Deer feeding selectivity for invasive plants

Native generalist herbivores might limit plant invasion by consuming invading plants or enhance plant invasion by selectively avoiding them. The role of herbivores in plant invasion has been investigated in relation to plant native/introduced status, however, a knowledge gap exists about whether food selection occurs according to native/introduced status or to species. We tested preference of the native herbivore white-tailed deer (Odocoileus virginianus) for widespread and frequently occurring invasive introduced and native plants in the northeastern United States. Multiple-choice deer preference trials were conducted for the species and relative preference was determined using biomass consumption and feeding behavior. While more native than introduced plant biomass was consumed overall, deer food selection varied strongly by plant species. Results show consistent deer avoidance of several invasive introduced plants (Alliaria petiolata, Berberis thunbergii, and Microstegium vimineum) and a native plant (Dennstaedtia punctilobula). Other invasive introduced plants (Celastrus orbiculatus, Ligustrum vulgare, and Lonicera morrowii) and a native plant (Acer rubrum) were highly preferred. These results provide evidence that herbivore impacts on plant invaders depend on plant species palatability. Consequently, herbivore selectivity likely plays an important role in the invasion process. To the extent that herbivory impacts population demographics, these results suggest that native generalist herbivores promote enemy release of some plant invaders by avoiding them and contribute to biotic resistance of others by consuming them.     

Reduced ant defenses in <Emphasis Type="Italic">Macaranga</Emphasis> myrmecophytes (Euphorbiaceae) infested with a winged phasmid <Emphasis Type="Italic">Orthomeria cuprinus</Emphasis>

Macaranga is a tree genus that includes many species of myrmecophytes, which are plants that harbor ant colonies within hollow structures known as domatia. The symbiotic ants (plant–ants) protect their host plants against herbivores; this defense mechanism is called ‘ant defense’. A Bornean phasmid species Orthomeria cuprinus feeds on two myrmecophytic Macaranga species, Macaranga beccariana and Macaranga hypoleuca, which are obligately associated with Crematogaster ant species. The phasmids elude the ant defense using specialized behavior. However, the mechanisms used by the phasmid to overcome ant defenses have been insufficiently elucidated. We hypothesized that O. cuprinus only feeds on individual plants with weakened ant defenses. To test the hypothesis, we compared the ant defense intensity in phasmid-infested and non-infested M. beccariana trees. The number of plant–ants on the plant surface, the ratio of plant–ant biomass to tree biomass, and the aggressiveness of plant–ants towards experimentally introduced herbivores were significantly lower on the phasmid-infested trees than on the non-infested trees. The phasmid nymphs experimentally introduced into non-infested trees, compared with those experimentally introduced into phasmid-infested trees, were more active on the plant surface, avoiding the plant–ants. These results support the hypothesis and suggest that ant defenses on non-infested trees effectively prevent the phasmids from remaining on the plants. Thus, we suggest that O. cuprinus feeds only on the individual M. beccariana trees having decreased ant defenses, although the factors that reduce the intensity of the ant defenses remain unclear.     

How do two specialist butterflies determine growth and biomass of a shared host plant?

Although insect herbivory can modify subsequent quantity and quality of their host plants, change in plant quantity following herbivory has received less attention than plant quality. In particular, little is known about how previous herbivore damage determines plant growth and biomass in an insect species-specific manner. We explored whether herbivore species-specific food demand influences plant growth and biomass. To do this, we conducted a series of experiments and field survey using two specialist butterflies, Sericinus montela and Atrophaneura alcinous, and their host plant, Aristolochia debilis. It is known that A. alcinous larva requires four times more food resources to fulfill its development than S. montela larva. Despite that A. alcinous larvae imposed greater damage on plants than S. montela larvae, plant growth did not differ due to herbivory by these species both in single and multiple herbivory events. On the other hand, total aboveground biomass of the plants was reduced more by A. alcinous than S. montela feeding regardless of the number of herbivory events. Feeding on plants with a history of previous herbivory neither decreased nor increased larval growth. Our results suggest that food demand of the two butterfly species determined subsequent plant biomass, although the plant response may depend on tolerance of the host plant (i.e., ability to compensate for herbivore damage). Such difference in the effects of different herbivore species on host plant biomass is more likely to occur than previously thought, because food demand differs in most herbivore species sharing a host plant.     

