Agasicles Hygrophila Research Articles

Effects of physiological integration on defense strategies against herbivory by the clonal plant Alternanthera philoxeroides

The plant–herbivore interaction is one of the most fundamental interactions in nature. Plants are sessile organisms, and consequently rely on particular strategies to avoid or reduce the negative impact of herbivory. Here, we aimed to determine the defense strategies against insect herbivores in the creeping invasive plant Alternanthera philoxeroides. We tested the defense response of A. philoxeroides to herbivory by a leaf-feeding specialist insect Agasicles hygrophila and a polyphagous sap-feeding insect Planococcus minor. We also tested the mechanisms triggering defense responses of A. philoxeroides by including treatments of artificial leaf removal and jasmonic acid application. Furthermore, we examined the effect of physiological integration on these defense strategies. The combination of artificial leaf removal and jasmonic acid application produced a similar effect to that of leaf-feeding by the real herbivore. Physiological integration influenced the defense strategies of A. philoxeroides against herbivores, and increased biomass allocation to aboveground parts in its apical ramets damaged by real herbivores. Our study highlights the importance of physiological integration and modular plasticity for understanding the consequences of herbivory in clonal plants.

Effects of clonal integration and nitrogen supply on responses of a clonal plant to short-term herbivory

Clonal integration, i.e. resource sharing between interconnected ramets, can help clonal plants tolerate abiotic stress. However, few studies have directly examined the ecological significance of clonal integration in the ability of clonal plants to tolerate biotic stress such as herbivory. We grew clonal fragments of an invasive clonal plant Alternanthera philoxeroides, each consisting of an apical part (relatively young) and a basal part (relatively old), at two levels (low and high) of soil nitrogen (N). For each fragment, the apical part was subjected to either no herbivory or herbivory for 2 weeks by a specialist insect, Agasicles hygrophila, and was either connected with or disconnected from the basal part not subjected to herbivory. Herbivory in the apical part severely reduced leaf growth (leaf mass, leaf number or leaf area) and ramet production of both apical and basal parts, and increased root to shoot ratio of the apical part. Irrespective of herbivory and soil N, stem connection between the apical and the basal part decreased root to shoot ratio of the apical part, but did not change its growth measures. Meanwhile, connection increased stem N concentration of the apical part growing under high-N supply and decreased stem carbon (C) concentration of the apical part under low-N supply. By contrast, connection increased root to shoot ratio of the basal part, but reduced its leaf and ramet production. Connection also increased leaf and stem C concentrations of the basal part under low-N supply. Thus, clonal integration can be beneficial for the early development of young A. philoxeroides ramets, but not for their local adaptation to herbivory damage by A. hygrophila.

Identification of cytochrome P450 monooxygenase genes and their expression in response to high temperature in the alligatorweed flea beetle Agasicles hygrophila (Coleoptera: Chrysomelidae)

Cytochrome P450 monooxygenases (P450s) are a large class of enzymes that play essential roles in metabolic processes such as hormone synthesis and the catabolism of toxins and other chemicals in insects. In the present study, we identified 82 P450 genes using comprehensive RNA sequencing in the flea beetle Agasicles hygrophila, and all of the sequences were validated by cloning and sequencing. Phylogenetic analysis showed that the P450 genes in A. hygrophila fell into the mitochondrial clan, CYP2 clan, CYP3 clan and CYP4 clan and were classified into 20 families and 48 subfamilies. Most A. hygrophila P450 genes had high sequence homology with those from other coleopteran insects. To understand the effects of high temperatures on the metabolic processes of female and male adults, we studied the effects of two temperature regimes (constant temperature of 28 °C for 20 h with a 4-h period of high temperatures of 30 °C and 39 °C) on the expression levels of P450 genes in A. hygrophila using RT-PCR and qRT-PCR. The results showed that there were no differences in expression in 30 P450 genes between the control and high-temperature-treated A. hygrophila adults, while 22 P450 genes showed up-regulated expression and 19 P450 genes were down-regulated in A. hygrophila female adults after high-temperature treatment. For A. hygrophila male adults exposed to high temperatures, we found that 8 P450 genes had higher expression levels and 12 P450 genes had lower expression levels under the same conditions. The P450 genes are candidates that showed significantly different expression levels after high-temperature treatments in A. hygrophila adults, and further studies are needed to determine their possible roles in metabolic processes during the response to elevated temperatures.

