Cold hardiness and overwintering survival of the cabbage seedpod weevil,Ceutorhynchus obstrictus

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is a serious pest of brassicaceous crops in temperate regions and a chronic member of the pest complex that attacks canola in Canada. We conducted several laboratory and field experiments to quantify winter survival and its role in the population dynamics of this insect. We estimated the supercooling point of the weevil at ?7 °C and its survival over 8 weeks decreased significantly at ?5 °C relative to 5 °C, but extending the overwintering period at 5 °C to 18.5 weeks had no effect on mortality. Cumulative sub‐freezing degrees estimated from air temperature, and especially from soil temperature, were highly correlated with weevil survival. Our linear regression model predicted poor survival of the weevils in typical winters in northern Alberta. Our results indicate that if milder winters prevail, as predicted by global warming, there is potential for the weevils to establish and become a serious pest in northern canola‐growing regions of Canada.     

Responses of Chalcidoidea (Hymenoptera) parasitoids to invasion of the cabbage seedpod weevil (Coleoptera: Curculionidae) in western Canada

Invasion of the European weevil, Ceutorhynchus obstrictus (Marsham), was investigated through surveys of its range and population densities in Alberta and Saskatchewan from 2001 to 2005. After it was first reported in southern Alberta, C. obstrictus rapidly expanded its range and abundance. Our more recent surveys indicate that its northward expansion has slowed, but that it has continued to extend its range eastward to southcentral Saskatchewan. The distribution and abundance of parasitoids of C. obstrictus in Alberta and Saskatchewan were investigated from 2003 to 2005 by mass rearing canola pods infested with C. obstrictus larvae. Although weevil populations were not parasitized for several years immediately following its introduction to southern Alberta, a surprisingly diverse assemblage of Chalcidoidea parasitoids, comprising 12 species from four families, were recently reared from weevil-infested canola siliques in Alberta and Saskatchewan. The Chalcidoidea fauna of C. obstrictus include species with both Nearctic and Holarctic distributions, with some species having restricted host ranges and others that are more niche than taxon-specific. These Chalcidoidea species appear to have expanded their host ranges to parasitize C. obstrictus in the region. Most parasitism is attributable to Trichomalus lucidus (Walker), Chlorocytus sp., and Pteromalus sp. (Pteromalidae), and Necremnus tidius (Walker) (Eulophidae). Parasitism levels varied considerably over the three years of this study. From 2003 to 2005 increases in parasitism occurred among all four of the species dominating the parasitoid fauna of C. obstrictus, but greater increases were observed for Chlorocytus sp. and Pteromalus sp. than for T. lucidus. Parasitoid species have sometimes caused substantial levels of host mortality, although current levels are usually less than 15% for all species combined and so are not sufficient to control weevil populations. Implementing a classical biological control program for C. obstrictus by reconstructing its European natural enemy complex is being considered, but it is still uncertain whether parasitism levels by native Chalcidoidea will increase over time since considerable year-to-year variation has been found. Parasitism levels of C. obstrictus should therefore continue to be monitored to assess whether a classical biological control program should be implemented.     

Mortality factors affecting the cabbage seedpod weevil,Ceutorhynchus obstrictus (Marsham), in its area of origin: A life table analysis

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is a widely distributed invasive pest of cruciferous crops in North America. Control measures rely mostly on the application of insecticides but alternative control strategies such as classical biological control are under evaluation. To investigate the impact of parasitoids and other mortality factors on C. obstrictus populations, life table studies were conducted between 2005 and 2007 in 13 winter oilseed rape fields in Switzerland, part of the native range. Under field conditions females only realized approximately 50% of their potential lifetime fecundity, varying between 96 and 631 eggs per individual. Total generational mortality was higher than 99.6% in each year. Overwintering mortality of adults was the major population limiting factor, contributing approximately 50% to the overall generational mortality of C. obstrictus, whereas factors acting on the immature stages were responsible for the remaining 50%. Among the mortality factors of the immature stages, egg, larval and pupal mortality contributed 9–12%, 25–28% and 4–6% to the generational mortality, respectively. Larval ecto-parasitism on its own accounted for 7–15% of the generational mortality. Comparison of life table data presented here with that from North America will be invaluable for elucidating the mortality factors that regulate C. obstrictus populations in Europe, the region of origin.     

