Host plant nutritional quality affects the performance of the parasitoid Diadegma insulare

The well documented biochemical profile of Brassicaceae, oligophagy of the herbivore Plutella xylostella (L.) (Lepidoptera: Plutellidae), and host specialization of the parasitoid Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) provide an ideal system for investigating tritrophic interactions mediated by nutritional quality of plants. We evaluated the bottom-up effects of five soil fertility regimes on nutritional quality of canola (Brassica napus L.) and then on several fitness correlates of female and male D. insulare as mediated through P. xylostella. Variation in soil fertility influenced the nutritional quality of host plants and this in turn affected the performance of D. insulare. In general, D. insulare performed best on plants grown with 3.0 g fertilizer pot⁻¹; these plants had 2.06-, 3.77-, and 1.02-fold more nitrogen, phosphorous and potassium, respectively than ones grown without any added fertilizer. P. xylostella escape from D. insulare was highest (32%) on plants grown at 1.0 g fertilizer, and this could be attributed to both physical and physiological defense mechanisms mediated by host plant nutritional quality. Plant stress and plant vigor are competing paradigms pertaining to the performance of herbivorous insects on their host plants. These hypotheses were originally proposed to predict responses of herbivores, but may also explain the effects of plant quality on koinobiont parasitoids, such as D. insulare.     

Effect of Bt broccoli and resistant genotype of <Emphasis Type="Italic">Plutella xylostella</Emphasis> (Lepidoptera: Plutellidae) on development and host acceptance of the parasitoid <Emphasis Type="Italic">Diadegma insulare</Emphasis> (Hymenoptera: Ichneumonidae)

The ecological implications on biological control of insecticidal transgenic plants, which produce crystal (Cry) proteins from the soil bacterium Bacillus thuringiensis (Bt), remain a contentious issue and affect risk assessment decisions. In this study, we used a unique system of resistant insects, Bt plants and a parasitoid to critically evaluate this issue. The effects of broccoli type (normal or expressing Cry1Ac protein) and insect genotype (susceptible or Cry1Ac-resistant) of Plutella xylostella L. (Lepidoptera: Plutellidae) were examined for their effects on the development and host foraging behavior of the parasitoid, Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) over two generations. Parasitism rate and development of D. insulare were not significantly different when different genotypes (Bt-resistant or susceptible) of insect host larvae fed on non-Bt broccoli plants. D. insulare could not discriminate between resistant and susceptible genotypes of P. xylostella, nor between Bt and normal broccoli plants with different genotypes of P. xylostella feeding on them. No D. insulare could emerge from Bt broccoli-fed susceptible and heterozygous P. xylostella larvae because these larvae were unable to survive on Bt broccoli. The parasitism rate, developmental period, pupal and adult weights of D. insulare that had developed on Bt broccoli-fed Cry1Ac-resistant P. xylostella larvae were not significantly different from those that developed on non-Bt broccoli-fed larvae. Female D. insulare emerged from Cry1Ac-resistant P. xylostella that fed on Bt plants could successfully parasitize P. xylostella larvae. The life parameters of the subsequent generation of D. insulare from P. xylostella reared on Bt broccoli were not significantly different from those from non-Bt broccoli. The Cry1Ac protein was detected in P. xylostella and in D. insulare when hosts fed on Bt broccoli. These results are the first to indicate that Cry1Ac did not harm the development or host acceptance of an important endoparasitoid after two generations of exposure. We suggest that using other Bt crops and resistant insect species would likely lead to similar conclusions about the safety of the presently used Bt proteins on parasitoids.     

Parasitoid diversity and impact on populations of the diamondback moth Plutella xylostella (L.) on Brassica crops in central México

Three experimental plots were established and maintained during one year at two sites in the state of Querétaro, México, in order to identify species of parasitoids attacking the diamondback moth (DBM) Plutella xylostella (L.) (Lepidoptera: Plutellidae), and to gather information concerning their relative importance and patterns of population fluctuation. At both sites, the plots were planted with broccoli, cabbage and cauliflower during three cropping seasons. Parasitoid species identified were: Diadegma insulare Cresson, Diadromus (= Thyraeella) collaris Gravenhorst (Hymenoptera: Ichneumonidae); Habrobracon sp. (Hymenoptera: Braconidae); Oomyzus (= Tetrastichus) sokolowoskii Kurdjumov (Hymenoptera: Eulophidae); and Spilochalcis (= Conura) sp. (Hymenoptera: Chalcididae). This last species is a hyperparasitoid of D. insulare. The most abundant and frequently occurring species was D. insulare, it occurred in both localities during all three cropping seasons. The highest levels of parasitism caused by D. insulare on DBM were registered in the spring-summer season of 1996 at `La Soledad' farm with averages of 42.7, 45.0 and 44.5% on cabbage, broccoli, and cauliflower, respectively. Because D. insulare was detected attacking the pest at very low population densities during the initial stages of the crop cycle, it is assumed that the parasitoid has a high searching capacity. Correlation (r) between DBM and D. insulare population numbers was positive and significant. The other species identified occurred sporadically and had little impact on pest populations. The identification of D. collaris represents the first record of this species in North America.     