An invasive plant provides refuge to native plant species in an intensely grazed ecosystem

Invasion by exotic plant species and herbivory can individually alter native plant species diversity, but their interactive effects in structuring native plant communities remain little studied. Many exotic plant species escape from their co-evolved specialized herbivores in their native range (in accordance with the enemy release hypothesis). When these invasive plants are relatively unpalatable, they may act as nurse plants by reducing herbivore damage on co-occurring native plants, thereby structuring native plant communities. However, the potential for unpalatable invasive plants to structure native plant communities has been little investigated. Here, we tested whether presence of an unpalatable exotic invader Opuntia ficus-indica was associated with the structure of native plant communities in an ecosystem with a long history of grazing by ungulate herbivores. Along 17 transects (each 1000 m long), we conducted a native vegetation survey in paired invaded and uninvaded plots. Plots that harboured O. ficus-indica had higher native plant species richness and Shannon–Wiener diversity H′ than uninvaded plots. However, mean species evenness J was similar between invaded and uninvaded plots. There was no significant correlation between native plant diversity and percentage plot cover by O. ficus-indica. Presence of O. ficus-indica was associated with a compositional change in native community assemblages between paired invaded and uninvaded plots. Although these results are only correlative, they suggest that unpalatable exotic plants may play an important ecological role as refugia for maintenance of native plant diversity in intensely grazed ecosystems.     

An Experimental Test of Trade-Offs Associated with the Adaptation to Alternate Host Plants in the Introduced Herbivorous Beetle, <Emphasis Type="Italic">Ophraella communa</Emphasis>

The adaptation to alternate host plants of introduced herbivorous insects can be vital to agriculture due to the emergence of crop pests. Historically, it is assumed that there are trade-offs associated with the adaptation to new host plants; a generalist genotype that adapts to an alternate host is expected to have a relatively lower fitness on the ancestral host than a specialist genotype (physiological cost) or a relatively lower host-searching ability for the ancestral host plant (behavioral cost). In this study, we tested the costs of adaptation to a new host plant in the introduced herbivorous insect, Ophraella communa LeSage (Coleoptera: Chrysomelidae). In its native range (United States), O. communa feeds mostly on Ambrosia artemisiifolia L. (Asterales: Asteraceae) and cannot utilize the related species, Ambrosia trifida L. (Asterales: Asteraceae), as a host plant. On the other hand, the introduced O. communa population in Japan utilizes A. trifida extensively, and is adapting to it, both physiologically and behaviorally. We compared larval performance on the ancestral and alternate plants and adult host-searching ability between the native and introduced beetle populations. The introduced O. communa showed higher larval survival and adult feeding preference for the alternate host plant A. trifida than did the native O. communa, indicating that the introduced O. communa has rapidly adapted to the alternate host plant. However, there are no differences in either larval performance on the ancestral host A. artemisiifolia or host-searching accuracy between the native and introduced O. communa.     

Differences in physiological traits and resistances of <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis> after herbivory by generalists and specialists

Many researchers have surveyed damages caused by natural enemies of invasive plants in both native and introduced ranges to test the enemy release hypothesis. In this study, we report our findings on the physiological and morphological impacts of a co-evolved specialist insect (Agasicles hygrophila) and two generalist insects (Atractomorpha sinensis and Hymenia recurvalis) in introduced ranges on an invasive plant, Alternanthera philoxeroides, in both field trials and controlled environments. The resistance of A. philoxeroides against the generalists and the specialist was also studied. We obtained consistent results in both the field trials and the controlled treatments: both the generalists and the specialist decreased leaf biomass, photosynthesis, leaf nitrogen content, and total leaf non-structural carbohydrate content in A. philoxeroides. However, the specialist decreased leaf mass, photosynthesis, and leaf nitrogen content more acutely than the generalists. Moreover, A. philoxeroides increased both leaf lignin and cellulose concentrations upon the generalists’ attack but only increased cellulose concentration in response to the specialist. Our results showed that even under the same population density, the co-evolved specialists from original ranges caused more severe morphological and physiological damage to A. philoxeroides than the generalists in introduced ranges. This revealed that invasive plants released some herbivory stress before their co-evolved specialists were introduced, which may contribute to the superior performance of invasive plants in introduced regions.     

