Responses of the parasitoids of Delia radicum (Diptera: Anthomyiidae) to the vegetational diversity of intercrops

Several natural enemies regulate populations of root maggots (Delia spp.) (Diptera: Anthomyiidae) in canola (Brassica napus L.) in western Canada, among them the rove beetles Aleochara bilineata Gyllenhal and Aleochara verna Say (Coleoptera: Staphylinidae) and the hymenopteran Trybliographa rapae Westwood (Hymenoptera: Figitidae). Intercrops of canola and wheat (Triticum aestivum L.) can be part of an integrated pest management strategy to reduce damage by Delia spp. to canola. We investigated several intercropping regimes of canola and wheat to determine effects on parasitism of Delia radicum (L.) and activity densities of adult A. bilineata and A. verna. Studies were conducted over four site-years in central Alberta, Canada in 2005 and 2006. Mean parasitism rates of D. radicum puparia by A. bilineata ranged from 7.27% to 81.69%. Increasing proportions of wheat in intercrops significantly reduced parasitism by A. bilineata in one site-year. Parasitism of D. radicum by T. rapae was not affected by intercropping; mean parasitism rates were between 2.17% and 14.55%. In one site-year combined parasitism by all parasitoids significantly increased with increasing canola as a proportion of total crop plant populations. Pitfall trap collections of adult A. bilineata increased with increasing proportions of canola in some site-years. Collections of A. verna adults were low relative to A. bilineata and were largely unaffected by intercropping. Although canola–wheat intercrops do not appear to favour parasitism of D. radicum, reductions in canola root damage by Delia larvae in intercrops, reported previously, suggest that canola–wheat intercrops may nevertheless be favourable as a crop protection strategy.     

Effects of fall and spring seeding date and other agronomic factors on infestations of root maggots, Delia spp. (Diptera: Anthomyiidae), in canola

Several agronomic benefits can result from fall seeding of canola (Brassica spp.), but extensive research data are lacking on the potential impact of this practice on infestations of root maggots (Delia spp.) (Diptera: Anthomyiidae), which are major pests of the crop in western Canada. Field experiments making up 13 location by year combinations were conducted in central Alberta, Canada, from 1998 to 2001 to determine the effect of fall versus spring seeding of canola on root maggot damage. Depending on the experiment, interactions with seeding rate, seed treatment, timing of weed removal, and canola species (cultivar) also were investigated. Root maggot damage declined with an increase in seeding rate for plots seeded in May but not in fall or April. Susceptibility to infestation was greater for plants of Brassica rapa L. than Brassica napus L., but seed treatment had no effect on damage by these pests. Combined analysis using data from all experiment by location by year combinations indicated that seeding date had no significant effect on root maggot damage. The extended emergence of Delia spp. adults, which spans the appearance of crop stages vulnerable to oviposition regardless of seeding date, prevented reduced root maggot attack. Covariance analysis demonstrated the importance of increasing seeding rate for reducing root maggot infestations, a practice that can be especially beneficial for May-seeded canola when growing conditions limit the ability of plants to compensate for root maggot damage. Results determined with the small plot studies described here should be validated in larger plots or on a commercial field scale, but both the combined and covariance analyses indicate that seeding canola in fall does not predispose plants to greater damage by larval root maggots than seeding in spring.     

Dispersal ofAleochara bilineata [Col.: Staphylinidae] following inundative releases in urban gardens

Aleochara bilineata (Gyllenhal) [Coleoptera: Staphylinidae] is a common predator and endoparasite of root maggot [Diptera: Anthomyiidae] in both commercial crops and home gardens. To test dispersal activity ofA. bilineata in home gardens, marked beetles were released at rates of 0 and 1,000 in 1987 and 0, 250, 500, and 1,000/gardens/wk in 1988. Three percent of marked beetles were recaptured in release gardens. NaturalA. bilineata populations were very small in all gardens, and there was no detectable increase from 1987 to 1988. Recaptures were proportional to release rates. There was no significant difference in recapture rates between sexes. Recaptures and non-uniform distributions of markedA. bilineata in control gardens revealed that they were capable of flying at least 5 km under urban conditions, and of selecting particular gardens as suitable mating, foraging, and oviposition sites.      

Pronounced effects of slug herbivory on seedling recruitment of Brassica cultivars and accessions,especially those with low levels of aliphatic glucosinolates

In demographic studies on Brassica cultivars and accessions we observed large genotypic variation in the ability of seedlings to establish. Here we quantify the role of slugs during establishment by including and excluding slug herbivory.Slug exclusion during the first 10 days after germination led to 26.5 times more surviving seedlings in modern canola varieties of Brassica napus, 4.3 times more in feral B. napus and 1.9 times more in wild Brassica rapa. While seed and seedlings of canola were low in aliphatic glucosinolates (AGS), feral B. napus and wild B. rapa had much higher AGS levels. Consequently, we find a positive correlation between the survival of young seedlings and AGS concentration. Concentrations of indole glucosinolates (IGS) in seeds were much lower than those of AGS and did not correlate significantly with survival. Subsequently, we exposed 10-day-old seedlings to herbivory. In the 4 days following exposure, slug herbivory was negatively correlated to AGS concentration but not to IGS.In choice experiments in the lab, the preference of the slug Arion lusitanicus also correlated negatively with AGS content. A. lusitanicus preferred to feed on B. napus rather than on wild B. rapa, but had no significant preference when presented with plants similar in AGS content.Slugs can be a limiting factor for seedling recruitment in populations of B. napus, especially for modern canola cultivars with a low AGS content.     

