Phenology of native weevils (Coleoptera: Curculionidae) in New Zealand pastures and parasitism by the introduced braconid,Microctonus aethiopoides Loan (Hymenoptera: Braconidae)

Abstract

The phenology of native brachycerine weevil species at seven pasture sites in Otago, Canterbury and Waikato was studied by regular quantitative sampling of adults. Weevils were identified to species, and dissected to record reproductive status and parasitism by introduced braconid parasitoids in the genus Microctonus. Climatic data assisted in the interpretation of some population density patterns. Weevil population density was estimated for periods of two to five years at the selected sites. Species in the Entimini (species of Irenimus and Nicaeana) were generally univoltine, with adults emerging in winter‐spring. The main period of reproductive activity was spring, and parasitism by Microctonus aethiopoides reached its highest incidence in January. Low level parasitism of native weevil species by M. aethiopoides was detected at all sites, and by M. hyperodae at two sites. At one site in Otago, parasitism by M. aethiopoides was higher and could have affected the population density of Irenimus aemulator (Broun) and Nicaeana sp. Most parasitism occurred after the main reproductive period of weevils in spring, but a putative second generation in some species might be more affected by parasitoid attack. A native rhytirhinine species, Steriphus variabilis, differed from the entimines because adults emerged in autumn and spring, and may be bivoltine. Mechanisms of M. aethiopoides parasitism of non‐target species in the field are discussed.     

Field release,establishment and initial dispersal of Irish <Emphasis Type="Italic">Microctonus aethiopoides</Emphasis> in <Emphasis Type="Italic">Sitona lepidus</Emphasis> populations in northern New Zealand pastures

The clover root weevil Sitona lepidus Gyllenhal (Coleoptera: Curculionidae) became an economically important pasture pest in New Zealand shortly after it was discovered in the Waikato region in 1996. A classical biological programme was initiated and an Irish biotype of Microctonus aethiopoides Loan (Hymenoptera: Braconidae) was released at four sites in the North Island in late summer 2006. These sites in Waikato, Hawke’s Bay and Manawatu (two sites) regions were monitored monthly and parasitoid establishment confirmed at all sites within four months. In the winter of 2007, parasitism exceeded 70%. A widespread North Island drought in summer 2008 had a severe impact on S. lepidus populations at the Waikato and the two Manawatu release sites, resulting in parasitism below detection levels in the following summer. However, populations recovered by autumn. Within three years at the Hawke’s Bay site, M. aethiopoides appears to be suppressing S. lepidus populations and has dispersed naturally over 60 km.     

Could research in the native range, and non-target host range in Australia, have helped predict host range of the parasitoid Microctonus aethiopoides Loan (Hymenoptera: Braconidae), a biological control agent introduced for Sitona discoideus Gyllenhal (Coleoptera: Curculionidae) in New Zealand?

It is generally accepted that knowledge of the natural and novel host range of proposed biological control agents can help to inform predictions of potential host range in new areas of introduction. To test this hypothesis, this paper describes a retrospective study conducted to contrast and compare the natural host range of Microctonus aethiopoides Loan (Hymenoptera: Braconidae) with its novel host range found in Australia and New Zealand, where it has been introduced to control the adult stage of the weevil Sitona discoideus Gyllenhal (Coleoptera: Curculionidae), a pest of lucerne. Surveys carried out in and near lucerne crops in Morocco and Australia each resulted in collections of over 3,000 weevils, of which respectively 84?% and 93?% were S. discoideus. The host ranges determined from these surveys for each M. aethiopoides population were then compared with information already available for field host range in New Zealand. In Morocco, species in the genera Sitona and Charagmus (Curculionidae: Entiminae: Sitonini) and Hypera (Curculionidae: Hyperinae: Hyperini) were found to be parasitised by M. aethiopoides. In Australia, an earlier record of non-target parasitism of ‘Prosayleus’ sp. 2 (Curculionidae: Entiminae: Leptopiini) is still the only known instance of non-target parasitism by M. aethiopoides. The known non-target field host range in New Zealand is much greater, comprising 19 native and introduced weevil species mainly in the subfamily Entiminae (tribe Leptopiini) but also in Curculioninae, Cyclominae and Lixinae. This is discussed in the context of predictions that could have been made at the time of introduction 30?years ago had the Moroccan and Australian data, modern molecular technologies and current understanding of weevil classification been available. The absence of Leptopiini in Morocco and the record of a native Australian leptopiine host could have indicated that native weevils in this tribe in New Zealand might be at risk of attack by M. aethiopoides.     