Differences in physiological traits and resistances of Alternanthera philoxeroides 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.

Heat sensitivity of eggs attributes to the reduction in Agasicles hygrophila population.

Agasicles hygrophila has been introduced worldwide as a control agent for the invasive weed Alternanthera philoxeroides. However, global warming has potential impact on its controlling efficacy. The aim of this research was to explore the primary factors responsible for the greatly reduced A. hygrophila population in hot summers. To imitate the temperature conditions in summers, different developmental stages of A. hygrophila were treated with high temperatures from 32.5°C to 45°C for 1-5h. Based on the survival rate, the heat tolerance of each developmental stage was ranked from lowest to highest as follows: egg, 1st, 2nd, 3rd instar larva, adult and pupa. Eggs showed the lowest heat tolerance with 37.5°C as the critical temperature affecting larval hatching. Heat treatment of the A. hygrophila eggs at 37.5°C for 1h decreased the hatch rate to 24%. Our results indicated that when compared with the control at 25°C, 1h treatment at 37.5°C prolonged the duration of the egg stage, shortened the duration of oviposition and total longevity, and changed the reproductive pattern of A. hygrophila. The net reproductive rate, intrinsic rate and finite rate were all significantly reduced. The results suggest that low heat tolerance of the eggs was the major factor responsible for the reduction of A. hygrophila populations, and the key temperature was 37.5°C. Therefore, appropriate measures should be taken to protect eggs in order to maintain the efficacy of A. hygrophila in the biological control of A. philoxeroides in hot summers.

Eco-Biology and Management of Alligator Weed [Alternanthera philoxeroides) (Mart.) Griseb.]: a Review

Exotic plant invasion, a global issue, has a tremendous impact on ecology, economy, human, and animal health. Alligator weed (the world’s first aquatic weed) is a serious invasive weed in 32 different countries of South America, Australia, Asia, and North America. Recently, it has been recorded as a threat weed of rice, maize, soybean, vegetables, fruit trees, and pastures, causing 19–45% yield losses in these crops in addition to its infestation in canals, lakes, and ditches. Alligator weed has the potential to ruin agricultural and natural ecosystems and recreational areas. Ability to propagate via vegetative fragmentation, water-borne dispersal of vegetative propagules, and allelopathic potential contribute towards its success as an invasive weed species of terrestrial, semi-aquatic, and aquatic environments. Application of glyphosate, metsulfuron-methyl, dichlobenil, fluridone, hexazinone, triclopyr amine, dimethylamine, imazapyr, diuron, and amitrole herbicides have been found most effective in controlling this weed in different habitats. Agasicles hygrophila, Vogtia malloi pastana, Amynothrips andersoni, and Nimbya alternanthera have been reported as bio-agents for the control of alligator weed. We present a comprehensive review of the biology, interference, and management options of an extremely dangerous invasive weed species. Although management of alligator weed through chemical, biological, and mechanical means are often effective, there is need for well-planned, long-term field experiments to evaluate the role of different factors that are stated to be responsible for its increasing infestation and distribution (e.g., regeneration after damage caused by herbicides, high soil fertility levels, soil disturbances, shallow vs. deep ploughing and grazing management). It is recommended that future research should focus more on the integration of different management approaches in both aquatic and terrestrial ecosystems, and in various ecological regions.