Natural enemies ofPlutella xylostella (Lep.: Plutellidae) on crucifers in Honduras

Three primary parasitoids in three genera were reared fromPlutella xylostella (L.) larvae and pupae collected in various crucifer producing regions of Honduras. The ichneumonidDiadegma insulare (Cresson) was by far the most abundant species. TwoSpilochalcis species, facultative hyperparasitoids attackingP. xylostella andD. insulare, were encountered as well as eleven species in nine genera of obligate hyperparasitoids attackingD. insulare. Three vespid predators are noted as predating on larvae.      

Do laboratory studies predict parasitoid interaction outcomes in the field?

Studying competitive interactions among natural enemies is important to elucidate the success and non-target impact of candidate biological control agents. Increased regulation of new introductions requires that studies on non-target species be carried out in confined conditions. Hypotheses about potential impacts of biological control agents in the field are based on data from Petri dish or small cage experiments conducted in the laboratory. This study compared the performance of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), parasitoids Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) and Microplitis plutellae (Muesebeck) (Hymenoptera: Braconidae) in experiments conducted in small cages in the laboratory and in large cages in the field. Results showed no significant differences between laboratory and field outcomes for D. insulare alone and when D. insulare and M. plutellae were combined. For M. plutellae alone, parasitism in the laboratory cages was significantly less than in the field cages. These results demonstrate that laboratory studies may be useful to develop hypotheses on competitive interactions of candidate parasitoid biological control agents.     

Studies on culture age versus exometabolite production in batch cultures of the cyanobacterium <Emphasis Type="Italic">Nostoc insulare</Emphasis>

Changes in the content of exometabolites excreted by the cyanobacterium Nostoc insulare during batch cultivation were determined. During linear growth, only the non-toxic compound N,N′-(4,5-dimethyl-1,2-phenylene)-bis-acetamide was detectable in appreciable quantities in the medium, whereas during stationary growth the antimicrobial and cytotoxic exometabolites 4,4′-dihydroxybiphenyl and 9H-pyrido(3,4-b)indole (norharmane) were also present to an increasing degree. Hence it is proposed that biosynthesis of N,N′-(4,5-dimethyl-1,2-phenylene)bis-acetamide in N. insulare is associated with cell proliferation and primary metabolism of this organism. 4,4′-Dihydroxybiphenyl and norharmane, however, are proposed to be products of secondary metabolism that are excreted by N. insulare primarily under nutrient-restricted conditions and under increased pressure of competition with other organisms.     

Leaf nutrient levels and the spatio-temporal distributions of <Emphasis Type="Italic">Plutella xylostella</Emphasis> and its larval parasitoids <Emphasis Type="Italic">Diadegma insulare</Emphasis> and <Emphasis Type="Italic">Microplitis plutellae</Emphasis> in canola

Seasonal distribution patterns of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), and its principal parasitoids Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) and Microplitis plutellae (Muesebeck) (Hymenoptera: Braconidae) were investigated over three site-years in commercial fields of canola (Brassica napus L.) in southern Alberta, Canada. The sampling of P. xylostella, D. insulare, and M. plutellae from points arranged in grid patterns, together with the mapping and analysis of their spatial distributions over time, generated a detailed picture of the pattern of crop infestation by the herbivore and its parasitoids. Plutella xylostella exhibited significant aggregations on different scales most often when its host plants were in early flowering. Diadegma insulare adults exhibited significant aggregated distributions during early flowering and distributions subsequently became more uniform as the wasps moved into the crop later in the season. However, M. plutellae distributions were aggregated in mid flowering in only one site-year. The close spatial associations between densities of D. insulare and P. xylostella indicated that host abundance was the main determinant of parasitoid distribution patterns. Spatial distributions of nutrient contents in leaf tissue and their spatial associations with the herbivore and parasitoids were also investigated. Significant spatial associations existed between certain nutrients (e.g. nitrogen, sulfur, and potassium) and P. xylostella distributions. Sulfur exhibited a positive effect on the distributions of D. insulare but not of M. plutellae. We observed similar relationships between nutrients and the distribution of P. xylostella parasitoids as for nutrients and P. xylostella, but these relationships lacked consistency and may be the results of the spatial associations between the parasitoids and their hosts. Aggregated distributions of adults and larvae of P. xylostella hold promise for spatially targeted insecticidal applications as a means for reducing the environmental impact of insecticides on nontarget and beneficial species in canola agroecosystems.     

