Cabbage Seedpod Weevil Research Articles

Incorporation of Novel Ceutorhynchus obstrictus-resistant Canola Genotypes Into Mixed Cropping Strategies: Evidence for Associational Resistance

Canola genotypes resistant to the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), have recently been developed through introgression of Sinapis alba L. to Brassica napus L. Several lines express antixenosis and antibiosis resistance and have been shown to be less attractive to weevils in visual and olfactory behavioral bioassays. This paper details a small-plot study that assessed the effects on distribution dynamics of weevil adults and larvae of interspersing susceptible among resistant genotypes relative to monocultures over two growing seasons. Results indicate that mixes reduced weevil numbers and oviposition in pods of susceptible genotypes. These results are consistent with associational resistance.

Plant Nutrients and the Spatiotemporal Distribution Dynamics of <I>Ceutorhynchus obstrictus</I> (Coleoptera: Curculionidae) and Its Parasitoids

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), is an alien invasive pest of canola, Brassica napus L., in North America. Field populations of both adults and larvae are known to be aggregated in their distributions, but the causal mechanism for this clustering is not understood. We investigated the hypothesis that spatial distributions of C. obstrictus adults, larvae, and their parasitoids were directly or indirectly related to host plant quality as indicated by leaf tissue nutrient contents. During the years of 2005 and 2007, these insect populations were sampled in a grid pattern in three commercial fields of B. napus located near Lethbridge, Alberta, Canada. Leaf samples were collected from plants in each grid plot and analyzed for nutrient contents. Spatial patterns and associations among the collected variables were analyzed using Spatial Analysis by Distance IndicEs software. We observed significant spatial associations among C. obstrictus adults, larvae, and parasitoids. The distributions of C. obstrictus adults and larvae were correlated with several plant nutrients, most notably nitrogen and sulfur. Evidence was found for a possible nitrogen-sulfur interaction in which ovipositioning C. obstrictus females appeared to prefer plants with high levels of sulfur and low levels of nitrogen. Spatial tracking of C. obstrictus larvae by its parasitoids was evident in only one field, and this appeared related to comparatively low parasitoid populations in two of the three fields. The associations between C. obstrictus and plant nutrients may have possible applications in fertility management, trap cropping, and precision insecticide application technology.

Flight Activity and Dispersal of the Cabbage Seedpod Weevil (Coleoptera: Curculionidae) Are Related to Atmospheric Conditions

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is an invasive pest of canola (Brassica napus L. and Brassica rapa L.) in western Canada. Under current climatic conditions, C. obstrictus is spreading from established populations in southwestern Alberta at ≈ 55 km/yr. We studied the influence of climatic conditions on C. obstrictus flight behavior in 2007 and 2008 and eastward dispersal from the western border of Saskatchewan from 2002 to 2007. Positive linear relationships between increases in mean temperature and flight height and between greater mean maximum temperature and expanded dispersal distances were significant. Increases in relative humidity were associated with reduced flight heights and dispersal distances. We developed models that predict the relationships of temperature and relative humidity with flight height and with dispersal distance. We also discuss implications for C. obstrictus dispersal under current climatic conditions and in the context of predicted climate change.

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.

Antixenosis and antibiosis resistance toCeutorhynchus obstrictusin novel germplasm derived fromSinapis albaxBrassica napus

AbstractIntrogression of cabbage seedpod weevil,Ceutorhynchus obstrictus(Marsham) (Coleoptera: Curculionidae), resistance fromSinapis albaL. to susceptibleBrassica napusL. (Brassicaceae) has produced genetic lines resistant to the weevil in replicated field trials. In the current study, weevil feeding and oviposition onS. albaand on resistant novel lines developed by crossingS. alba×B. napuswere less frequent than on susceptible germplasm.Development times were greater and biomass was less when larvae were reared on resistant lines orS. alba. Oocyte development was faster in post-diapause springtime adult female weevils caged on susceptible plants than in those on a resistant line,S. alba, or an early-season food host,Thlaspi arvenseL (Brassicaceae). Our results suggest that antixenosis resistance and antibiosis resistance are expressed by resistant lines. These results and previous chemical analyses of these lines also suggest that resistance is potentially influenced by attractive and (or) feeding-stimulant effects of 2-phenylethyl glucosinolate and antifeedant or toxic effects of 1-methoxy-3-indolylmethyl glucosinolate.

Responses of Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae) to olfactory cues associated with novel genotypes developed by Sinapis alba L. × Brassica napus L.