Determining the host specificity of the biological control agent Trichomalus perfectus (Hymenoptera: Pteromalidae): the importance of ecological host range

We determined the host range of the parasitoid Trichomalus perfectus (Walker), a candidate for classical biological control of cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), an important pest of canola in Canada. Studies were conducted in Europe and in North America. In laboratory experiments, the levels of parasitism (acceptance) of Ceutorhynchus turbatus Schultze, C. cardariae Korotyaev, C. omissus Fall and C. querceti (Gyllenhal) by T. perfectus were not significantly different than of the target host C. obstrictus. Although C. typhae (Herbst), C. pallidactylus (Marsham), C. americanus Buchanan, C. neglectus Blatchely and Ceutorhynchus sp. nr. nodipennis were parasitised by T. perfectus, the levels of parasitism were significantly lower on these species than on C. obstrictus. Ceutorhynchus peyerimhoffi Hustache, C. erysimi (Fabricius), C. alliariae H. Brisout, C. roberti Gyllenhal, Mogulones borraginis (Fabricius), Mononychus vulpeculus (Fabricius) and the leaf-mining fly Scaptomyza flava (Fallén) were not attacked. Ecological host range surveys in Europe corroborated the prediction that T. perfectus would attack C. cardariae at similar rates to C. obstrictus. In North America, the recent discovery of T. perfectus in a C. omissus population suggests that laboratory findings predicting that C. omissus is a preferred host may be the case in the field. We found that T. perfectus attacks larvae of some Ceutorhynchus spp. feeding on Brassicaceae and does not attack species outside of that host range. Thus, the parasitoid can be defined as narrowly oligophagous. These results demonstrate the value of ecological host range studies in the area of origin to validate hypotheses generated through laboratory host range experiments.     

Nutritional Effects on the Appearance of Canola and its Attractiveness to the Cabbage Seedpod Weevil

To investigate visual cues as a mechanism to explain phytophagous insect intraspecific host selection and to explain the effects of plant nutrition on visual cues associated with host selection we assessed the response of the cabbage seedpod weevil, Ceutorhynchus obstrictus, to plants of canola (oilseed rape), Brassica napus supplied with varied nitrogen (N) rates and sulfur to nitrogen (S:N) ratios. We assessed the effects of these treatments on B. napus appearance using photography and spectrophotometry. Plant nutrition had a significant effect on C. obstrictus; host plant attractiveness declined with increasing S:N ratios. We found strong evidence that attractiveness of host plants to C. obstrictus was mediated by several components of B. napus appearance supporting the role of visual cues in intraspecific host selection.     

Bottom-up effects of Brassica napus nutrition on the oviposition preference and larval performance of Ceutorhynchus obstrictus (Coleoptera: Curculionidae)

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is a serious pest of canola, Brassica napus L. and Brassica rapa L., both in Europe and North America. Nitrogen and sulfur levels can be readily manipulated by growers and could potentially be exploited to improve current pest management strategies. In an effort to better understand the relationship between host plant nutrition and C. obstrictus oviposition preference and its larval developmental biology, we exposed gravid females to host plants grown under differing regimes of nitrogen and sulfur. Results indicated that plants grown with a higher supply of nitrogen were preferred as hosts. Plants with a higher sulfur supply were also preferred but only in plants grown at lower nitrogen levels. In contrast, larval development time increased with increasing nitrogen levels although larval dry weights were unaffected. The results were inconsistent with the preference-performance hypothesis. The inconsistencies between oviposition choice and larval growth and development may be due to a conflict between maternal and larval fitness. Observed changes in larval development time may not represent a significant fitness cost for larval C. obstrictus, as no corresponding change in larval weight was observed. Possible causal mechanisms for the increase in development time include differences in oil, protein and glucosinolate contents of the seed. Fertilizer management regimes currently recommended were considered to be optimal for management of C. obstrictus as the yield benefits from higher rates of nitrogen fertilization would more than compensate for increased level of infestation.     