Evolutionary and environmental effects on the geographical adaptation of herbivory resistance in native and introduced <Emphasis Type="Italic">Solidago altissima</Emphasis> populations

To understand rapid evolution in plant resistance to herbivory, it is critical to determine how the genetic correlation among resistances varies genetically and/or environmentally. We conducted a reciprocal transplant experiment of tall goldenrod, Solidago altissima with multiple replicates within the native range (USA) and the introduced range (Japan) to explore the differences in phenotypic traits of resistance to multiple herbivorous insects and their relationships between and within the countries. The Japanese plants were more resistant to the lace bug, Corythucha marmorata, which had recently invaded Japan, but were more susceptible to other herbivorous insects compared to the USA plants. An antagonistic relationship was found between plant resistances to lace bugs and other herbivorous insects in both USA and Japanese plants. In addition, this relationship was more obvious in gardens with a high level of foliage damage than in gardens with a low level of foliage damage by other herbivorous insects. An antagonistic relationship between resistances to aphids and lace bugs was also observed in USA gardens, but not in Japanese garden. These results suggest that the strength of constraints on the evolution of plant resistance due to genetic trade-offs may differ among biotic environments, including community structure of herbivorous insects. Therefore, differences in herbivorous insect communities between the native and introduced ranges can result in the rapid evolution of greater resistance in plants in the introduced range than in the native range.     

Effects of herbivory and mistletoe infection by <Emphasis Type="Italic">Psittacanthus calyculatus</Emphasis> on nutritional quality and chemical defense of <Emphasis Type="Italic">Quercus deserticola</Emphasis> along Mexican forest fragments

Mistletoes are parasitic plants that show effects that can parallel or contrast with those caused by herbivores to their host plants, particularly on aspects such as host biomass, resource allocation patterns, and interspecific interactions at the community level. In this study, we evaluated the potential synergistic effects of herbivory and infection by the mistletoe Psittacanthus calyculatus on nutritional quality and chemical defense of the white oak Quercus deserticola along forest fragments in Mexico. For this, we sampled leaves of parasitized oaks, unparasitized oaks and mistletoes at four forest fragments in the Cuitzeo basin, Michoacán state, Mexico, and measured herbivory levels and foliar water content, total nonstructural carbohydrates, phenols, flavonoids, and hydrolyzable tannins in each sample. Higher levels of infection by P. calyculatus were found in the smaller forest fragments, while foliar damage by herbivores was higher in larger forest fragments. At all sites, levels of herbivory were lower in the mistletoe than in both parasitized and unparasitized oaks. However, there was a positive relationship between herbivory levels in parasitized oaks and their mistletoes. Also, foliar water content and total phenol concentration were positively correlated between the oaks and the mistletoes. The results suggest that herbivory levels in parasitized hosts and mistletoes depend on the close physiological interaction between the nutritional quality and the chemical defense of the two plants involved. This is one of the few studies analyzing the chemical ecology of the interaction between plant hosts and plant parasites.     

Can enemy release explain the invasion success of the diploid <Emphasis Type="Italic">Leucanthemum vulgare</Emphasis> in North America?

Enemy release is a commonly accepted mechanism to explain plant invasions. Both the diploid Leucanthemum vulgare and the morphologically very similar tetraploid Leucanthemum ircutianum have been introduced into North America. To verify which species is more prevalent in North America we sampled 98 Leucanthemum populations and determined their ploidy level. Although polyploidy has repeatedly been proposed to be associated with increased invasiveness in plants, only two of the populations surveyed in North America were the tetraploid L. ircutianum. We tested the enemy release hypothesis by first comparing 20 populations of L. vulgare and 27 populations of L. ircutianum in their native range in Europe, and then comparing the European L. vulgare populations with 31 L. vulgare populations sampled in North America. Characteristics of the site and associated vegetation, plant performance and invertebrate herbivory were recorded. In Europe, plant height and density of the two species were similar but L. vulgare produced more flower heads than L. ircutianum. Leucanthemum vulgare in North America was 17 % taller, produced twice as many flower heads and grew much denser compared to L. vulgare in Europe. Attack rates by root- and leaf-feeding herbivores on L. vulgare in Europe (34 and 75 %) was comparable to that on L. ircutianum (26 and 71 %) but higher than that on L. vulgare in North America (10 and 3 %). However, herbivore load and leaf damage were low in Europe. Cover and height of the co-occurring vegetation was higher in L. vulgare populations in the native than in the introduced range, suggesting that a shift in plant competition may more easily explain the invasion success of L. vulgare than escape from herbivory.     