Many herbivorous insects use olfactory cues for host location. Extracts from Brassica napus L. have been shown to elicit electrophysiological and behavioural responses in the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (syn. C. assimilis (Paykull)) (Coleoptera: Curculionidae). These include volatile products of the hydrolysis of glucosinolates. Here we present results of a laboratory olfactometer study examining the attractiveness of odours from flowering racemes and foliage of Sinapis alba L. (an inappropriate host for larval development), B. napus (an excellent host for larval development) and lines derived from S. alba × B. napus selected from colonization studies to demonstrate resistance or susceptibility. Results of this study indicate differential attraction of C. obstrictus to the odours of resistant and susceptible lines and suggest the role of hydrolysis products of glucosinolates, particularly the attractive effects of 2-phenylethyl isothiocyanate.

Contributions of visual cues to cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), resistance in novel host genotypes

Herbivorous insects can demonstrate great selectivity among visual cues. The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae) (syn. Ceutorhynchus assimilis (Paykull)) is also influenced by visual cues. Here, we present results of a study examining the attractiveness of flowers and foliage of Sinapis alba (a suboptimal host for larval development), Brassica napus (an excellent host for larval development) and novel lines derived from S. alba x B. napus; several of these have been demonstrated to be resistant in field and laboratory tests. Attractiveness, reflectance properties and resistance demonstrated in field trials were related. Responses to host plants differed among plant genotypes and were related to the amounts of UV and yellow reflected from flowers. Moderate UV reflectance greatly increased the attractive effects of yellow. Fewer weevils responded to test plants with greater or lesser floral UV reflectance.

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

AbstractThe 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.

The importance of taxonomy in responses to invasive alien species

Invasive alien species (IAS) cause major economic losses and threaten biodiversity worldwide. Taxonomic information is essential to initiating defence against IAS, including early detection and identification of new IAS outbreaks. We demonstrate how current IAS strategies (i.e. prevention of new invasions, early detection and rapid response to new invaders and management of established invaders) require strong taxonomic infrastructure. Examples are given from terrestrial molluscs (Gastropoda) and insects such as the Gypsy Moth, Lymantria dispar Linnaeus (Lepidoptera: Lymantriidae), disease vector flies of the families Culicidae and Calliphoridae (Diptera:), Sirex noctilio (Fabricius) (Hymenoptera: Siricidae) woodwasps and Dendroctonus bark beetles and the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), that demonstrate the role of taxonomy in IAS response. Further, we show through the example of the Emerald Ash Borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), how international co-operation is essential to identify initial outbreaks of IAS. Such international co-operation is especially important because few countries can fund comprehensive pest identification organizations alone.

Responses of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), to seed treatments of canola (Brassica napus L.) with the neonicotinoid compounds clothianidin and imidacloprid

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), is a major pest in the production of canola (Brassica napus L.) in North America and Europe, and effective population control is often essential for economical crop production. In North America, neonicotinoid insecticides have been used for several years in canola as seed treatments for reducing herbivory by flea beetles. The neonicotinoids clothianidin and imidacloprid were investigated to determine their effects on preimaginal development and on emergence of new-generation adults of C. obstrictus in comparison with effects of lindane, a chlorinated hydrocarbon seed treatment. Mean numbers of second- and third-instar larvae were significantly higher in plants seed-treated with lindane than in plants treated with the neonicotinoid compounds, even though weevil oviposition was similar for all treatments. Emergence of new-generation adults was reduced by 52 and 39% for plants seed-treated with clothianidin and imidacloprid, respectively, compared with emergence from plants treated with lindane. Seed treatment with both clothianidin and imidacloprid produced systemic insecticidal effects on larvae of C. obstrictus, with clothianidin slightly more effective than imidacloprid. Use of clothianidin or imidacloprid as seed treatments can comprise an important component in the integrated management of cabbage seedpod weevil in canola.

Biochemical markers for cabbage seedpod weevil (Ceutorhynchus obstrictus (Marsham)) resistance in canola (Brassica napus L.)