Overwintering conditions affect cold hardiness,survival, and post-overwintering fitness of the pea leaf weevil

Overwintering conditions affect the physiological state of ectotherms, and therefore, their cold hardiness and survival. A measure of the lethal and sublethal impacts of overwintering conditions on pest populations is crucial to predict population dynamics and to manage pests the following spring. The impact of winter conditions can be most intense for invasive insects undergoing range expansion. Insect herbivores can display plastic host use behaviours that depend on their body condition following winter. The pea leaf weevil, Sitona lineatus L. (Coleoptera: Curculionidae), is an invasive pest of field peas, Pisum sativum L., and faba bean, Vicia faba L. (Fabaceae). Pea leaf weevil has expanded its range in North America to include the Prairie Provinces of Canada. This study investigated the effects of temperature and microhabitat on overwintering survival and cold hardiness of pea leaf weevil in its expanded range. Further, we investigated the sublethal effect of overwintering temperature and duration on post-overwintering survival, feeding, and oviposition of pea leaf weevil. We also investigated the role of juvenile hormone in modulating body condition of overwintering weevils. The overwintering survival of pea leaf weevil adults increased with soil temperature and varied with region and microhabitat. More weevils survived winters when positioned near tree shelterbelts compared to open alfalfa fields. The supercooling point of pea leaf weevil varied throughout its expanding range but did not differ for weevils held in the two microhabitats. The average threshold lethal temperature of pea leaf weevil at all three sites was −9.4 °C. Weevils that overwintered for a longer duration and at a higher temperature subsequently fed more on faba bean foliage and laid more eggs compared to those which overwintered for a shorter duration at a lower temperature. Our findings highlight that warm winters would increase overwintering survival and post-overwintering fitness, facilitating further pea leaf weevil invasion northward in the Prairie Provinces of Canada.     

The effect of temperature on the development and life cycle regulation of the pine weevil Hylobius abietis and the potential impacts of climate change

• 1 The pine weevil Hylobius abietis is widely distributed in the Palaearctic region where it is a major pest. Although predominantly semi‐voltine, with a 2‐year life cycle, the generation time across its range can vary from 1 to 4 years. The duration of the life cycle and the seasonal timing of weevil activity affect the economic impact and management of this pest, all of which are likely to change in a warming climate.
• 2 To determine the effect of temperature and tree species on weevil growth and development, laboratory experiments were performed with eggs, larvae, prepupae, pupae and adults, using, as appropriate, the host species Scots pine Pinus sylvestris L. and Sitka spruce Picea sitchensis (Bong.) Carr. under constant or alternating temperatures.
• 3 The development rate was linearly related to temperature, with developmental thresholds for eggs, larvae and pupae of 8, 4.5 and 7.3 °C, respectively. Day‐degrees were estimated for each life stage. Larval development was affected by tree species, being slower on Sitka spruce than on Scots pine, and was faster under alternating than constant temperatures.
• 4 The development time for prepupae was highly variable, with an apparent facultative prepupal diapause initiated by temperature. The temperature range 20–17.5 °C marked the transition between median prepupal development times of approximately 25 and 90 days. The prepupal stage may serve to minimize the risk of overwintering mortality in the pupal stage and help to synchronize the life cycle.
• 5 Larval and adult mass was positively related to developmental temperature, demonstrating an inverse temperature size rule, and weevils were heavier when developing on Scots pine than Sitka spruce. Development in alternating temperatures reduced weevil mass on Scots pine. The influence of temperature on weevil mass is likely to have a positive effect on fecundity and overwintering survival. The effects of climate change on development, voltinism and weevil mass are discussed.