Moroccan specimens of Microctonus aethiopoides spice our understanding of genetic variation in this internationally important braconid parasitoid of adult weevils

Microctonus aethiopoides Loan (Hymenoptera: Braconidae) was introduced from Morocco to Australia and New Zealand for biological control of the lucerne pest, Sitona discoideus. Previous research has indicated that M. aethiopoides intraspecific genetic variation is more strongly associated with weevil host species than geographic origin. Cytochrome c oxidase subunit 1 (COI) sequences from parasitoids dissected from weevils collected during a survey of lucerne-growing areas in Morocco allowed us to further test this hypothesis. As found previously, there were two strong clades in M. aethiopoides with no geographical basis to this structure. Earlier research suggested that intraspecific variability within M. aethiopoides was related to weevil host genus (Sitona vs. Hypera), and the analysis confirmed that one of the clades corresponded strongly with the host Sitona discoideus. The other clade, however, previously characterised by parasitoids from Hypera postica also included parasitoids dissected from Charagmus spp., which is a sister genus to Sitona. It is suggested that food plant associations of the host weevils might have had an influence on the evolutionary history of the parasitoid.     

Parasitism of the lucerne pest Sitona discoideus Gyllenhal (Coleoptera: Curculionidae) and non-target weevils by Microctonus aethiopoides Loan (Hymenoptera: Braconidae) in south-eastern Australia, with an assessment of the taxonomic affinities of non-target hosts of M. aethiopoides recorded from Australia and New Zealand

Abstract  The braconid parasitoid Microctonus aethiopoides Loan has been released in Australia and New Zealand for biological control of the lucerne pest Sitona discoideus Gyllenhal. In New Zealand, the parasitoid attacks a number of endemic weevil species. A survey of Curculionoidea found in and near lucerne in south-eastern Australia was carried out to investigate whether similar non-target parasitism was occurring, and to relate this to levels of parasitism found in the target host, S. discoideus . Some of the original M. aethiopoides release sites were particularly targeted in the survey of 25 sites in Victoria, New South Wales and South Australia. Almost 2500 weevils were collected, of which over 90% were S. discoideus , with the remaining 197 other weevils comprising 29 species found at 15 of the 25 sites. Parasitism of S. discoideus by M. aethiopoides occurred at 12 lucerne sites, with levels ranging from 0 to 25%. A single incidence of parasitism of a species of an Australian native weevil Prosayleus sp. by M. aethiopoides was recorded. No parasitism of any other weevil species was observed. The taxonomic affinities between Sitona and native Australian and New Zealand weevils are discussed, concluding that non-target host range in M. aethiopoides may be determined more by ecological factors than by taxonomic affinities among its hosts.     

Infochemical-mediated preference behavior of the parasitoid Microctonus hyperodae when searching for its adult weevil hosts

Infochemicals are the most important cues used by parasitoids for host location. The attractiveness of infochemicals in a tritrophic context is expected to be determined by the degree of specialization of the parasitoid and its host(s). Microctonus hyperodae Loan (Hymenoptera: Braconidae) is an oligophagous parasitoid that attacks adult Curculionidae of the Brachycerinae subfamily, especially Listronotus bonariensis Kuschel, on Gramineae. In 1996, a new host–parasitoid association between the carrot weevil Listronotus oregonensis LeConte and M. hyperodae was created in the laboratory. In this study, the infochemicals used by M. hyperodae when searching for its adult weevil hosts were determined using a Y‐shaped olfactometer. Three curculionid species (L. oregonensis, Listronotus sparsus Say, and Neydus flavicaudis Boheman) and one bruchid species [Callosobruchus maculatus (Fabricius)], and their feces, were tested. It was expected that hosts phylogenetically and ecologically close to L. bonariensis would be more attractive than species less related but in fact, M. hyperodae responded only to L. oregonensis and its feces. When feces and host insects were tested separately, M. hyperodae responded to the odors emitted by L. oregonensis adults but not to their feces, suggesting that most of the kairomones came from the host itself. Host plants were also tested, but M. hyperodae responded neither to Lolium multiflorum Lamark (Gramineae) nor to Daucus carota L. (Umbelliferae) leaves.     