In the last decade, the cabbage seedpod weevil (Ceutorhynchus obstrictus (Marsham)) has become a major insect pest of canola (Brassica napus L.) in Canada reducing seed yields up to 35%. Therefore, the benefits of developing weevil resistant germplasm to canola breeders and the industry would reduce input costs, pesticide use, environmental degradation and increase yield. Yellow mustard (Sinapis alba L.) is resistant to C. obstrictus (CSPW), although the exact mechanism is not known (McCaffrey et al. 1999). A unique canola population was generated at the University of Guelph from a cross between B. napus and S. alba through embryo rescue and backcrossed to canola several times prior to double haploid (DH) production. Approximately one-half of this DH population had canola quality glucosinolate concentration (<16 μmol/g) and was used for further breeding. The hypothesis was that some DH progeny from this cross inherited resistance to CSPW from S. alba. Weevil infestation levels were assessed for the B. napus × S. alba BC2 and BC3 DH populations in the field over 7 years in Alberta where weevil pressure is strong to establish the resistant or susceptible status of these lines. The basic objectives for this study were to confirm field resistance in the B. napus × S. alba germplasm in Ontario and to identify any biochemical markers associated with resistance/susceptibility. Canola doubled haploid lines derived from BC2 or BC3 families were field screened for resistance (R) followed by chemical analysis of glucosinolates to detect biochemical polymorphisms correlated with CSPW resistance using High Performance Liquid Chromatography (HPLC). Two polymorphic peaks were found, one each, from extracts of upper cauline leaves and Stage 3 pod seed, with retention times of ~23 and 19 min, respectively. These HPLC peaks consistently correlated with larval infestation data and the peak differences between R and S DH lines were significant. Therefore, these two peaks can be considered as biochemical markers in this breeding germplasm and may play a role in rapid and early detection of CSPW resistance.

Cabbage seedpod weevil resistance in canola (Brassica Napus L.) yellow mustard (Sinapis alba L.) and canola × yellow mustard hybrids

Canola, yellow mustard and yellow mustard × canola hybrids were screened for resistance to Ceutorhynchus obstrictus (Marsham) in a series of greenhouse and laboratory choice tests. Tests were conducted using small and large cages designed to hold Brassica pods or whole plants, respectively, with ovipositing female C. obstrictus. Pods were examined for feeding punctures, eggs laid and exit holes that resulted from emerging larvae. All yellow mustard cultivars examined were highly resistant to C. obstrictus feeding and egg laying. In addition, hybrid lines were found with significantly reduced feeding punctures, oviposition, and exit holes compared with canola. A negative relationship was observed between total glucosinolate content of seed and C. obstrictus resistance; however, in both detached pod and whole plant choice tests, hybrids were identified with high seed glucosinolate content that were more susceptible than the most susceptible canola. The relatively poor association between total seed meal glucosinolate content and C. obstrictus resistance is highly important and plant breeders should be able to develop canola-quality oil and seed meal characteristics with improved C. obstrictus resistance using yellow mustard × canola hybrids. The effect of specific glucosinolate types on C. obstrictus resistance may be more complex than previously thought, and further research will be needed to better understand the relationship between specific glucosinolate types and resistance to C. obstrictus. Key words: Brassica napus, Sinapis alba, Ceutorhynchus obstrictus, cabbage seedpod weevil, insect resistance, glucosinolate

Contributions to the Life History, Host Range, and Distribution of &lt;I&gt;Necremnus tidius&lt;/I&gt; (Hymenoptera: Eulophidae)

Abstract Field and laboratory studies investigated the preimaginal developmental time, adult emergence phenology, mating behavior, and weevil hosts of Necremnus tidius (Walker) (Hymenoptera: Eulophidae), a solitary ectoparasitoid of coleopteran larvae. In addition, changes in parasitism levels of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), by N. tidius were determined during 2003–2005 in commercial fields of canola (Brassica rapa L. and Brassica napus L.) in southern Alberta and Saskatchewan, Canada. Mean ± SE developmental time from egg to adult was 18.3 ± 0.23 d for specimens reared at 21°C and a photoperiod of 12:12 (L:D) h. Emergence began in late July in commercial fields near Lethbridge, AB, and 20 d later 81% of the population had emerged. Emergence was completed by the end of August. Mating was preceded by a complex courtship behavior with the male standing on the dorsum of the mesosoma and gaster of the female, flapping its wings without flight, lea...

Resistance to cabbage seedpod weevil among selected Brassicaceae germplasm

AbstractBrassicaceae germplasm (Brassica napus L., Brassica rapa L., Brassica juncea (L.) Czern., Brassica carinata A. Braun, Sinapis alba L., Camelina sativa (L.) Crantz, Crambe species) with cultivar development potential for the prairies was evaluated for resistance to cabbage seedpod weevil (Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae)) infestation under field conditions in southern Alberta from 2001 to 2005. Relative susceptibilities were as follows: B. rapa and B. napus &gt; B. juncea = B. carinata &gt; S. alba. Although some of the germplasm evaluated varied within species in terms of glucosinolate profile, erucic acid level, oil content, and plant height, none of these factors appeared to influence the degree of C. obstrictus damage. A subset of this germplasm was also evaluated in the greenhouse. Although the canola species B. rapa sustained the highest level of damage in the field tests, it was similar to other Brassica genotypes in the greenhouse. All lines of S. alba were virtually immune to weevil attack in both field and greenhouse tests, regardless of glucosinolate profile, suggesting that other factors confer resistance in this species. Synchrony of crop development with weevil activity in the field explains some of the variability in damage observed among genotypes and illustrates the importance of combining field and greenhouse studies when assessing resistance.