     

Inferring the introduction history of the invasive apomictic grass Cortaderia jubata using microsatellite markers

Aim Reconstructing the introduction history of exotic species is critical to understanding ecological and evolutionary processes that underlie invasive spread and to designing strategies that prevent or manage invasions. The aims of this study were to infer the introduction history of the invasive apomictic bunchgrass Cortaderia jubata and to determine if molecular data support the postulated horticultural origin of invasive populations. Location Invaded areas in the USA (California, Maui) and New Zealand; native areas in Bolivia, Ecuador and Peru. Methods We used nuclear microsatellite markers to genotype 281 plants from invaded areas in California, Maui and New Zealand, and 77 herbarium specimens from native South America, and compared the genotypic and clonal variation of C. jubata from the invaded and native ranges. Clonal diversity was determined from genotypic diversity using two analytical methods. Results Invasive C. jubata from invaded regions in California, Maui and New Zealand consisted of the same single clone that probably originated from a single introduced genotype. In contrast, 14 clones were detected in herbarium specimens from the native areas of Bolivia, Ecuador and Peru. The invasive clone matched the most common clone identified in herbarium specimens from southern Ecuador where horticultural stock is presumed to have originated. Main conclusions The lack of clonal and genotypic diversity in invasive plants, but moderately high diversity detected in native plants, indicates a significant reduction in genetic variation associated with the introduction of C. jubata outside of its native range. Based on historical accounts of the horticultural introduction of C. jubata and the results of this study, a severe founder effect probably occurred during deliberate introduction of C. jubata into cultivation. Our results are consistent with the postulated horticultural origin of invasive C. jubata and point to southern Ecuador as the geographical source of invasive populations.     

Twenty‐five years after: post‐introduction association of Mecinus janthinus s.l. with invasive host toadflaxes Linaria vulgaris and Linaria dalmatica in North America

Linaria vulgaris, common or yellow toadflax, and Linaria dalmatica, Dalmatian toadflax (Plantaginaceae), are Eurasian perennial forbs invasive throughout temperate North America. These Linaria species have been the targets of classical biological control programmes in Canada and the USA since the 1960s. The first effective toadflax biological control agent, the stem‐mining weevil Mecinus janthinus (Coleoptera: Curculionidae) was introduced from Europe in the 1990s. This weevil has become established on L. dalmatica and L. vulgaris in both countries, although it has shown greater success in controlling the former toadflax species. Genetic and ecological studies of native range M. janthinus populations revealed that weevils previously identified as a single species in fact include two cryptic species, now recognised as M. janthinus, associated with yellow toadflax, and the recently confirmed species Mecinus janthiniformis, associated with Dalmatian toadflax. The results of a comprehensive study characterising haplotype identities, distributions and frequencies within M. janthinus s.l. native range source populations were compared to those populations currently established in the USA and Canada. The presence of both Mecinus species in North America was confirmed, and revealed with a few exceptions a high and consistent level of host fidelity throughout the adopted and native ranges. Genetic analysis based on mitochondrial cytochrome oxidase subunit II gene (mtCOII) defined the origin and records the subsequent North American establishment, by haplotype, of the European founder populations of M. janthinus (northern Switzerland and southern Germany) and M. janthiniformis (southern Macedonia), and provided population genetic indices for the studied populations. This analysis together with existing North American shipment receipt, release and rearing records elucidates probable redistribution routes and sources of both weevil species from initially released and established adopted range populations.     

Phenological synchrony affects interaction strength of an exotic weevil with Platte thistle,a native host plant

Phenological synchrony of a consumer population with its resource populations is expected to affect interaction intensity. We quantified phenological variation and synchrony of populations of an invasive Eurasian flower head weevil, Rhinocyllus conicus, that consumes florets, ovules, and seeds of developing flower heads of a native North American thistle, Cirsium canescens, in Sand Hills prairie in Nebraska, USA. Variation in timing of adult activity among weevil populations was larger than variation in timing of flower head development among C. canescens populations, and it drove the observed variation in the phenological synchrony between weevil and host plant populations. Furthermore, the degree of phenological synchrony between populations was significant in explaining variation in weevil egg load on the newly acquired host plant. Because population growth of C. canescens is limited by predispersal seed losses to floral herbivores, variation in the synchrony of herbivore and plant flowering will affect the density of the plant population. These results provide strong quantitative support for the hypothesis that the synchrony of insect activity with plant resources can determine the magnitude of impact of floral herbivores on their host plant populations.     

