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 Premise of the study: A growing number of studies document effects of inbreeding on plant interactions with insect herbivores, including deleterious effects on direct and indirect plant defenses. However, our understanding of the specifi c mechanisms mediating such effects remains limited. Here we examine how inbreeding affects constitutive and induced expression of structural defenses (spines and trichomes) in common horsenettle, Solanum carolinense .  Methods: Inbred and outbred progeny from nine maternal families of horsenettle were assigned to three treatments: control, Manduca sexta caterpillar damage, or mechanical damage. Numbers of internode spines and the density of abaxial and adaxial trichomes were assessed before and after (21 d) damage treatments. Data on internode length, fl owering time, and total fl ower production was also collected to explore the costs of defense induction.  Key results: Inbreeding adversely affected constitutive and induced physical/structural defenses: undamaged outbred plants produced more abaxial and adaxial leaf trichomes and internode spines than did inbred plants. Foliar damage by M. sexta larvae also induced more trichomes (on new leaves) and internode spines on outbred plants. Both inbred and outbred plants exposed to mechanical or caterpillar damage had shorter internodes than did control plants, but inbred damaged plants had longer internodes than did outbred damaged plants. Control outbred plants produced signifi cantly more fl owers than did control inbred plants or damaged plants of either breeding type.  Conclusions: Constitutive and induced structural defenses in horsenettle were negatively affected by inbreeding. Reduced fl ower production and internode length on damaged plants compared to controls suggests that defense induction entails signifi cant costs.

Plant volatiles serve as key foraging and oviposition cues for insect herbivores as well as their natural enemies, but little is known about how genetic variation within plant populations influences volatile-mediated interactions among plants and insects. Here, we explore how inbred and outbred plants from three maternal families of the native weed horsenettle ( Solanum carolinense ) vary in the emission of volatile organic compounds during the dark phase of the photoperiod, and the effects of this variation on the oviposition preferences of Manduca sexta moths, whose larvae are specialist herbivores of Solanaceae. Compared with inbred plants, outbred plants consistently released more total volatiles at night and more individual compounds—including some previously reported to repel moths and attract predators. Female moths overwhelmingly chose to lay eggs on inbred (versus outbred) plants, and this preference persisted when olfactory cues were presented in the absence of visual and contact cues. These results are consistent with our previous findings that inbred plants recruit more herbivores and suffer greater herbivory under field conditions. Furthermore, they suggest that constitutive volatiles released during the dark portion of the photoperiod can convey accurate information about plant defence status (and/or other aspects of host plant quality) to foraging herbivores.