A review of the species of Mesopolobus (Chalcidoidea: Pteromalidae) associated with Ceutorhynchus (Coleoptera: Curculionidae) host-species of European origin

Four species of Mesopolobus Westwood were reared as parasitoids of Ceutorhynchinae hosts in Europe during surveys in 2000-2004. An illustrated key is given to differentiate the four species, M. gemellus Baur & Muller sp. n., M. incultus (Walker), M. morys (Walker) and M. trasullus (Walker), plus M. moryoides Gibson, a parasitoid of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), in North America. Pteromalus clavicornis Walker is recognized as a junior synonym of M. incultus syn. n., and Pteromalus berecynthos Walker (also a junior synonym of M. incultus) is considered a correct original spelling. For Disema pallipes Förster (a junior synonym of Mesopolobus morys), a lectotype is designated. Mesopolobus morys is for the first time accurately associated with the seed weevil Ceutorhynchus turbatus (Schultze), a potential agent for classical biological control, of hoary cress, Lepidium draba L. (Brassicaceae), in North America. Mesopolobus gemellus is associated with another seed weevil, Ceutorhynchus typhae (=C. floralis) (Herbst), in pods of shepherd's purse, Capsella bursa-pastoris (L.) Medik. (Brassicaceae). Implications of the host-parasitoid associations are discussed relative to the introduction of species to North America for classical biological control of the cabbage seedpod weevil.

Insect Pest Incidence and Injury to Herbicide‐Tolerant Canola in Western Canada

Genetically modified herbicide‐tolerant (HT) canola (Brassica napus L.) cultivars dominate the acreage planted to this oilseed crop in western Canada. We conducted a 3‐yr (2000–2002) field study near Lethbridge, AB, Canada to compare the incidence and damage of three key insect pests in Roundup Ready, Liberty Link (both HT), and Q2 (a conventional cultivar). Flea beetle (Phyllotreta cruciferae and P. striolata [Coleoptera, Chrysomelidae]) damage in 2000 ranged from 31 to 32% of foliage consumption at the cotyledon stage, which surpassed economic thresholds but was not related to cultivar. Cabbage seedpod weevil (Ceutorhynchus obstrictus Marsham [Coleoptera, Curculionidae]) damage to canola pods ranged from 58 to 70 holes per 100 pods in 2001, and lygus plant bugs (Lygus spp. [Heteroptera, Miridae]) at the early pod stage reached densities of &gt;37 individuals per 10 sweeps in 2000 and 2002. For both insect pests, their damage or abundance was similar among the conventional and HT cultivars. We conclude that, in the short term, the insect pests common in southern Alberta do not exhibit a preference to invade and damage the transgenic herbicide canola cultivars commonly planted in the prairies.

Managing cabbage seedpod weevil in canola using a trap crop—A commercial field scale study in western Canada

Cabbage seedpod weevil is a relatively new pest of canola (oilseed rape) in the Prairie Ecozone of western Canada. In some years, in southern Alberta, farmers have had to resort to costly aerial insecticide applications to protect their canola crops. In this study, we evaluated perimeter trap cropping as a strategy to manage weevils in large commercial fields of canola. A total of 11 experimental fields (with perimeter trap crops) and nine conventional fields without trap crops were studied from 2000 to 2003. Field perimeters of Brassica rapa planted at the same time as the main crop of B. napus flowered ∼1 week earlier and effectively concentrated weevil populations that were sprayed with insecticide. The perimeter trap crop kept weevil populations below threshold levels in the main crops when fields were large and square (1600m×1600m). However, further research is required to develop management tools that augment the efficacy of trap crops in situations where fields are small or narrow (<400m wide), especially when populations are several times the economic threshold. Pod damage was less than 12% in the main crops of experimental fields where only the trap crops were sprayed which was comparable to that of conventionally managed fields. The savings in chemical insecticides, labor and time, as well as possible integration with other IPM strategies such as biocontrol, make perimeter trap cropping an important tool for cabbage seedpod weevil management.