Population genetics of Ageratina adenophora using inter-simple sequence repeat (ISSR) molecular markers in China

Abstract

Understanding distribution and diversity of invasive weeds is essential for the development of efficient control measures against it. In the present study, inter-simple sequence repeat (ISSR) markers were used to assess the biogeographic relationships among populations of the invasive Crofton weed (Ageratina adenophora (Spreng.)) during 2004–2006 in China. A total of 100 ISSR primers with di-, tri-, tetra- and penta-nucleotide repeats were screened, from which 20 polymorphic and informative primers were selected. Amplification of the 20 primers generated a total of 479 polymorphic bands among the 64 weed populations, and a high level of genetic diversity (H _E = 0.1541 ± 0.0193) was detected in A. adenophora. Neighbor-joining (NJ) cluster analysis based on genetic distances among populations grouped the populations according to their geographical origin, i.e. (1) populations of southwestern Guizhou, (2) populations of Liangshan city in Sichuan, (3) populations of western Guizhou, (4) Guangxi populations plus Chongqing populations, (5) populations of southern Yunnan, and (6) populations of Yangtze River Valleys in Sichuan plus populations of western Yunnan. A significant positive correlation between geographical and genetic distance was detected by the Mantel test (r = 0.183, p = 0.0012). Based on the divergence relationships revealed by ISSR markers, it was assumed that A. adenophora mainly dispersed through wind and water in China.     

Persistence of the flowerbud weevil Anthonomus santacruzi in optimal versus marginal areas: implications for the biological control of the invasive tree Solanum mauritianum in South Africa

Anthonomus santacruzi Hustache (Coleoptera: Curculionidae) was released in South Africa to offset the extensive reproductive output of the invasive tree Solanum mauritianum Scopoli (Solanaceae). Widespread establishment has occurred predominantly in the coastal areas of KwaZulu-Natal province, with limited success in higher-altitude inland areas. Irrespective of location, populations exhibit peaks in the austral autumn and decline during winter. In this study, we evaluated the persistence of A. santacruzi populations in climatically-optimal coastal areas versus climatically-marginal inland areas. The weevil’s pre- and post-winter abundance was surveyed at six coastal and six inland sites during 2018, and compared between areas and seasons. The 2018 pre-winter data were also compared to 2016 pre-winter data collected at the same sites. During 2018, pre- and post-winter numbers were six times and 22 times higher, respectively, at optimal sites than at marginal sites, with substantial winter declines at all sites. Post-winter weevil numbers at optimal sites were significantly higher than pre-winter numbers at marginal sites. Pre-winter numbers at optimal sites were not significantly different between 2016 and 2018, but at marginal sites were significantly lower in 2018. Inflorescences of S. mauritianum at marginal sites contained significantly more floral material and fruit than those at optimal sites, during both seasons in 2018. Significant negative correlations between A. santacruzi numbers and floral/fruit production suggest some impact on the reproductive output of S. mauritianum. Since A. santacruzi populations are barely persisting in marginal areas, releases in other South African provinces should target locations that are below 300?m in altitude.     

