The evolution of increased competitive ability (EICA) hypothesis predicts that invasive plant species may escape their specialized natural enemies in their introduced range and subsequently evolve with a decrease in investment in anti-herbivore chemical defenses relative to native conspecifics. We compared the chemical profile of 10 populations of US native and 20 populations of European invasive Solidago gigantea. To test for differences in inducibility between native and invasive populations, we measured secondary chemistry in both damaged and undamaged plants. We also performed bioassays with three specialist and two generalist insect herbivores from four different feeding guilds. There was no evidence that invasive populations had reduced concentrations of sesquiterpenes, diterpenes, or short-chain hydrocarbons (SCH), although significant variation among populations was detected. Sesquiterpene and diterpene concentrations were not influenced by damage to the host plant, whereas SCH concentrations were decreased by damage for both native and invasive plants. Performance of the three specialist insects was not affected by the continental origin of the host plant. However, larval mass of the generalist caterpillar Spodoptera exigua was 37% lower on native plants compared to invasive plants. The other generalist insect, a xylem-tapping spittlebug that occurs on both continents, performed equally well on native and invasive plants. These results offer partial support for the defense predictions of the EICA hypothesis: the better growth of Spodoptera caterpillars on European plants shows that some defenses have been lost in the introduced range, even though our measures of secondary chemistry did not detect differences between continents. Our results show significant variation in chemical defenses and herbivore performance across populations on both continents and emphasize the need for analysis across a broad spatial scale and the use of multiple herbivores.

Inbreeding in the form of self-fertilization is widespread among plants and typically results in broad, detrimental changes in plant morphology and physiology. Phenotypic changes associated with inbreeding are likely to alter interactions between inbred plants and other organisms, but few studies have investigated this potential. We found that inbreeding in the entire-leaf morning glory, Ipomoea hederacea var. integriuscula, altered this plant's ability to resist and tolerate attack by insect herbivores. The effects of inbreeding on plant defense, however, varied among insect species, and plant defense theory helped explain this variation. If the effects of inbreeding on plant phenotype are analogous to those of environmental stresses, then the plant vigor hypothesis predicts specialist herbivores will perform better on outbred plants, and the plant stress hypothesis predicts that generalist herbivores will perform better on inbred plants. We conducted a series of greenhouse experiments in which we reared two species of specialist tortoise beetles, a generalist moth species, and a generalist aphid species on inbred and outbred morning glories to test these hypotheses. We found that specialist tortoise beetles performed significantly better when reared on outbred plants and that aphid populations grew significantly faster on inbred plants as predicted by the plant vigor and plant stress hypotheses, respectively. Beet armyworm caterpillars, however, performed better on outbred plants, not inbred plants as predicted. These results suggest that plant defense theories may be useful for predicting the effects of inbreeding on plant-herbivore interactions, but differences in herbivore feeding habit (leaf chewing vs. phloem feeding) may also help explain variation in the effects of plant inbreeding on insect herbivores.

Addendum to: Hull-Sanders HM, Johnson RH, Owen H, Meyer GA. Effects of polyploidy on secondary chemistry, physiology and performance of native and invasive genotypes of Solidago gigantea (Asteraceae). Am J Bot 2009; 96:762-70; DOI: 10.3732/ajb.0800200. H erbivores are sensitive to the genetic structure of plant populations, as genetics underlies plant phenotype and host quality. Polyploidy is a widespread feature of angiosperm genomes, yet few studies have examined how polyploidy influences herbivores. Introduction to new ranges, with consequent changes in selective regimes, can lead to evolution of changes in plant defensive characteristics and also affect herbivores. Here, we examine how insect herbivores respond to polyploidy in Solidago gigantea, using plants derived from both the native range (USA) and introduced range (Europe). S. gigantea has three cytotypes in the US, with two of these present in Europe. We performed bioassays with generalist (Spodoptera exigua) and specialist (Trirhabda virgata) leaf-feeding insects. Insects were reared on detached leaves (Spodoptera) or potted host plants (Trirhabda) and mortality and mass were measured. Trirhabda larvae showed little variation in survival or pupal mass attributable to either cytotype or plant origin. Spodoptera larvae were more sensitive to both cytotype and plant origin: they grew best on European tetraploids and poorly on US diploids (high mortality) and US tetraploids (low larval mass). These results show that both cytotype and plant origin influence insect herbi-vores, but that generalist and specialist insects may respond differently. Polyploidy, or the possession of more than two sets of homologous chromosomes, is a fundamental force in angiosperm evolution. 1,2 Many plant species or species complexes consist of multiple cytotypes that may occur sympatrically; 3 this is an important source of genetic structure in plant populations that is often overlooked. 4 Possession of multiple genomes may confer advantages to polyploid plants such as increased heterozygosity, a decreased probability of inbreeding depression, or a greater gene pool available for selection; these traits contribute to the widespread success of polyploids and may make them prone to invasive-ness. 5,6 In a recent article, 7 we examined the functional consequences of poly-ploidy for different cytotypes of Solidago gigantea Ait. (Asteraceae), collected from both its native range (North America) and its introduced range (Europe). In this addendum, we show how cytotype and continent of origin influence interactions of S. gigantea with insect herbi-vores. Interactions with herbivores are expected to vary with cytotype because of phenotypic changes associated with polyploidy, but this area has received little study (reviewed in refs. 8-11). Plant origin, from either the native range or an introduced range, should also influence herbivores. Plants may escape from their specialist natural enemies in the introduced range, thereby experiencing reduced herbivore pressure from an insect comm...

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