Comparing traditional methods of test species selection with the PRONTI tool for host-range testing of Eadya daenerys (Braconidae)

A computer-based tool (PRONTI; Priority Ranking Of Non-Target Invertebrates) has been developed to assist the selection of invertebrate species for risk-assessment testing with entomophagous biological control agents (BCAs). PRONTI was used to produce a prioritised list of taxa for host-range testing with the braconid parasitoid Eadya daenerys, a potential BCA for the eucalypt pest, Paropsis charybdis. The resulting list was compared with a list developed using traditional species selection methods. Seven of the nine species on the traditional list were in the PRONTI top 20. The remaining two species (Agasicles hygrophila and Cassida rubiginosa) may not have been selected if the PRONTI tool had been used. These two species were on the traditional list because they are in the same family as the target and are considered valuable BCAs. Alternative BCAs were prioritised by PRONTI. The other 13 taxa prioritised by PRONTI were not on the traditional list: the taxa are little-known natives in the target subfamily or sister subfamily but larvae are presumed to be root-feeders, whereas target larvae are leaf-feeders. PRONTI can support the traditional approach by providing transparent evidence to support the selection (or rejection) of non-target species for host-range testing.

Population dynamics and overwintering of a biological control beetle, Agasicles hygrophila, on a nontarget plant Alternanthera sessilis, along a latitudinal gradient

Assessing the impact of temperature changes on insect population and overwintering on nontarget hosts is important for prediction of nontarget effects in weed biological control. Agasicles hygrophila is a beetle used for biological control of the invasive plant alligator weed, Alternanthera philoxeroides, with nontarget damage to a native plant, Alternanthera sessilis. In this study, we monitored plant growth and phenology along with beetle population and overwintering on these two hosts along a latitudinal gradient from subtropical to temperate climates (Guilin, Wuhan and Kaifeng) in China. We found only annual A. sessilis seedlings in temperate Kaifeng, but both annual seedlings and perennial ramets of A. sessilis were found in Guilin and Wuhan. However, in winter, living A. sessilis plants were found in only subtropical Guilin. Beetles at the Guilin site could successfully maintain their populations and overwinter on A. sessilis. Although the beetle could sustain its populations on A. sessilis at the other two higher-latitude sites during the growing season, it failed to overwinter on either species, indicating that the temperatures in different climate zones may directly and/or indirectly affect the development of insect biological control agents and their population sizes on nontarget hosts. Therefore, it is important to consider the shifting plant–insect interactions induced by climate when assessing potential nontarget effects of species introduced as biological control agents.

Direct and legacy effects of herbivory on growth and physiology of a clonal plant

The ability to tolerate novel herbivores is widely considered to influence plant invasion success. For clonal plants that have reduced capacity to evolve in response to novel herbivores, legacy effects of herbivory on parental plants might be translated to offspring ramets, resulting in pre-adaptation to tolerate herbivory for new vegetative growth. Using the invasive clonal plant Alternanthera philoxeroides, we first exposed plants to herbivory by Planococcus minor, a widespread and generalist piercing-sucking insect. Herbivory decreased above- and below-ground plant biomass by approximately 50% with a concomitant 134% increase in root N concentration but no changes in concentrations of soluble sugars, starch or non-structural carbohydrates related to herbivory tolerance. Offspring ramets were then exposed to herbivory by three different herbivore species: (1) P. minor, (2) the specialist leaf-beetle Agasicles hygrophila, and (3) the stenophagous tortoise-beetle Cassida piperata. There was no evidence of interactive effects between herbivory on parental plants and herbivory on offspring plants on growth, biomass allocation patterns, or physiological responses, suggesting that pre-adaptation to herbivory did not occur in A. philoxeroides with these herbivores. There were, however, species-specific herbivore tolerance responses. In the offspring generation, herbivory by A. hygrophila strongly suppressed growth and biomass allocation, but patterns were generally weaker for other herbivores. Tolerance effects could be explained by stimulatory effects of grazing by C. piperata and P. minor on taproot biomass along with idiosyncratic increases of starch and non-structural carbohydrate concentration in some storage organs. Our results highlight the importance of A. hygrophila in controlling the aboveground spread of A. philoxeroides. However, herbivory by other species was largely tolerated and accompanied by increased allocation to underground storage organs and altered physiological reserves, both of which could allow this invasive plant to tolerate herbivory and successfully invade new areas in the face of new herbivore pressure.