Serratia marcescens as a rapid indicator of Microctonus hyperodae oviposition activity in Listronotus maculicollis and potential application of the technique to host-specificity testing

Listronotus maculicollis (Dietz) (Coleoptera: Curculionidae) is a potential novel host of the braconid parasitoid Microctonus hyperodae Loan, but initial studies have shown that levels of parasitism are lower than in the natural host L. bonariensis (Kuschel). A novel bacterial indicator test was used to determine whether the lower level of parasitism was due to behavioural factors, lack of oviposition, or host resistance. The incidence of ovipositor penetration by the parasitoid M. hyperodae into adult L. maculicollis was measured by immersing the ovipositor of the parasitoid in the facultative pathogen, Serratia marcescens Bizio. Adult weevils were then exposed to parasitoids for up to 72 h and rapid mortality used as an indicator of oviposition penetration. Survival was assessed after six days and surviving weevils were dissected and examined for parasitoid larvae. Mortality among L. maculicolis exposed to parasitoids treated with S. marcescens was significantly higher (P<0.001) than the controls but significantly lower (P<0.001) than in the natural host, L. bonariensis. Dissection of weevils exposed to uncontaminated parasitoids revealed that parasitism in L. maculicolis was significantly (P<0.001) less than parasitism in L. bonariensis. Serratia marcescens-induced mortality plus parasitism of surviving weevils in the parasitoid plus bacteria treatments produced a similar overall effect. Application of bacteria to the parasitoid ovipositor provided a rapid, simple test for ovipositor penetration, which shows potential for separation of behavioural and physiological defence mechanisms in parasitoid/host range studies.     

Effect of various pesticides on the non-target species Microctonus hyperodae, a biological control agent of Listronotus bonariensis

The parasitoid Microctonus hyperodae Loan (Hymenoptera: Braconidae) was introduced into New Zealand to control the weevil Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae), a major pest of graminaceous plants. Four experiments were conducted to investigate the effects of various pesticides that are commonly used in the pastoral environments of L. bonariensis and M. hyperodae. Topical applications of aqueous solutions prepared from commercial formulations of five herbicides were not toxic, but the surfactant Silwett L‐77 increased M. hyperodae mortality relative to the water‐treated controls. Laboratory assays showed that M. hyperodae adults were susceptible to chlorpyrifos residues on pasture foliage following application of the insecticide to field plots at ≥5 g a.i. ha^?1. Maintenance of L. bonariensis on ryegrass in the laboratory showed that treatment of the food plants with chlorpyrifos at ≥96 g a.i. ha^?1 reduced L. bonariensis survivorship and ultimately reduced M. hyperodae prepupal emergence from those hosts. Initially, mortalities of non‐parasitized L. bonariensis were significantly greater than for parasitized L. bonariensis. Maintenance of parasitized L. bonariensis on diflubenzuron‐treated ryegrass plants arrested M. hyperodae larval development in the host and ultimately reduced prepupal emergence of M. hyperodae from those hosts. Despite the arrested development of M. hyperodae, the mortality of L. bonariensis hosts was increased. Adult M. hyperodae successfully reared from hosts maintained on diflubenzuron (12.5 g a.i. ha^?1) treated food plants had reduced reproductive potential. The consequences of pasture management strategies that employ pesticides are discussed in relation to biocontrol of L. bonariensis by M. hyperodae.     