Introgression of Resistance to Cabbage Seedpod Weevil to Canola from Yellow Mustard

The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham), is an insect pest of major economic importance in the production of canola (Brassica napus L. and B. rapa L.) in Europe and North America. Studies were conducted to test intergeneric germplasm, produced by crossing Sinapis alba L. (yellow mustard) × B. napus and then backcrossing progeny to the B. napus parent, as potential sources of resistance to the weevil. Of 230 intergeneric genotypes evaluated in field trials in 2001, 18 had an average of fewer than 0.05 weevil exit holes per pod, a level of resistance selected as conferring appropriate resistance and used for further testing. Subsequent tests confirmed several genotypes that evidently carried genes for resistance to C. obstrictus from the S. alba parent. In field assessments, mean exit holes per pod in the common commercial variety, B. napus cv. Q2, ranged from approximately 2.5 to 8.3 times more than those observed in the resistant lines. Some genotypes appeared to exhibit both antixenotic and antibiotic resistance to C. obstrictus, as indicated by fewer eggs laid per pod and larval development that was significantly lengthened in the resistant germplasm compared with the B. napus check. Introgression produced several genotypes with resistance to cabbage seedpod weevil that can now be crossed with agronomically superior B. napus germplasm; when the weevil‐resistant canola is available to producers it can comprise an important component in the integrated management of this pest, resulting in substantial reductions in insecticide use in this crop.

Glucosinolate profile and oviposition behavior in relation to the susceptibilities of Brassicaceae to the cabbage seedpod weevil

AbstractUnderstanding how host‐plant characteristics affect behavioral and physiological responses of insect herbivores is of considerable importance in the development of resistant crop germplasm. Feeding, oviposition preference, larval development, and oviposition behavior of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (= Ceutorhynchus assimilis Payk.) (Coleoptera: Curculionidae), were investigated on eight Brassicaceae species that differed in their glucosinolate profiles. The least preferred host plants for feeding and oviposition were the Sinapis alba L. lines while the Brassica carinata L. line was most preferred. Larval development occurred most rapidly on Brassica rapa L. and slowest on S. alba. Larval weight was highest on B. napus L. and lowest on S. alba. Total glucosinolate levels did not influence C. obstrictus larval growth or development; however high levels of specific glucosinolates such as p‐hydroxybenzyl and 3‐butenyl glucosinolate were associated with increased developmental time or reduced weight. The time required for oviposition behavioral events was measured on different host‐plant species: B. rapa, B. napus, B. napus×S. alba, B. tournefortii Gouan., B. juncea (L.) Czern, B. carinata, B. nigra (L.) Koch., and S. alba. The early steps in the sequence were completed faster on more susceptible host plants (B. carinata, B. napus, and B. rapa) than on relatively resistant ones (B. tournefortii and B. juncea). Females explored pods of B. nigra and S. alba, but oviposition occurred only rarely on these species. There was no significant difference in the location on the pod on which oviposition occurred among the different plant species. Mean eggs laid per female weevil were highest on the B. napus×S. alba hybrid and lowest on B. nigra and S. alba.

The spatio-temporal distribution dynamics of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Coleoptera: Curculionidae), and its larval parasitoids in canola in western Canada

Distribution patterns of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), and its larval parasitoids were investigated in commercial fields of spring canola (Brassica rapa L. and Brassica napus L.) in southern Alberta, Canada, from 2002 to 2004 in relation to developmental stages of its host plants. Adult weevils invaded fields along one or more fronts when crops were in bud to early flower. Significant clustering of adults along field edges in early stages of invasion was followed by more homogeneous distributions as canola reached the mid to late flowering and pod enlargement stages. Larval weevil distributions, as indicated by exit holes in siliques at the end of the season, were often aligned spatially with adult distributions, but they did not coincide in all regions of the fields. The primary ectoparasitoid species attacking weevil larvae comprised Necremnus tidius (Walker) (Hymenoptera: Eulophidae), and Trichomalus lucidus (Walker), Chlorocytus sp., and Pteromalus sp. (Hymenoptera: Pteromalidae). Parasitism rates increased from 0.1 to 5.0% over the three years of study. Parasitoid distributions were often, but not consistently, spatially associated with high densities of C. obstrictus larvae. Lack of close spatial alignment of parasitoids and their hosts probably reflects low parasitoid numbers in comparison with an abundant resource of weevil larvae, and a lack of co-evolutionary history between host and parasitoids. Some parasitoids invaded fields early in host plant development, at the same time that weevils invaded. Unfortunately the synchronous invasions of host and parasitoids indicate that insecticidal applications to reduce adult weevil infestations may be detrimental to these beneficial species.