Latitudinal variation in soil biota: testing the biotic interaction hypothesis with an invasive plant and a native congener

Soil biota community structure can change with latitude, but the effects of changes on native plants, invasive plants, and their herbivores remain unclear. Here, we examined latitudinal variation in the soil biota community associated with the invasive plant Alternanthera philoxeroides and its native congener A. sessilis, and the effects of soil biota community variation on these plants and the beetle Agasicles hygrophila. We characterized the soil bacterial and fungal communities and root-knot nematodes of plant rhizospheres collected from 22 °N to 36.6 °N in China. Soil biota community structure changed with latitude as a function of climate and soil properties. Root-knot nematode abundance and potential soil fungal pathogen diversity (classified with FUNGuild) decreased with latitude, apparently due to higher soil pH and lower temperatures. A greenhouse experiment and lab bioassay showed native plant mass, seed production, and mass of beetles fed native foliage increased with soil collection latitude. However, there were no latitudinal patterns for the invasive plant. These results suggest that invasive and native plants and, consequently, their herbivores have different responses to latitudinal changes in soil-borne enemies, potentially creating spatial variation in enemy release or biotic resistance. This highlights the importance of linking above- and below-ground multitrophic interactions to explore the role of soil biota in non-native plant invasions with a biogeographic approach.

Strong effects of hydrologic environment and weak effects of elevated CO2 on the invasive weed Alternanthera philoxeroides and the biocontrol beetle Agasicles hygrophila

Global change, such as elevated CO2, may alter interactions between invasive plants and biocontrol agents, impacting biocontrol efficacy. Here, we conducted four experiments in Texas, USA to test how elevated CO2 influences an invasive plant (Alternanthera philoxeroides) and its interactions with an introduced biocontrol beetle (Agasicles hygrophila) in terrestrial (well-watered) and flooded environments. We grew plants for 9 months in ambient or elevated CO2 (800 ppm) chambers in continuously flooded or well-watered conditions. In no-choice trials, flooding increased leaf toughness and decreased beetle consumption but beetles only oviposited on ambient CO2 leaves. In choice trials, beetles preferred to feed and oviposit on terrestrial plants but were also less likely to damage elevated CO2 leaves. Caged beetle populations were larger in terrestrial conditions than aquatic conditions for a second set of plants grown in the chambers. With a third set of plants grown in the ambient or elevated CO2 chambers, damage for plants placed in the field (aquatic setting) was higher for plants grown in terrestrial conditions vs. flooded conditions at ambient CO2. Our results suggest that elevated CO2 will have minor effects on the efficacy of this biocontrol agent by decreasing oviposition and number of leaves damaged, and hydrologic environment may affect invasive plant performance by altering herbivore oviposition and feeding preferences. A broader understanding of the effects of global change on biocontrol will help prevent and manage future spread of invasive plants.

Response to Short-Term Cold Storage for Eggs of Agasicles hygrophila (Coleoptera: Chrysomelidae), a Biological Control Agent of Alligator Weed Alternanthera philoxeroides (Caryophyllales: Amaranthaceae).

The alligator weed flea beetle, Agasicles hygrophila Selman & Vogt (Coleoptera: Chrysomelidae) has been used very successfully for the biological control of the widely-distributed invasive weed Alternanthera philoxeroides (Mart.) Griseb (Caryophyllales: Amaranthaceae). In order to extend the 'shelf life' of natural enemies released in biological control programs, cold storage has proven to be a valuable commercial procedure. To determine a suitable low temperature for storage of A. hygrophila, we conducted short-term cold storage treatments of eggs (4°C for 0.5, 1, 2, 5 d, and 7.5, 10, 15°C for 5 d and a control of 25°C; all eggs were returned to 25°C after the treatments). We evaluated the effects of these treatments on the subsequent fitness of the populations based on a demographic analysis using group-reared age-stage two-sex life tables. For 5 d storage, temperatures below 10°C had lethal effects, which were also observed at 4°C for 2 d storage. Storage at 4°C for 0.5 d did not affect the fitness of A. hygrophila, but it did not prolong the developmental time. Storage at 10°C for 5 d significantly decreased rates of population increase compared with 25°C. A. hygrophila stored at 15°C for 5 d had similar age-(stage) specific survival rates, rates of population increase, increased longevity and reproductive capability to the controls at 25°C. It is concluded that there were no significant fitness costs after 5 d storage at 15°C, which is therefore potentially a suitable storage temperature for A. hygrophila eggs.

