> 2. Experimental exposure to moth eggs significantly increased fruit abortion. Fruits built after attack on exposed plants did not have significantly thicker fruit walls compared with fruits on control plants. The proportion of N and C within seeds was significantly, but only slightly, modified in exposed plants. <br> 3. Fruits with thicker fruit walls were not significantly less profitable, nor did they require longer handling time when exposed to larvae. <br> 4. Populations varied significantly in fruit wall thickness. When flowers were infested experimentally, we found significant among-family variation in larval growth, suggesting genetic (or maternal environmental) variation. <br> 5. We conclude that fruit abortion is an important defence mechanism. Constitutive investment in fruit wall thickness does not provide a strong protection against secondary attack. Plants do not appear to respond to attack by modifying fruit wall thickness or seed provisioning in later fruits. Future studies may compare fruit abortion and palatability of European and American plants with their greater variation, to address whether relaxed selection away from the seed predator modifies defences to this natural enemy.