Variation in the composition and activity of ants on defense of host plant <Emphasis Type="Italic">Turnera subulata</Emphasis> (Turneraceae): strong response to simulated herbivore attacks and to herbivore’s baits

Plants with extrafloral nectaries attract a variety of ant species, in associations commonly considered mutualistic. However, the results of such interactions can be context dependent. Turnera subulata is a shrub widely distributed among disturbed areas which has extrafloral nectaries at the base of leaves. Here, we evaluated whether the ants associated with T. subulata (i) vary in space and/or time; (ii) respond to simulated herbivory, and (iii) reduce herbivory rates. For this, we quantified the abundance and species richness of ants associated with T. subulata throughout the day in six different sites and the defensive capability of these ants under simulated herbivory in the leaves and stems of T. subulata plants (N = 60). We also checked the proportion of the lost leaf area and quantified leaf damage by chewing herbivores in the host plant. We found that a total of 21 ant species associated with the host plant. Species composition showed significant variation across the sampled sites and throughout the day. Visitation rates and predation by ants were higher in plant stems than in leaves. In general, herbivory rates were not correlated with ant association or activity, with the exception of the proportion of leaf area consumed; there was a significant lower herbivory rate on plants in which ants defended the leaves. Our results suggest that the benefits of association may depend on the ecological context. This context dependence may mask the correlation between the defense of ants and herbivory rates.     

Oviposition Preferences of <Emphasis Type="Italic">Plutella xylostella</Emphasis> are Influenced by the Type of Plant Induction and Glucosinolate Hydrolysis Profiles

Effects of inducing plants by exposing them to insect herbivory, mechanical damage or damaged neighboring plants were evaluated on the oviposition preferences of Plutella xylostella. The role of plant genotypes differing in their glucosinolate hydrolysis profiles was also evaluated using a wild ecotype (Col-0) and a genetically modified line (tgg1tgg2) of Arabidopsis thaliana. While the Col-0 line has normal production of glucosinolate hydrolysis products, the double myrosinase knockout (tgg1tgg2) is defective in the production of these volatiles. Dual choice oviposition assays were performed using naïve P. xylostella females, and the two A. thaliana lines, which were exposed to the three types of induction treatments. Female oviposition preferences were significantly influenced by both the type of plant induction and the plant genotypes differing in their volatile profiles. Plutella xylostella females significantly preferred to oviposit on herbivore-damaged plants (versus undamaged controls) when Col-0 plants were used, but chose control plants over the double myrosinase knockout tgg1tgg2. However, plant genotype did not influence oviposition choices between plant-plant primed or mechanically damaged plants and paired undamaged controls. Given the prevalent use of genetically modified plants and the potential differences in their responses to different types of induction, these factors may be important to consider in the management of specialist pests such as the diamondback moth P. xylostella.     

Soybean defense induction to <Emphasis Type="Italic">Spodoptera cosmioides</Emphasis> herbivory is dependent on plant genotype and leaf position

Plants have evolved a diverse array of defensive mechanisms against biotic and abiotic stresses, which can be either constitutive or inducible. Variation in plant-intrinsic factors such as the genotype and the leaf position coupled with insect herbivory can affect the expression of resistance to insects. We investigated if soybean defense induction triggered by Spodoptera cosmioides herbivory varies in function of the genotype and leaf position. This hypothesis was tested in two bioassays using leaf discs or entire leaflets collected from the upper and lower trifoliates of S. cosmioides-injured and uninjured V3-V4 soybean plants. We used one genotype that was constitutively resistant and one that was constitutively susceptible to S. cosmioides based on previous screening. Third-instar larvae were fed one of the treatments and assayed for leaf consumption, larval growth, and efficiency of conversion of ingested food. Genotype and leaf position significantly interacted with herbivory and affected soybean-induced resistance to S. cosmioides. Negative responses on S. cosmioides larvae consumption and growth rates were only observed when leaf material was originated from the upper soybean trifoliate. The susceptible soybean genotype did not exhibit induced resistance characteristics. Food offered as leaf disc was better at demonstrating induced resistance in previously injured soybean, whereas offering entire leaflet the induced effects were less pronounced. Here we provide new findings on soybean resistance by demonstrating that resistance induction to S. cosmioides herbivory is dependent on the plant genotype and leaf position where injury took place, with negative effects better evinced in bioassays using leaf discs than entire leaflets.     

Differences in interactions of aboveground and belowground herbivores on the invasive plant <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis> and native host <Emphasis Type="Italic">A. sessilis</Emphasis>

Plant invasions may result in novel plant-herbivore interactions. However, we know little about whether and how invasive plants can mediate native above- and belowground herbivore interactions. In this study, we conducted greenhouse experiments to examine the interaction between a native defoliating beetle, Cassida piperata, and a native root-knot nematode, Meloidogyne incognita, on the invasive alligator weed, Alternanthera philoxeroides. We also included their native host A. sessilis in the experiments to examine whether the patterns of above- and belowground herbivore interaction vary with host plants (invasive vs. native). We analyzed total carbon and nitrogen in leaves and roots attacked by M. incognita and C. piperata. M. incognita slightly negatively affected feeding by C. piperata on A. philoxeroides, and the leaf area damaged decreased as the number of M. incognita increased. M. incognita had a negative impact on total leaf nitrogen, but had no impact on total leaf carbon. M. incognita egg production on A. philoxeroides roots decreased as the amount of damage caused by C. piperata increased. Herbivory by C. piperata did not affect total root carbon or nitrogen. M. incognita and C. piperata did not affect each other on the native plant A. sessilis. These results suggest that invasive plants can mediate native above- and belowground herbivore interactions. The knowledge of how invasive plants affect those interactions is crucial for better understanding the impacts of biological invasions on native above- and belowground organisms.     