Diadegma insulare development is altered by Plutella xylostella reared on water‐stressed host plants

Natural enemies of herbivores function in a multitrophic context, and their performance is directly or indirectly influenced by herbivores and their host plants. Very little is known about tritrophic interactions between host plants, pests and their parasitoids, particularly when the host plants are under any stress. Herbivores and their natural enemies’ response to plants under stress are diverse and variable. Therefore, in this study we investigated how diamondback moth, Plutella xylostella (L.), reared on water‐stressed host plants (Brassica napus L. and Sinapis alba L.) influenced the development of its larval parasitoid, Diadegma insulare (Cresson). No significant differences were observed in development of P. xylostella when reared on water‐stressed host plants. However, all results indicated that water stress had a strong effect on developmental parameters of D. insulare. Development of D. insulare was delayed when the parasitoid fed on P. xylostella, reared on stressed host plants. Egg to adult development of D. insulare was faster on non‐stressed B. napus than non‐stressed S. alba followed by stressed B. napus and S. alba. Female parasitoids were heavier on non‐stressed host plants than stressed counterparts. Furthermore, the parasitoid lived significantly longer on stressed B. napus. However, body size was not affected by water treatment. Most host plant parameters measured were significantly lower for water‐stressed than non‐stressed treatments. Results suggest that development of this important and effective P. xylostella parasitoid was influenced by both water stress and host plant species.     

Comparing floral nectar and aphid honeydew diets on the longevity and nutrient levels of a parasitoid wasp

We compared the effects of floral nectar from buckwheat, Fagopyrum esculentum Moench, and honeydew produced by the soybean aphid, Aphis glycines Matsumura (Homoptera: Aphididae), on longevity, nutrient levels, and egg loads of the parasitoid Diadegma insulare Cresson (Hymenoptera: Ichneumonidae). Diadegma insulare lived for 2 days in control treatments of water or clean soybean leaves, for 6–7 days with honeydew, and in excess of 2 weeks with buckwheat nectar. Potential reasons for the superiority of buckwheat nectar over soybean aphid honeydew for extending the longevity of parasitoids include: (i) parasitoids ingest more sugars from floral sources, (ii) oligosaccharides in honeydew have a lower nutritional value than nectar sugars, and (iii) honeydew has antagonistic compounds. Overall sugar levels were lower in honeydew‐ vs. nectar‐fed female wasps, suggesting a lower feeding rate, but other explanations cannot be excluded. Diadegma insulare eclosed with high levels of lipids and glycogen, and low levels of gut and storage sugars. All carbohydrates increased over the life of both nectar‐ and honeydew‐fed wasps, but remained low or decreased in starved wasps. Lipid levels declined over the lifespan of female wasps, but females fed floral nectar showed the slowest rate of lipid decline. Diet did not affect egg load, probably because the females were not given hosts in the experiment.     

Coexistence in the intertidal: interactions between the non‐indigenous New Zealand mud snail Potamopyrgus antipodarum and the native estuarine isopod Gnorimosphaeroma insulare