Host specificity testing and suitability of the parasitoidMicroctonus hyperodae (Hym.: Braconidae,Euphorinae) as a biological control agent ofListronotus bonariensis (Col.: Curculionidae) in New Zealand

The behaviour of the parasitoidMicroctonus hyperodae Loan was studied under quarantine conditions to determine its likely host range in New Zealand. The species was imported from South America as a potential biological control agent of Argentine stem weevil,Listronotus bonariensis (Kuschel). The study involved systematic evaluation of the parasitoid's behaviour when exposed to 24 non-host weevil species; all but three of these were native to New Zealand. Of those tested, four were found to sustain someM. hyperodae development. However, further examination showed that in all but one species,Irenimus aequalis (Broun), parasitoid development was impeded, with up to 50% of the larvae becoming encapsulated. Overall, those weevil species that were attacked produced only 19% of the parasitoids derived fromL. bonariensis controls. As an adjunct to this quarantine study, a review of the habitats of the native weevil and target pest populations indicated that refugia would probably exist for native alpine species. I. aequalis was not considered to be threatened byM. hyperodae as this weevil has benefited from the advent of European agricultural systems to the extent that it is now recognised as a minor pest. In view of its relatively oligophagous behaviour, the parasitoid was recommended as suitable for release.      

Short Distance Cues Used by the Adult Parasitoid <Emphasis Type="Italic">Microctonus hyperodae</Emphasis> Loan (Hymenoptera: Braconidae,Euphorinae) for Host Selection of a Novel Host <Emphasis Type="Italic">Listronotus oregonensis</Emphasis> (Coleoptera: Curculionidae)

Insect parasitoids use host kairomone to detect their hosts. However, in parasitoid species that attack adult hosts, the mobility of adult insect may mean that the host can move away for kairomone sources. The effect of Listronotus oregonensis LeConte (Coleoptera: Curculionidae) adult sex, feces and movement on host selection behavior by Microctonus’’ hyperodae Loan (Hymenoptera: Braconidae; Euphorinae) females was evaluated in the laboratory. We hypothesized that, in addition of using host kairomones, parasitoids of adult stage should use host movement for host selection. The sex of L. oregonensis did not affect the host selection behavior of M. hyperodae. However, host feces decreased the number of weevil antennations done by M. hyperodae. Microctonus hyperodae stopped less frequently near immobile L. oregonensis than near walking ones and these latter were frequently pursued by M. hyperodae. Host movement was the stimulus that elicited oviposition by M. hyperodae. The adaptive implications of these results are discussed.     

Influence of Clavicipitaceous Endophyte Infection in Ryegrass on Development of the ParasitoidMicroctonus hyperodaeLoan (Hymenoptera: Braconidae) inListronotus bonariensis(Kuschel) (Coleoptera: Curculionidae)

Laboratory experiments were conducted to examine the effect of ryegrass infection by the endophytic fungusAcremonium loliiLatch, Christensen and Samuels onMicroctonus hyperodaeLoan, a parasitoid ofListronotus bonariensis(Kuschel). Progression of parasitoids through the larval instar stages was shown to depend on adequate nutrition of the weevil host. Compared to confinement on endophyte-free ryegrass, parasitized weevils held on nonpreferred diets comprising leaf segments from endophyte-infected ryegrass and switchgrass contained parasitoid larvae with retarded development. Similarly, development of parasitoid larvae was retarded in hosts feeding on artificial diet containing diterpenes and alkaloids ofA. loliiorigin. Several diterpenes incorporated into the diet reduced survival of the parasitoid larvae. Attack rate of parasitoids was reduced when the quality of potential host weevils was compromised by confinement on nonpreferredA. lolii-infected ryegrass or without food for 14 days.     