Trail Chemicals of the Convergens Ladybird Beetle, <Emphasis Type="Italic">Hippodamia convergens</Emphasis>, Reduce Feeding and Oviposition by <Emphasis Type="Italic">Diaphorina citri</Emphasis> (Hemiptera: Psyllidae) on Citrus Plants

We investigated feeding and oviposition behavior of the Asian citrus psyllid, Diaphorina citri, when exposed to the foraging trails of the convergens ladybird beetle, Hippodamia convergens. Diaphorina citri females feeding on citrus leaves directly exposed to the ladybird adults or treated with trail extract excreted significantly less honeydew droplets than controls. The trail chemicals of the ladybird beetle also decreased oviposition by D. citri females on citrus. In a no-choice experiment, D. citri females preferred to oviposit on control flush and plants than those with ladybird trail-extract treatments. In two-choice experiments, 68.0% of D. citri released into cages exhibited strong selection preference for settling and eventual oviposition on control plants than plants treated with ladybird trail extract. Diaphorina citri eggs were found on all new leaf flush of control plants, whereas only 29.5% of flush on treatment plants were selected for oviposition. The trail chemical deposited by the convergens ladybird beetle elicits repellency of D. citri feeding and oviposition. Therefore, the trail chemicals my contain components that could be useful for behavior-based management of D. citri and HLB disease by reducing psyllid feeding and oviposition.     

The never ending story of <Emphasis Type="Italic">rol</Emphasis> genes: a century after

The genes rolA, B, C, and D, derived from Agrobacterium rhizogenes and naturally engineered in plants, are being investigated for a long time about their function, molecular mechanism, origin and evolution and, more recently, the perspectives they offer in plant biotechnology. Evidences point to these genes as important regulators in a wide field of plant endeavors, from hormone control to morphology, from physiological status to defense and from metabolism to signaling. However, in spite of the extant insight on rol genes mechanism and function, a comprehensive picture is still lacking. Recent data suggest that additional research could lead to significant advancement in the knowledge of the role of these genes mainly in plant-bacterium coevolution, and in the development of rol genes—based applications. Through a comprehensive critical review of literature we present a picture of rol genes functions in different plant species, focusing on the relationship between individual genes and plant physiology and metabolism. A possible scenario for their evolution is outlined.     

Long-term ungulate exclusion reduces fungal symbiont prevalence in native grasslands

When symbionts are inherited by offspring, they can have substantial ecological and evolutionary consequences because they occur in all host life stages. Although natural frequencies of inherited symbionts are commonly <100 %, few studies investigate the ecological drivers of variation in symbiont prevalence. In plants, inherited fungal endophytes can improve resistance to herbivory, growth under drought, and competitive ability. We evaluated whether native ungulate herbivory increased the prevalence of a fungal endophyte in the common, native bunchgrass, Festuca campestris (rough fescue, Poaceae). We used large-scale (1 ha) and long-term (7–10 year) fencing treatments to exclude native ungulates and recorded shifts in endophyte prevalence at the scale of plant populations and for individual plants. We characterized the fungal endophyte in F. campestris, Epichloë species FcaTG-1 (F. campestris taxonomic group 1) for the first time. Under ungulate exclusion, endophyte prevalence was 19 % lower in plant populations, 25 % lower within plant individuals, and 39 % lower in offspring (seeds) than in ungulate-exposed controls. Population-level endophyte frequencies were also negatively correlated with soil moisture across geographic sites. Observations of high within-plant variability in symbiont prevalence are novel for the Epichloë species, and contribute to a small, but growing, literature that documents phenotypic plasticity in plant-endophyte symbiota. Altogether, we show that native ungulates can be an important driver of symbiont prevalence in native plant populations, even in the absence of evidence for direct mechanisms of mammal deterrence. Understanding the ecological controls on symbiont prevalence could help to predict future shifts in grasslands that are dominated by Epichloë host plants.