Nonindigenous species (NIS) that achieve high densities in their recipient communities are expected to have negative effects on native competitors. However, recent reviews have shown that competitors, unlike predators and parasites, have rarely been documented to cause extinctions. There is a need for better understanding of competitive interactions between nonindigenous species and native competitors across systems where NIS reach high densities. In this paper we examine competitive interactions between the exotic, invasive New Zealand mud snail Potamopyrgus antipodarum and native estuarine invertebrates. P. antipodarum is often described as a freshwater snail but can occur in brackish water as well; it has been established in the Columbia River Estuary for over ten years. We addressed competition in this estuarine system using three approaches: field surveys of the benthic invertebrate community, stable isotope analysis of overlap in resource use by common invertebrates, and a laboratory competition experiment that tested the strength of competition between P. antipodarum and the isopod, Gnorimosphaeroma insulare, which overlap in resource use. We found no evidence of negative competitive impacts of P. antipodarum on native benthic invertebrates in this estuarine system. Densities of P. antipodarum and common native epibenthic invertebrates are positively correlated in the intertidal, likely due to shared habitat preferences. The competition experiment showed that the effect of interspecific competition from P. antipodarum on the foraging and survival of G. insulare was significantly less than the effect of intraspecific competition between isopods. The presence of the isopod G. insulare reduced foraging in P. antipodarum, but this change did not result in reduced survival and growth of snails. Hence, interspecific competition is weak despite high densities achieved by the invading species. Finally, we discuss several possible explanations for the observation that P. antipodarum does not have an obvious negative competitive impact on native benthic invertebrates.     

Phylogeography of Eastern Polynesian sandalwood (Santalum insulare), an endangered tree species from the Pacific: a study based on chloroplast microsatellites

Aim Patterns of genetic variation within forest species are poorly documented in island ecosystems. The distribution of molecular variation for Santalum insulare, an endangered tree species endemic to the islands of eastern Polynesia, was analysed using chloroplast microsatellite markers. The aims were to quantify the genetic diversity; to assess the genetic structure; and to analyse the geographical distribution of the diversity within and between archipelagoes. The ultimate goal was to pre‐define evolutionary significant units (ESUs) for conservation and restoration programmes of this species, which constitutes a natural resource on small, isolated islands. Location Eleven populations, each representative of one island, covering most of the natural occurrence of S. insulare were sampled: five populations from the Marquesas Archipelago; three from the Society Archipelago; and three from the Cook–Austral Archipelago. These South Pacific islands are known for their high degree of plant endemism, and for their human occupation by Polynesian migrations. The extensive exploitation of sandalwood by Europeans nearly 200 years ago for its fragrant heartwood, used overseas in incense, carving and essential oil production for perfume, has dramatically reduced the population size of this species. Methods We used chloroplast microsatellites, which provide useful information in phylogeographical forest tree analyses. They are maternally inherited in most angiosperms and present high polymorphism. Among the 499 individuals sampled, 345 were genotyped successfully. Classical models of population genetics were used to assess diversity parameters and phylogenetic relationships between populations. Results Four microsatellite primers showed 16 alleles and their combinations provided 17 chlorotypes, of which four exhibited a frequency > 10% in the total population. The gene diversity index was high for the total population (H_e = 0.82) and varied among archipelagoes from H_e = 0.40 to 0.67. Genetic structure is characterized by high levels of differentiation between archipelagoes (36% of total variation) and between islands, but differentiation between islands varied according to archipelago. The relationship between genetic and geographical distance confirms the low gene flow between archipelagoes. The minimum spanning tree of chlorotypes exhibits three clusters corresponding to the geographical distribution in the three main archipelagoes. Main conclusions The high level of diversity within the species was explained by an ancient presence on and around the hotspot traces currently occupied by young islands. Diversity in the species has enabled survival in a range of habitats. Relationships between islands show that the Cook–Austral chlorotype cluster constitutes a link between the Marquesas and the Society Islands. This can be explained by the evolution of the island systems over millions of years, and extinction of intermediary populations on the Tuamotu Islands following subsidence there. Based on the unrooted neighbour‐joining tree and on the genetic structure, we propose four ESUs to guide the conservation and population restoration of Polynesian Sandalwood: the Society Archipelago; the Marquesas Archipelago; Raivavae Island; and Rapa Island.     

Screening of microalgal culture media for the presence of algicidal compounds and isolation and identification of two bioactive metabolites,excreted by the cyanobacteria <Emphasis Type="Italic">Nostoc insulare</Emphasis> and <Emphasis Type="Italic">Nodularia harveyana</Emphasis>

Culture medium extracts obtained from 115 culture media of 35 different microalgae species were screened for the presence of algicidal compounds, in particular for compounds which are cytotoxic to Arthrospira (Spirulina) laxissima. In agar plate diffusion tests and in a test system combining thin layer chromatography (TLC) with the use of an aqueous suspension of living A. laxissima cells as spray reagent, 14 microalgae species were found with cytotoxic activity of different intensity to A. laxissima. In a so-called TLC plate diffusion test, using A. laxissima and other microalgae as test organisms, the culture medium extracts of Nodularia harveyana and Nostoc insulare possessed the highest strength and range of algicidal activity. The algicidal compound in the culture medium extracts of Nodularia harveyana was shown to be norharmane (9H-pyrido(3,4-b)indole), a known indole alkaloid. The main algicidal compound in culture medium extracts of Nostoc insulare was identified as 4,4′-dihydroxybiphenyl. The possible applicability of both compounds as therapeutics or as useful agents for removing cyanobacterial water blooms or for developing new antifouling systems is discussed.     