Efficacy of plant essential oils against two major insect pests of coffee (Coffee berry borer,Hypothenemus hampei,and antestia bug,Antestiopsis intricata) and maize weevil,Sitophilus zeamais

Coffee berry borer, antestia bug and maize weevil are serious pest of coffee and maize, respectively. Bioassays of plant essential oils were conducted with coffee berry borer, antestia bug and the maize weevil. Essential oils of Thymus vulgaris, Aloysia sp., Ruta chalepensis, Chenopodium ambrosioides and Cymbopogon nardus resulted in 80%–90% mortality of coffee berry borer, whereas essential oils of C. ambrosioides, T. vulgaris and R. chalepensis achieved 87.5%–92.5% mortality of antestia bug. Essential oils of C. ambrosioides caused significantly the highest percentage mortality of the maize weevils (95%) with “Lethal Dose” (LD₅₀) values of 2.202 ml 100 ml^?1 within 24 h after treatment, whereas essential oils of Mentha spicata, T. vulgaris and R. chalepensis resulted in 82.5%, 77.5% and 73.5% mortality, respectively. The potential of plant essential oils for the management of coffee berry borer, antestia bug and maize weevil for the small-scale farmers is discussed.     

Understanding invasion history: genetic structure and diversity of two globally invasive plants and implications for their management

Aim Resolving the origin of invasive plant species is important for understanding the introduction histories of successful invaders and aiding strategies aimed at their management. This study aimed to infer the number and origin(s) of introduction for the globally invasive species, Macfadyena unguis‐cati and Jatropha gossypiifolia using molecular data. Location Native range: Neotropics; Invaded range: North America, Africa, Europe, Asia, Pacific Islands and Australia. Methods We used chloroplast microsatellites (cpSSRs) to elucidate the origin(s) of introduced populations and calculated the genetic diversity in native and introduced regions. Results Strong genetic structure was found within the native range of M. unguis‐cati, but no genetic structuring was evident in the native range of J. gossypiifolia. Overall, 27 haplotypes were found in the native range of M. unguis‐cati. Only four haplotypes were found in the introduced range, with more than 96% of introduced specimens matching a haplotype from Paraguay. In contrast, 15 haplotypes were found in the introduced range of J. gossypiifolia, with all invasive populations, except New Caledonia, comprising multiple haplotypes. Main conclusions These data show that two invasive plant species from the same native range have had vastly different introduction histories in their non‐native ranges. Invasive populations of M. unguis‐cati probably came from a single or few independent introductions, whereas most invasive J. gossypiifolia populations arose from multiple introductions or alternatively from a representative sample of genetic diversity from a panmictic native range. As introduced M. unguis‐cati populations are dominated by a single haplotype, locally adapted natural enemies should make the best control agents. However, invasive populations of J. gossypiifolia are genetically diverse and the selection of bio‐control agents will be considerably more complex.     

Dynamics of infection by Leptosphaeria maculans on canola (Brassica napus) as influenced by crop rotation and tillage

Abstract

This study, intending to understand the effects of crop rotation and tillage on blackleg disease, was conducted in a field at Carman, Manitoba, Canada, from 1999 – 2002. Canola, wheat and flax were among the rotated crops. Rotations were performed under conventional or zero-till conditions. The number of infected plants, infected leaves per plant, lesions per plant, and percentage of leaf coverage with lesions decreased when canola was rotated with wheat and flax under zero till. The number of lesions per plant and percentage of leaf coverage with lesions were strongly correlated with stem disease severity, and the number of infected plants with stem disease incidence. Ascospores and pycnidiospores of Leptosphaeria maculans were reduced by crop rotation and tillage. This study suggests that the appropriate combination of rotation and tillage may lower airborne inoculum and reduce infection of canola plants by L. maculans.     

Field and laboratory studies to determine the suitability of Cissoanthonomus tuberculipennis (Coleoptera: Curculionidae) for release against Cardiospermum grandiflorum (Sapindaceae) in South Africa