SMRT sequencing of full-length transcriptome of flea beetle Agasicles hygrophila (Selman and Vogt)

This study was aimed at generating the full-length transcriptome of flea beetle Agasicles hygrophila (Selman and Vogt) using single-molecule real-time (SMRT) sequencing. Four developmental stages of A. hygrophila, including eggs, larvae, pupae, and adults were harvested for isolating total RNA. The mixed samples were used for SMRT sequencing to generate the full-length transcriptome. Based on the obtained transcriptome data, alternative splicing event, simple sequence repeat (SSR) analysis, coding sequence prediction, transcript functional annotation, and lncRNA prediction were performed. Total 9.45 Gb of clean reads were generated, including 335,045 reads of insert (ROI) and 158,085 full-length non-chimeric (FLNC) reads. Transcript clustering analysis of FLNC reads identified 40,004 consensus isoforms, including 31,015 high-quality ones. After removing redundant reads, 28,982 transcripts were obtained. Total 145 alternative splicing events were predicted. Additionally, 12,753 SSRs and 16,205 coding sequences were identified based on SSR analysis. Furthermore, 24,031 transcripts were annotated in eight functional databases, and 4,198 lncRNAs were predicted. This is the first study to perform SMRT sequencing of the full-length transcriptome of A. hygrophila. The obtained transcriptome may facilitate further exploration of the genetic data of A. hygrophila and uncover the interactions between this insect and the ecosystem.

Differences in Physiological Traits and Resistances of Alternanthera philoxeroides after Herbivory by Generalists and Specialists

Many researchers have surveyed damages caused by natural enemies in invasive plants in both native and introduced ranges to testing enemy release hypothesis. Although the physiological impact of natural enemies on invasive plants is important, little research has been conducted to compare the impacts between generalists and specialists. In this study, we report our findings on the physiological and morphological impacts of a native specialist insect (Agasicles hygrophila) and two generalist insects (Atractomorpha sinensis and Hymenia recurvalis) in introduced ranges on an invasive plants Alternanthera philoxeroides in both field trials and controlled environments. Resistances of A. philoxeroides against the generalists and the specialist were 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 revealed that even under the same population density, the specialists from native ranges caused more severe morphological and physiological damages on A. philoxeroides than the generalists in introduced ranges. Which magnified the consequence of invasive plants suffered lower richness and abundance of natural enemy and contributed to superior performance of invasive plants in introduced regions.

Specificity of herbivore-induced responses in an invasive species, Alternanthera philoxeroides (alligator weed).

Herbivory‐induced responses in plants can both negatively affect subsequently colonizing herbivores and mitigate the effect of herbivory on the host. However, it is still less known whether plants exhibit specific responses to specialist and generalist herbivores in non‐secondary metabolite traits and how specificity to specialists and generalists differs between invasive and native plant populations. We exposed an invasive plant, Alternanthera philoxeroides, to Agasicles hygrophila (Coleoptera, Chrysomelidae; specialist), Spodoptera litura (Lepidoptera, Noctuidae; generalist), manual clipping, or application of exogenous jasmonic acid and examined both the specificity of elicitation in traits of fitness (e.g., aboveground biomass), morphology (e.g., root:shoot ratio), and chemistry (e.g., C/N ratio and lignin), and specificity of effect on the subsequent performance of A. hygrophila and S. litura. Then, we assessed variation of the specificity between invasive and native populations (USA and Argentina, respectively). The results showed S. litura induced higher branching intensity and specific leaf area but lower C/N ratio than A. hygrophila, whereas A. hygrophila induced higher trichome density than S. litura. The negative effect of induction on subsequent larval growth was greater for S. litura than for A. hygrophila. Invasive populations had a weaker response to S. litura than to A. hygrophila in triterpenoid saponins and C/N ratio, while native populations responded similarly to these two herbivores. The specific effect on the two herbivores feeding on induced plants did not vary between invasive and native populations. Overall, we demonstrate specificity of elicitation to specialist and generalist herbivores in non‐secondary metabolite traits, and that the generalist is more susceptible to induction than the specialist. Furthermore, chemical responses specific to specialist and generalist herbivores only exist in the invasive populations, consistent with an evolutionary change in specificity in the invasive populations.

