Summary 1.Invasive plants often have novel biotic interactions in their introduced ranges. These interactions, including less frequent herbivore attacks, may convey a competitive advantage over native plants. Invasive plants may vary in defence strategies (resistance vs. tolerance) or in response to the type of herbivore (generalists vs. specialists), but no study to date has examined this broad set of traits simultaneously. 2. Here, we examined resistance and tolerance of Chinese tallow (Triadica sebifera) populations from the introduced and native ranges to generalist (Cnidocampa flavescens) and specialist herbivores (Gadirtha inexacta) in the native range. 3. In a field common-garden test of resistance, caterpillars of each species were raised on plants from native and invasive populations. We found the specialist grew larger on and consumed more mass of invasive plant populations than native populations, while the generalist showed the same performance between them. The results were consistent with our laboratory bioassay using excised leaves. Chemical analyses showed that the invasive plants had lower tannin content and higher ratio of carbohydrate to protein than those of their native counterparts, suggesting that plants from invasive populations have altered chemistry that has a larger impact on specialist than on generalist resistance. 4. To test for differences in herbivore tolerance, plants were first defoliated by specialist or generalist herbivory and then allowed to regrow for 100 days in a field common garden. We found that plants from invasive populations had greater herbivore tolerance than native populations, especially for tolerance to generalists. They also grew more rapidly than native counterparts in the absence of herbivory. 5. Synthesis. The results of these experiments indicate that differences in selective pressures between ranges have caused dramatic reductions in resistance to specialist herbivores and those changes in plant secondary chemistry likely underlie these differences. The greater tolerance of invasive populations to herbivory appears to at least partly reflect an increase in growth rate in the introduced range. The greater tolerance to generalist herbivores suggests the intriguing possibility of selection for traits that allow plants to tolerate generalist herbivores more than specialist herbivores.

Summary 1.Plants produce a variety of secondary metabolites such as flavonoids or tannins that vary in effectiveness against different herbivores. Because invasive plants experience different herbivore interactions in their introduced versus native ranges, they may vary in defence chemical profiles. 2. We subjected tallow tree (Triadica sebifera) seedlings from native (China) and introduced (US) populations to induction by leaf clipping or one of three Chinese caterpillars (two generalists and one specialist). We measured the concentrations of five flavonoids and four tannins in leaves produced before or after damage. We measured growth of caterpillars fed these leaves from plants of each induction treatment or undamaged controls. 3. Plants from introduced populations had higher flavonoids and lower tannins than plants from native populations, especially in new leaves following induction. Caterpillar responses to changing chemical concentrations varied in direction and strength, so overall performance varied from significantly lower (generalist Grammodes geometrica), unchanged (generalist Cnidocampa flavescens), to significantly higher (specialist Gadirtha inexacta) on introduced populations. 4. Synthesis. Together, such a trade-off in secondary metabolism in invasive plants and the effect on herbivores suggest divergent selection may favour different chemical defences in the introduced range where co-evolved natural enemies, especially specialists, are absent.

Many herbivores increase their consumption rate as dietary nutrient concentration declines. This compensatory response can mitigate the fitness‐lowering impact of reduced food quality, but little is known about its costs. In this study we tested the hypothesis that one cost to a faster consumption rate can be the ingestion of a toxic dose of an allelochemical occurring in the food. We fed velvetbean caterpillars a diet with progressively diluted nutrient levels but containing the same concentration (% fresh mass, fm) of caffeine, a methylxanthine alkaloid. Larvae compensated for the reduced nutrient level, with those fed the most diluted diet increasing their biomass‐relative consumption rate (fm) 2.6‐fold over larvae fed the undiluted diet. Consequently, their rate of caffeine ingestion increased to a pharmacologically effective dose, interfering with food utilization, slowing growth, reducing subsequent feeding and lowering survival. These results suggest that greater allelochemical ingestion can be one cost of an increased consumption rate, although additional studies with other allelochemicals and species are necessary to more broadly evaluate whether insects can adaptively balance their intake of nutrients and allelochemicals through adjustments in consumption rate. In addition, these results highlight the importance of measuring consumption rates of allelochemicals and other ingested biocides, not just their dietary concentration, when assessing efficacy against herbivores.

Almost 200 species of orchid bees are the exclusive pollinators of nearly 700 specialized orchids in the neotropics. This well-known mutualism involves orchids, called perfume orchids, which produce species-specific blends of floral fragrances, and male orchid bees, which collect and use these fragrance compounds during their courtship. We report here the naturalization of an orchid bee, Euglossa viridissima, in southern Florida, USA, where perfume orchids are absent. Chemical analysis of the contents of the fragrance storage organs in the hind tibias of 59 male bees collected in Florida identified 55 fragrance compounds, including 27 known from the perfumes of nine species of E. viridissima's orchid mutualists in Mesoamerica. Aromatic leaves, such as basil, were found to be important surrogate sources of needed fragrance compounds in Florida. The bee's ability to live and become abundant in the absence of its orchid mutualists suggests that the orchid bee-perfume orchid mutualism may be facultative for the bees, even though it is obligatory for the orchids. This invasive bee visits and potentially pollinates the flowers of many plants in Florida, behavior that could promote the abundance of selected exotic and native species.