Evidence for parasitoid‐induced premature mortality in the Argentine stem weevil

The Argentine stem weevil Listronotus bonariensis Kuschel (Coleoptera: Curculionidae) is an exotic pest of New Zealand ryegrass and the adult‐stage is parasitized by the introduced solitary endoparasitoid Microctonus hyperodae Loan (Hymenoptera: Braconidae: Euphorinae). This biological control agent is effective, although, under both laboratory and field conditions, an unexplained source of premature mortality in the weevils is observed after exposure to M. hyperodae. This premature mortality may be affected to varying degrees by the length of time of parasitoid exposure, the physiological conditions of the host, and the host to parasitoid ratios, although it occurs naturally without any physical interruption to the parasitoid ovipositional process. In the present study, the premature mortality reported in earlier studies is confirmed and it is conjectured to be the result of injection of parasitoid venom without an egg. Moreover, the lack of premature mortality resulting from longer exposure periods indicates that there might be a curative effect resulting from subsequent oviposition; the egg reverses the toxic effect induced by the injection of venom only. As discussed, this phenomenon may not be restricted to the L. bonariensis/M. hyperodae system and, accordingly, there are evolutionary, biosecurity and general pest management questions to be considered.     

Host selection and reproductive success of French and Moroccan populations of the parasitoid, Microctonus aethiopoides (Hymenoptera: Braconidae)

French and Moroccan populations of the parasitoid Microctonus aethiopoides Loan were studied in the laboratory for their host selection, mating behavior, and reproductive success. The French strain, collected on Hypera postica (Gyllenhal), although capable of parasitizing and producing viable offspring on Sitona weevils, preferred Hypera weevils, its known target host. The Moroccan strain, collected on Sitona discoideus Gyllenhal, exhibited host specificity for Sitona. A partial reproductive isolation was observed between the two strains. Moroccan females mated more frequently with French males than did French females with Moroccan males. The pre-copulation time for mating pairs of opposite strains was significantly longer than that for mating pairs of the same strain. There was no significant difference in copulation time nor in larval and pupal duration between French and Moroccan strains. In summary, the French and Moroccan strains of M. aethiopoides are clearly separable by biological, behavioral, and morphometric traits and the preferred host for Hypera is the French strain and Sitona for the Moroccan strain. Consequently, geographic location and host source become important when considering this parasitoid as a potential biological control agent.     

Field parasitism of nontarget weevil species (Coleoptera: Curculionidae) by the introduced biological control agent Microctonus aethiopoides Loan (Hymenoptera: Braconidae) over an altitude gradient

The parasitoid, Microctonus aethiopoides Loan (Hymenoptera: Braconidae) was introduced into New Zealand in 1982 to control the alfalfa pest, Sitona discoideus Gyllenhal (Coleoptera: Curculionidae). Studies have shown that a number of nontarget weevil species are attacked in the field by this parasitoid. A field study was carried out to investigate nontarget parasitism by M. aethiopoides over an altitudinal sequence from the target host habitat (alfalfa) into native grassland. Three locations were selected for the study, and at each, the alfalfa growing in the valley floor was sampled annually for parasitism of the target pest that ranged between 17 and 78%. At progressively higher altitudes, three further grassland sites at each location were sampled monthly during spring to autumn for up to 6 yr. Weevil densities were estimated, species identified, and dissections carried out to determine reproductive status and parasitism. Almost 12,000 weevils were collected during the study, which were identified as 36 species in total from the three locations. Eight weevil species were found to be parasitized, including S. discoideus, the target host that was found at all sites. Parasitism of nontarget species was approximately 2% overall but varied with location, site, and season. Substantial nontarget parasitism was found at only one of the locations, with up to 24% parasitism of a native weevil, Nicaeana fraudator Broun (Coleoptera: Curculionidae), recorded. Another species, Irenimus egens (Broun) (Coleoptera: Curculionidae), was also found at this location at similar population densities but was attacked far less by M. aethiopoides. Results are discussed in relation to weevil phenology.     