Manipulating two olfactory cues causes a biological control beetle to shift to non‐target plant species

Summary Olfactory cues can determine the host preferences of herbivorous insects, but their role in host shifting is unclear. Host specificity and the potential for host shifts are important criteria for screening and post‐release evaluation of biological control agents for invasive plants. However, the role of olfactory cues in mediating host shifts in biological control agents is not well understood. To investigate the role of olfactory cues in host selection of a reportedly monophagous flea beetle (Agasicles hygrophila), an important biocontrol agent for invasive alligator weed (Alternanthera philoxeroides), we extracted and analysed the volatiles produced by the host‐plant A. philoxeroides and the non‐host plants Alternanthera sessilis, Beta vulgaris and Amaranthus mangostanus. Moreover, we used electrophysiological techniques, behavioural bioassays and field trials to test the antennal responses and behavioural preferences of A. hygrophila to combinations of different plant volatiles and treatments, and pure compounds in different dosages and combinations. We show that A. hygrophila female beetles indeed use olfactory cues to select plants for feeding and oviposition and that the survivorship of larvae on the second preferred non‐host plant A. sessilis, a close relative of the first preferred host plant A. philoxeroides, was over 75% in a field trail. Although female beetles responded to many volatile compounds from host and non‐host plants, (E)‐4,8‐dimethyl‐1,3,7‐nonatriene (DMNT) positively encouraged the beetle's feeding and oviposition preferences, whereas (Z)‐3‐hexenol displayed repellent effect. Remarkably, complementation assays with (Z)‐3‐hexenol on host plant or DMNT on non‐host plants significantly shifted A. hygrophila host preferences to non‐host plants and resulted in oviposition and egg hatching on the non‐host plant A. sessilis in field trials. Synthesis. We demonstrate an olfactory mechanism by which a specialized herbivorous beetle uses the ratio of two common plant volatiles, (E)‐4,8‐dimethyl‐1,3,7‐nonatriene and (Z)‐3‐hexenol, to discriminate between its host and non‐host plants in nature. This study highlights an important mechanism by which olfactory cues could lead to undesired host range expansion in biocontrol agent, thus representing an important warning of the potential for a host shift and development of invasiveness in a common biocontrol agent, the flea beetle.

Effects of Periodically Repeated Heat Events on Reproduction and Ovary Development of Agasicles hygrophila (Coleoptera: Chrysomelidae).

Insect development occurs within a specific temperature range. Constant temperature studies may produce misleading information on the eco-physiological impacts of temperature on the population dynamics of an insect species, as in most natural environments, temperature usually undergoes daily variation. In China, field surveys showed that the decline in the Agasicles hygrophila (Selman & Vogt) (Coleoptera: Chrysomelidae) population from early August to late September in summer resulted in difficulties in effectively controlling the population of Alternanthera philoxeroides (Mart.) Griseb (Amaranthaceae). Previous studies have largely ignored more natural, fluctuating conditions. In our study, we first investigated the impacts of different temperature conditions (25°C constant temperature for 20 h with a 4-h period of a high temperature of either 30°C, 33°C, 36°C, or 39°C) on adult reproduction and longevity, egg development time, egg hatch rate, female ovarian development, and oogenesis of A. hygrophila. Our results indicated that high temperatures of 30°C and 33°C did not affect the female ovarian development and oogenesis of A. hygrophila Contrarily, high temperatures of 36°C and 39°C negatively affected the population development of A. hygrophila. At 36°C and 39°C, the egg hatch rates were very low, and the egg development times significantly lengthened. The frequency of abnormal ovaries significantly rose at 39°C. We concluded that the decline in the A. hygrophila population during August and September may be related to the extreme high temperatures that frequently occur in summer. These results help provide a better understanding of A. hygrophila population dynamics under natural conditions.