Cardiospermum grandiflorum is an invasive creeper that was targeted for biological control in South Africa in 2003. To determine ecological host range of its natural enemies, surveys were conducted on C. grandiflorum and 11 other Sapindaceae at 40 sites in the weed's native range (Argentina). These surveys indicated that the seed-feeding weevil Cissoanthonomus tuberculipennis was restricted to C. grandiflorum, and that it was among the common natural enemies, occurring at most sites where C. grandiflorum was recorded. Open-field tests were conducted under natural conditions in Argentina to determine the host preference of C. tuberculipennis and other natural enemies of C. grandiflorum among three Cardiospermum species. These tests revealed that C. tuberculipennis and the bug Gargaphia sp. were restricted to C. grandiflorum though the latter subsequently developed on non-target Cardiospermum species in the laboratory. C. tuberculipennis was found to be highly damaging, destroying up to 44% of the seeds per plant in Argentina. In all the host-specificity tests, including no-choice, paired-choice and multi-choice tests, C. tuberculipennis only fed and developed on C. grandiflorum. Failure of C. tuberculipennis to feed and develop on all congeners of C. grandiflorum shows that the weevil is highly host-specific to the target weed. Results of host-specificity tests, open field tests and long-term monitoring of C. tuberculipennis populations demonstrate that the weevil poses no threat to non-target plant species, and therefore safe for release against C. grandiflorum in South Africa. Permission to release C. tuberculipennis in South Africa has been granted by the relevant regulatory authorities.     

Effects of seeding rate and plant density on sclerotinia stem rot incidence in canola

Abstract

Sclerotinia stem rot (Sclerotinia sclerotiorum Lib. deBary) affects canola wherever it is grown. Seeding rates, which are believed to affect the microclimate beneath the canopy, were evaluated for their impact on sclerotinia stem rot incidence. A study with five seeding rates (2.2 kg/ha, 3.3 kg/ha, 6.7 kg/ha, 13.3 kg/ha and 20.0 kg/ha) and four canola cultivars chosen for their variation in canopy structure and lodging resistance was conducted in Carman, Manitoba, Canada, in 2001 to 2003. A significant relationship between sclerotinia stem rot disease incidence (DI) and seeding rate was found. With an increase in seeding rate, the DI was significantly increased in the mean of the canola cultivars, and individually, only in the lodging-prone cultivar AC Excel. Lodging significantly increased for all cultivars when seeding rates exceeded the standard 6.7 kg/ha. Multiple regression analysis revealed that both plant density and lodging explain the majority of the variation in DI. Both plant density and lodging resistance varied in having a larger influence on DI depending on the year and cultivar analysed. Our results indicate that increasing seeding rate does modify the microenvironment and increases the potential for lodging, which may be responsible for plant-to-plant spread of this disease.     

The influence of plant size on attack rates of Oxyops vitiosa,an introduced herbivore of Melaleuca quinquenervia

Oxyops vitiosa Pascoe (Coleoptera: Curculionidae) was released in Florida in 1997 as a biological control agent of the invasive tree Melaleuca quinquenervia (Cav.) S.T. Blake. Three years after release, the weevil’s dispersal into three replicated M. quinquenervia stands was monitored to determine if O. vitiosa preferentially selects larger trees during patch colonisation. Data indicated that tree size did not influence the likelihood of an attack by O. vitiosa and smaller individuals within host patches did not escape herbivory through enemy-free space.     

Invaded habitat incompatibility affects the suitability of the potential biological control agent Listronotus sordidus for Sagittaria platyphylla in South Africa

Sagittaria platyphylla (Engelmann) J.G. Smith (Alismataceae) was first recorded in South Africa in 2008 and is considered to be an emerging weed with naturalised populations occurring throughout the country. A biological control programme was initiated in Australia and surveys conducted between 2010 and 2012 yielded potential agents, including the crown feeding weevil, Listronotus sordidus Gyllenhal (Coleoptera: Curculionidae). The potential of L. sordidus as a candidate biological control agent against S. platyphylla in South Africa was examined. Although adult feeding was recorded on a number of plant species, oviposition and larval development indicated a narrow host range restricted to the Alismataceae. In South Africa, S. platyphylla populations are primarily found in inundated systems. However, laboratory studies showed that L. sordidus did not oviposit on inundated plants, potentially nullifying the impact of the insect on South African populations. It is suggested that even though L. sordidus is a damaging, specific agent, its limited impact on inundated plant populations in South Africa does not justify the inherent risk associated with the release of a biological control agent.