Suitability of the maize weevil and angoumois grain moth as hosts for the parasitoidsAnisopteromalus calandrae andPteromalus cerealellae

Two parasitoids,Pteromalus cerealellae (Ashmead) andAnisopteromalus calandrae (Howard) (Hymenoptera: Pteromalidae), were compared for their ability to parasitize two important internally-developing insect pests of stored maize (Zea mays L.). Parasitism byP. cerealellae was greater on Angoumois grain moth,Sitotroga cerealella (Olivier), than on maize weevil,Sitophilus zeamais Motschulsky, in no-choice experiments.Anisopteromalus calandrae parasitized more maize weevils than didP. cerealellae. The former parasitoid parasitized only a few Angoumois grain moths successfully in maize, but parasitized many in wheat if the hosts were younger than 3 weeks old. Thus, both host age and type of grain affect suitability for parasitism. The effects of parental host (species on which the female developed) and experimental host (species exposed to parasitism) on parasitism rate ofP. cerealellae were tested in a host-switching experiment. Parasitism by parasitoids reared on maize weevils was 23% lower than that of parasitoids reared on Angoumois grain moth. This effect was independent of which host the filial generation of parasitoids was tested on. However, the experimental host species had a much greater effect on parasitoid fecundity than the parental host species. Female progeny had smaller body sizes when emerging from maize weevil than from Angoumois grain moth, which may explain the parental host effect on fecundity. There was also a slight intergenerational effect of host species on parasitoid body size.     

An Immunoassay Method for Detecting Microctonus aethiopoides (Hymenoptera: Braconidae: Euphorinae) Parasitism in the Alfalfa Weevil, Hypera postica (Coleoptera: Curculionidae)

A simple immunological assay was developed as an alternative to the dissection/visualization method for detecting the presence of the parasitoid, Microctonus aethiopoides Loan, in the alfalfa weevil, Hypera postica (Gyllenhal). The dot-blot assay was validated using laboratory-reared and field-collected adult weevils of known parasitization status. The dot-blot assay was also used to estimate the developmental stage of the parasitoid within parasitized adult hosts. The assay results can be used to forecast parasitoid emergence dates and estimate the parasitism rate of M. aethiopoides in alfalfa weevil populations.     

Biology of Centistes delusorius, a parasitoid of adult apple blossom weevil

Abstract

• 1 Natural control of apple blossom weevil, Anthonomus pomorum (L.), deserves attention, as the pest is regaining importance with the declining use of non‐selective pesticides in apple and pear orchards. In this study the biology of Centistes delusorius (Förster), a specific parasitoid of adult apple blossom weevil, is investigated.
• 2 The parasitoid hibernates as young larva in an adult weevil, and juvenile development is resumed in early spring. The fully grown parasitoid larvae leave their hosts during full bloom at the end of April and early May, to pupate. The adults emerging in May oviposit into the newly emerged weevils, which initially feed on apple leaves.
• 3 Centistes delusorius was detected in six out of 15 host‐weevil infested orchards, but was only common in two with larger apple trees standing in grass. There, parasitism levels of around 30% were usual in hosts taken from treebands in winter.
• 4 The delicate larva is vulnerable, and the thin cocoon provides little protection against either desiccation or drowning on a weedless orchard floor. Observations indicate that successful pupation of C. delusorius demands stable humid conditions and some shelter, such as that found in grass or woodland soils.
• 5 Parasitoid females, provided with honey, lived for a mean of 6.3 ± 2.1 days under outdoor conditions in June. Their life span was similar whether they had access to and oviposited in hosts, or not. The species is pro‐ovigenic, and potential fecundity is about 40 eggs. Oviposition usually takes a few seconds. Parasitized female hosts do not reproduce.
• 6 Up to 95% of the parasitoid eggs laid in May develop into a second generation, the adults of which appear in July, when the host has entered aestivation. Older (British) records of C. delusorius outside orchards suggest that some parasitized hosts, like the healthy ones, leave the orchard prior to aestivo‐hibernation, so that the latter do not escape parasitoid attack in July.
• 7 A trapping sample in late June, when most non‐parasitized weevils have gone into aestivo‐hibernation, is probably the most efficient method to detect parasitized weevils.
• 8 The (near‐)absence of C. delusorius in many orchards is probably due not only to pesticide side‐effects, or scarcity of its host, but also to the absence of suitable pupation sites for the wasp.