Demography and Population Projection of Flea Beetle, Agasicles hygrophila (Coleoptera: Chrysomelidae), Fed on Alligator Weed Under Elevated CO 2.

The flea beetle, Agasicles hygrophila Selman and Vogt, was introduced into China in 1987. For a more comprehensive understanding of the effect of elevated CO 2 concentration on the population dynamics, we collected the life table data of the flea beetle, A. hygrophila , at two different CO 2 concentration conditions, i.e., ambient (420 µl/liter) and elevated (750 µl/liter). The raw data were analyzed using the age-stage, two-sex life table theory. At 750 µl/liter CO 2 , shorter developmental durations of the egg, first instar, and pupa were observed, while the duration of the third instar and the total developmental duration of the larva were prolonged. The generation length of A. hygrophila was significantly shortened at the higher concentration. It was observed that the intrinsic rate of increase ( r ), finite rate (λ), and net reproduction rate ( R0 ) were higher and the mean generation time ( T ) was shorter at 750 µl/liter compared with that at 420 µl/liter. The bootstrap techniques were adopted to estimate the variances and standard errors of the developmental time, longevity, fecundity, and the population parameters. The bootstrap technique generated a normal distribution that was consistent with the central limit theorem and critical for following statistical analysis and comparison. Population projections based on age-stage, two-sex life tables could reveal the stage structure of A. hygrophila population and the leaf consumption capacity. Data collected in this study can potentially be used to evaluate the efficacy of A. hygrophila as a biological control agent of the alligator weed.

Warming benefits a native species competing with an invasive congener in the presence of a biocontrol beetle

Climate warming may affect biological invasions by altering competition between native and non-native species, but these effects may depend on biotic interactions. In field surveys at 33 sites in China along a latitudinal and temperature gradient from 21°N to 30.5°N and a 2-yr field experiment at 30.5°N, we tested the role of the biocontrol beetle Agasicles hygrophila in mediating warming effects on competition between the invasive plant Alternanthera philoxeroides and the native plant Alternanthera sessilis. In surveys, native populations were perennial below 25.8°N but only annual populations were found above 26.5°N where the invader dominated the community. Beetles were present throughout the gradient. Experimental warming (+1.8°C) increased native plant performance directly by shifting its lifecycle from annual to perennial, and indirectly by releasing the native from competition via disproportionate increases in herbivory on the invader. Consequently, warming shifted the plant community from invader-dominated to native-dominated but only in the presence of the beetle. Our results show that herbivores can play a critical role in determining warming effects on plant communities and species invasions. Understanding how biotic interactions shape responses of communities to climate change is crucial for predicting the risk of plant invasions.

Specialist Insect Herbivore and Light Availability Do Not Interact in the Evolution of an Invasive Plant.

Release from specialist insect herbivores may allow invasive plants to evolve traits associated with decreased resistance and increased competitive ability. Given that there may be genetic trade-off between resistance and tolerance, invasive plants could also become more tolerant to herbivores. Although it is widely acknowledged that light availability affects tolerance to herbivores, little information is available for whether the effect of light availability on tolerance differ between the introduced and native populations. We conducted a common garden experiment in the introduced range of Alternanthera philoxeroides using ten invasive US and ten native Argentinean populations at two levels of light availability and in the presence or absence of a specialist stem-boring insect Agasicles hygrophila. Plant biomass (total and storage root biomass), two allocation traits (root/shoot ratio and branch intensity, branches biomass/main stem biomass) and two functional traits (specific stem length and specific leaf area), which are potentially associated with herbivore resistance and light capture, were measured. Overall, we found that A. philoxeroides from introduced ranges had comparable biomass and tolerance to specialist herbivores, lower branch intensity, lower specific stem length and specific leaf area. Moreover, introduced populations displayed higher shade tolerance of storage root biomass and lower plastic response to shading in specific stem length. Finally, light availability had no significant effect on evolution of tolerance to specialist herbivores of A. philoxeroides. Our results suggest that post-introduction evolution might have occurred in A. philoxeroides. While light availability did not influence the evolution of tolerance to specialist herbivores, increased shade tolerance and release from specialist insects might have contributed to the successful invasion of A. philoxeroides.