Fresh weight (fw) food consumption by caterpillars of Anticarsia gemmatalis Htibner (Lepidoptera: Noctuidae) increased almost 2-fold as the nutrients in an artificial diet were increasingly diluted with water (diets contained 65, 79, 86 or 897o fw water). Nonetheless, dry weight (dw) relative consumption rate (RCR) declined with diet dilution. The efficiency at which the consumed food is digested and assimilated (approximate digestibility, AD) increased on the 3 diluted diets, and the efficiency at which digested food is converted to biomass (ECD) increased on the 79 and 86~o fw diets. As a consequence, dw gained and relative growth rate (RGR), which is the product of RCR x AD x ECD, on the 797o fw diet were similar to those on the 65 ~o fw diet, but they declined on the more diluted diets. Relative nitrogen consumption rate also declined with dietary dilution, but this was compensated by an increase in nitrogen utilization efficiency such that the product of these, relative nitrogen accumulation rate, was similar on all four diets. Insect lipid content declined from 327o on the undiluted diet to 137O dw on the most diluted diet, and was primarily responsible for the decline in RGR. The increases in fw consumption and AD, while not preventing a decline in RGR on the two most diluted diets, mitigated the impact of dietary dilution (e.g., without these increases, RGR on the most diluted diet would have been only 43 7O of that attained). These results indicate that the consumption and utilization of food are dynamic processes, and that caterpillars ofA. gemmatalis, like many other insects, exhibit compensatory responses to changes in dietary quality.

Brazilian peppertree (Schinus terebinthifolius Raddi, Anacardiaceae) was introduced into Florida from South America in the 1800s and commercialized as an ornamental plant. Based on herbaria records and available literature, it began to escape cultivation and invade ruderal and natural habitats in the 1950s, and is now considered to be one of Florida's most widespread and damaging invasive plants. Historical records and molecular evidence indicate that two genetic lineages of Brazilian peppertree were established in Florida, one in Miami on the east coast and a second near Punta Gorda on the west coast. Since arriving, the distributions of these two types have greatly expanded, and they have extensively hybridized. Principal component analysis and reciprocal niche fitting were used to test the equivalency of climatic niches of the Florida populations with the climatic niches of the two South American chloroplast haplotype groups which established in Florida. Both approaches indicated a significant shift in niches between the parental populations in the native range and the invasive populations in Florida. The models, however, closely predicted the areas of initial establishment. We hypothesize that (1) Brazilian peppertree was able to gain an initial foothold in Florida due to niche similarity and (2) the current dissimilarity in native and exotic niches is due to hybridization followed by rapid selection of genotypes adapted to Florida's climate. In addition, to examine the potential consequence of the introduction of additional genetic diversity from the native range on invasion success, a niche model constructed with occurrences of all native genotypes was projected onto the continental United States. The result of this test indicated that under such an event, the potential invasive range would greatly expand to cover most of the southeastern USA. Our study suggests that multiple introductions from disjunct regions in the native range can facilitate invasion success.Electronic supplementary material The online version of this article (

Invasive plants often have novel biotic interactions in their introduced ranges. Their defense to herbivory may differ from their native counterparts, potentially influencing the effectiveness of biological control. If invasive plants have decreased resistance but increased tolerance to enemies, insect herbivores may rapidly build up their populations but exert weak control. Moreover, resource availability to plants may affect the efficacy of biological control agents. We tested these predictions using Chinese tallow tree (Triadica sebifera) and two specialist herbivores (Heterapoderopsis bicallosicollis and Gadirtha inexacta) that are candidates for biological control. We performed a pair of field common garden experiments in China in which Triadica seedlings from the native or introduced range were grown in low or high light conditions and subjected to different levels of herbivory by each herbivore in a factorial design. We found that Heterapoderopsis achieved greater densities on tallow trees from the introduced range or when trees were grown in high light conditions. When Gadirtha was raised in the lab on tallow tree foliage we found that it performed better (larger pupal size) when fed foliage from introduced populations. However, introduced populations generally had greater herbivore tolerance such that the impact of each agent on plant performance was lower than on native populations despite higher herbivore loads. Tallow trees grew more slowly and achieved smaller sizes in lower light levels, but the impact of biological control agents was comparable to that found for higher light levels. Plants from introduced populations grew larger than those from native populations in all conditions. Our results suggest that reduced resistance and increased tolerance to herbivory in introduced populations may impede success of biological control programs. Biological control practitioners should include plants from the introduced range in the prerelease evaluation, which will help predict insect impact on target weeds.

To date, it remains unclear how herbivore-induced changes in plant primary and secondary metabolites impact above-ground and below-ground herbivore interactions. Here, we report effects of above-ground (adult) and belowground (larval) feeding by Bikasha collaris on nitrogen and secondary chemicals in shoots and roots of Triadica sebifera to explain reciprocal aboveground and below-ground insect interactions. Plants increased root tannins with below-ground herbivory, but above-ground herbivory prevented this increase and larval survival doubled. Above-ground herbivory elevated root nitrogen, probably contributing to increased larval survival. However, plants increased foliar tannins with above-ground herbivory and belowground herbivory amplified this increase, and adult survival decreased. As either foliar or root tannins increased, foliar flavonoids decreased, suggesting a trade-off between these chemicals. Together, these results show that plant chemicals mediate contrasting effects of conspecific larval and adult insects, whereas insects may take advantage of plant responses to facilitate their offspring performance, which may influence population dynamics.