     

Sex-biased parasitism,seasonality and sexual size dimorphism in desert rodents

We investigated seasonality of gender differences in the patterns of flea infestation in nine rodent species to test if sex-biased parasitism in terms of mean abundance, species richness, prevalence and the level of aggregation (a) varies among hosts and between seasons, and (b) is linked to sexual size dimorphism. Sexual size differences were significant in both summer and winter in Acomys cahirinus, Gerbillus pyramidum and Meriones crassus, and in winter only in Acomys russatus, Gerbillus dasyurus, Gerbillus nanus and Sekeetamys calurus. Sexual size dimorphism was male biased except for A. russatus in which it was female biased. Manifestation of sexual differences in flea infestation was different among hosts between seasons. A significant effect of sex on mean flea abundance was found in six hosts, on mean flea species richness in five hosts and on prevalence in two hosts. Male-biased parasitism was found in summer in one host only and in winter in five hosts. Female-biased parasitism occurred in winter in A. russatus. Gender differences in the slopes of the regressions of log-transformed variances against log-transformed mean abundances occurred in three hosts. No relationship was found between sexual size dimorphism and any parasitological parameter in any season using both conventional regressions and the method of independent contrasts. Our results suggest that sex-biased parasitism is a complicated phenomenon that involves several different mechanisms.     

Influence of release size on establishment and impact of a root weevil for the biocontrol of houndstongue (Cynoglossum officinale)

The root-boring weevil, Mogulones cruciger, was introduced into Canada to control the weed, houndstongue (Cynoglossum officinale). To optimise its use as a biocontrol agent, a 2-year study was performed in British Columbia, Canada to test if the number of M. cruciger released at sites predicted subsequent declines in weed populations. No, 100, 200, 300 or 400 weevils were released in 1999 at field sites (five replicates) corresponding to discrete populations of houndstongue separated by distances of 0.3–3 km. The sites were subsequently monitored for weevil establishment, population change, and host attack, and houndstongue population change. By 2001, M. cruciger had established at all 20 release sites and was present in low numbers in three of five control sites. The year following release, release size was positively correlated with number of adult weevils collected, their damage to host plants, and with subsequent numbers of larvae per plant. In contrast, houndstongue populations were reduced at the same rate and amount, regardless of the experimental release size, within 2 years of release. Significant release treatment×time interactions indicated that factors other than M. cruciger contributed to houndstongue reductions (e.g. drought). However, overall the addition of weevils accelerated the reductions relative to sites with no weevils added. Our study demonstrated that the lowest number within a range of release sizes typically used in weed biocontrol programmes (i.e. 100) was as effective as 200–400 weevils in achieving a consistent amount and rate of houndstongue reduction, and thus, could be implemented to optimise weevil use and achieve predictable biocontrol.     

Host plant preference and performance of the vine weevil Otiorhynchus sulcatus

Abstract 1 The relationship between reproductive performance and preference for potential host plants of the vine weevil is investigated, as shown in tests on contact (or feeding) preference, presented herein, and tests on olfactory preference, published elsewhere. 2 Assessment of reproductive performance shows that the host‐plant range of the adult vine weevil Otiorhynchus sulcatus in Europe is limited to one gymnosperm genus (Taxus sp.) and a broad range of angiosperm plants in two subclasses of the Dicotyledonae, namely Dilleniidae and Rosidae. The successful reproduction on very distantly related plant taxa suggests that the original weevil‐ and plant‐habitat has mediated the current host‐plant range of the vine weevil. 3 Contact‐preference tests with equally suitable hosts, such as Aronia, Fragaria, Euonymus and Taxus, and one less suitable host, Humulus, indicate a mismatch between contact preference and performance and, as far as olfactory preferences are known, these match neither the contact preferences nor the performance. This mismatch may arise because (i) host plant species offered do not occur in weevil habitat in Europe (e.g. Aronia and the cultivated Fragaria come from North America) and (ii) predation (or disease) risks differ among host plants, thereby altering effective reproductive performance. 4 With respect to performance on novel hosts (Thuja, Prunus) and bad hosts (Rhododendron), some between‐individual variation is found within a single population, suggesting that local populations harbour (possibly genetic) variation for adaptation to new hosts. How this variation is maintained in the face of strong selection pressures on local populations of flightless and thelytokous weevils, is an important question for understanding the broad host plant range in the vine weevil.