Contrary choices: possible exploitation of enemy-free space by herbivorous insects in cultivated vs. wild crucifers

Summary The pressure to escape natural enemies may shape how herbivorous insects use their plant resources. On wild crucifers, ovipositional preferences of the diamondback moth (Plutella xylostella; DBM) were similar to searching preferences of its main parasitoid, an ichneumonid wasp (Diadegma insulare). But on cultivated crucifers, these species had opposite preferences. In addition, DBM ovipositional preferences did not correlate with growth or reproduction on several foodplants. We interpret these patterns as evidence of evolution for use of enemy-free space in agricultural systems.     

A REVISION OF THE GENUS GYMNODERMA

Gymnoderma, which has been considered a monotypic genus, is composed of three species. G. insulare is described from Japan and differs from G. coccocarpum of southeastern Asia in being smaller and having a thicker cortex with didymic acid present. G. lineare from the Southern Appalachians is transferred from Cladonia because the podetia lack symbiotic algae, are solid, and are located on the marginal lower side of the thallus. Gymnoderma, including three species and possibly representing an ancestral form of Cladonia, is disjunctively distributed in eastern Asia and the Southern Appalachians. G. coccocarpum is reported from Thailand and Borneo for the first time.     

Notes on the variation,geographical distribution and ecology ofEuastrum insulare var.basichondrum (Chlorophyta,Desmidiaceae)

Euastrum insulare var.basichondrum, morphologically taking an intermediate position in respect ofCosmarium trilobulatum var.basichondrum andEuastrum bipapillatum, appeared to exhibit such a wide range of variation that it may be considered identical to the other two above-mentioned taxa. This taxon, after having been studied with both light- and scanning electron microscopy, is re-diagnosed. This alga occurs in wet moss cushions in weakly acid to neutral environments. The relation between such a terraqueous habitat and the principally arctic-alpine distribution and a conceivably associated process of speciation is discussed.     

Cytotoxic and non-cytotoxic exometabolites of the cyanobacterium Nostoc insulare

The isolation, identification and quantification of exometabolites from culture media of the cyanobacterium Nostoc insulare are described. Besides the known exometabolite 4,4′-dihydroxybiphenyl (I), two more compounds, the β-carboline 9H-pyrido(3,4-b)indole (norharmane, II) and N,N′-(4,5-dimethyl-1,2-phenylene)bis-acetamide (III), were discovered. Concentrations of all three compounds in media and biomass of five 250 L cultures of N. insulare were determined. Culture medium values for I ranged between 200 and 1,250 μg L⁻¹ (1.1–6.7 μmol L⁻¹), for III between 115 and 390 μg L⁻¹ (0.5–1.8 μmol L⁻¹), whereas concentrations of II were conspicuously lower (2–16 μg L⁻¹ or 0.014–0.094 μmol L⁻¹). Amounts of III per volume of culture medium were about tenfold higher than in the biomass contained in an equal culture volume. This difference is an indication for an active excretion of III. Amounts of I and II in biomass and media were of no significant difference. In the neutral red uptake assay, I and II were found to be toxic against eukaryotic cells as follows: I was of considerable cytotoxicity in concentrations from 1,000 to 10 mg L⁻¹ and of lower cytotoxicity (causing a 27 % decrease of cell viability) in a concentration of 1,000 μg L⁻¹, whereas II was merely of considerable cytotoxicity in concentrations from 1,000 to 100 mg L⁻¹. Because of the cytotoxicity of I and because of the many known pharmacological effects of II there is a possibility that a certain amount of risk to humans and livestock comes from cultures or even from biomass- free culture media of N. insulare. The natural function of the examined exometabolites is discussed.     

Impact of flowering buckwheat on Lepidopteran cabbage pests and their parasitoids at two spatial scales

We assessed the potential of annual buckwheat, Fagopyrum esculentum Moench, to lead to improved parasitism of lepidopteran cabbage pests over four years. Pest, parasitism, and hyperparasitism rates were monitored in replicated cabbage plots (12 × 20 m) with or without 3 m wide buckwheat borders from 2000 to 2003. Floral borders did not significantly increase egg, larval, or pupal densities of cabbage looper, Trichoplusia ni (Hübner), imported cabbageworm, Pieris rapae (L.), or diamondback moth, Plutella xylostella (L.). Buckwheat increased parasitism rates by Voria ruralis (Fallen) on T. ni larvae and Cotesia rubecula (Marshall) on P. rapaelarvae over four years. Parasitism by Diadegma insulare (Cresson) on P. xylostella larvae was higher in buckwheat than control plots in the first year, and parasitism by Euplectrus plathypenae (Howard) on T. ni larvae was lower in buckwheat than control plots in the second year. The hyperparasitoid Conura side (Walker) attacked D. insulare all four years, but buckwheat did not affect hyperparasitism rates. The effect of spatial scale on pest densities and parasitism in 2001 was evaluated by comparing plots separated at least 67 m (nearby) versus 800 m apart (isolated). T. ni pupae and P. rapae eggs and pupae were more abundant in plots in closer proximity, whereas P. xylostella densities did not vary by the spatial separation of plots. Tachinids and Pteromalus puparum (L.) attacked more P. rapae in nearby plots. E. plathypenae responded to the treatment × scale interaction, parasitizing more in control than buckwheat when plots were isolated but not when plots were nearby.     

Fitness of the parasitoid Diadegma insulare is affected by its host's food plants

The diamondback moth, Plutella xylostella (L.) is considered a specialist on Brassicaceae, but it is capable of expanding its food range by incorporation of non-brassicaceous plants into its diet. The use of a new food plant may change food availability and vulnerability of P. xylostella to its natural enemies, especially specialist parasitoids. In this laboratory study, we evaluated the bottom-up effects of two Brassicaceae viz. Brassica napus L. and Descurainia sophia (L.) Webb ex Prantl and two non-Brassicaceae viz. Tropaeolum majus L. and Cleome hassleriana Chodat on several fitness correlates of the specialist parasitoid, Diadegma insulare (Cresson) reared on P. xylostella larvae. Percentage of parasitism of host larvae by D. insulare varied among the plant species and was highest on T. majus and lowest on D. sophia. Values of several fitness correlates for D. insulare differed when the host was feeding on various plant species. Egg to adult development was fastest on B. napus followed by C. hassleriana, D. sophia and T. majus. Female wasps reared on C. hasseleriana lived the longest in absence of food. The response of D. insulare to potential food-plant expansion by its host P. xylostella is discussed.     

The impacts of rats on the endangered native flora of French Polynesia (Pacific Islands): drivers of plant extinction or coup de grâce species?

Although rats have clearly contributed to bird extinctions on islands, their role in plant extinctions is not as clear. Paleoenvironmental studies suggest rats were responsible for the demise of several island palm species. French Polynesia’s islands provide an opportunity to evaluate “modern” impacts of rats on native flora. Our study shows that 15 threatened taxa (nine families) are damaged by rats. All 12 subjected to seed predation are woody plants with large-seeded drupes. Three experience severe predation and recruitment depression (Santalum insulare, Ochrosia tahitensis, Nesoluma nadeaudii). Three-year monitoring of Polynesian sandalwood (Santalum insulare) populations in Tahiti during rat control suggested that over 99% of fruits were eaten before ripening. Seed predation on sandalwood appeared to be lower on islands without black rats Rattus rattus. Studies from Indo-Pacific islands document rat impact on at least 56 taxa (28 families). Certain families (Arecaceae, Elaeocarpaceae, Rubiaceae, Santalaceae, and Sapotaceae) are particularly vulnerable to seed predation. Other soft-barked trees (Araliaceae, Euphorbiaceae, and Malvaceae) suffer from stem or bark damages, especially during dry seasons. Although rats depress seedling recruitment and alter vegetation dynamics, no evidence demonstrates that they are solely responsible for current plant extinctions. Most of French Polynesia’s endangered species impacted by rats occur in severely degraded habitats. We therefore suggest that rats can be viewed more as coup de grace species (i.e., that give the final stroke of death), rather than as main drivers of plant extinctions. More research is needed to clarify the impacts of rat species and their importance in plant population decline or demise.