Canola (Brassica napus) germplasm shows variable allelopathic effects against annual ryegrass (Lolium rigidum)

Aims

The allelopathic activity of canola (Brassica napus) germplasm was investigated using the important Australian weed, annual ryegrass (Lolium rigidum) as the target species.

Methods

Three different canola plant densities (10, 20, and 30 seedlings/beaker) of each of 70 world-wide genotypes were tested in vitro in close proximity to annual ryegrass seedlings.

Results

The allelopathic activity of canola, as measured by reduction in annual ryegrass root and shoot growth, increased with canola crop seedling densities. Density did not consistently influence shoot length of annual ryegrass. Greater shoot length suppression was observed in genotype cv. Rivette and BLN3343CO0402. The Australian genotype cv. Av-opal and the breeding line Pak85388-502 suppressed root length of ryegrass more than other genotypes, even at low densities. At the lowest density, the least allelopathic genotypes were cv. Barossa and cv. Cescaljarni-repka, although they became more allelopathic at higher density. An overall inhibition index was calculated to rank each of the canola genotypes. There were significant differences between canola genotypes in their ability to inhibit root and shoot growth in ryegrass.

Conclusion

Considerable genetic variation exists among canola genotypes for their allelopathic effects on annual ryegrass. Further investigation is required to determine the allelopathic mechanisms, particularly to identify the responsible allelochemical(s) and the gene(s) controlling the trait. This research suggests that highly allelopathic canola genotypes can be potential for controlling weeds such as annual ryegrass in integrated weed management programs.     

Characterization of directly transformed weedy <Emphasis Type="Italic">Brassica rapa</Emphasis> and introgressed <Emphasis Type="Italic">B. rapa</Emphasis> with Bt <Emphasis Type="Italic">cry1Ac</Emphasis> and <Emphasis Type="Italic">gfp</Emphasis> genes

Crop to weed transgene flow, which could result in more competitive weed populations, is an agricultural biosafety concern. Crop Brassica napus to weedy Brassica rapa hybridization has been extensively characterized to better understand the transgene flow and its consequences. In this study, weedy accessions of B. rapa were transformed with Bacillus thuringiensis (Bt) cry1Ac- and green fluorescence protein (gfp)-coding transgenes using Agrobacterium to assess ecological performance of the wild biotype relative to introgressed hybrids in which the transgenic parent was the crop. Regenerated transgenic B. rapa events were characterized by progeny analysis, Bt protein enzyme-linked immunosorbent assay (ELISA), Southern blot analysis, and GFP expression assay. GFP expression level and Bt protein concentration were significantly different between independent transgenic B. rapa events. Similar reproductive productivity was observed in comparison between transgenic B. rapa events and B. rapa × B. napus introgressed hybrids in greenhouse and field experiments. In the greenhouse, Bt transgenic plants experienced significantly less herbivory damage from the diamondback moth (Plutella xylostella). No differences were found in the field experiment under ambient, low, herbivore pressure. Directly transformed transgenic B. rapa plants should be a helpful experimental control to better understand crop genetic load in introgressed transgenic weeds.     

Use of life tables to predict the impact of introducing exotic parasitoids,against the cabbage seedpod weevil in North America

ABSTRACT

Cabbage seedpod weevil, Ceutorhynchus obstrictus (Coleoptera: Curculionidae), is an invasive alien species that impacts seed production of canola, Brassica napus and B. rapa (Brassicaceae), one of Canada’s major agricultural crops. To determine the need for and potential impact of introduction of one or more natural enemies for biological control it is essential to understand the mortality factors associated with C. obstrictus both in the native range and in the introduced range. Life tables were constructed for Ontario and British Columbia populations to determine the mortality factors affecting C. obstrictus. These were compared with life tables generated in Switzerland, the area of origin of the weevil. Results indicated that introduction of an exotic specialist parasitoid into canola growing regions of Canada would substantially reduce C. obstrictus populations. This study is the first to compare life tables of an invasive pest in the areas of origin and introduction.     

Gene transfer between canola (Brassica napus L. and B. campestris L.) and related weed species

Brassica species are particularly receptive to gene transformation techniques. There now exists canola genotypes with transgenic herbicide resistance for glyphosate, imidazolinone, sulfonylurea and glufosinate herbicides. The main concern of introducing such herbicide resistance into commercial agriculture is the introgression of the engineered gene to related weed species. The potential of gene transfer between canola (Brassica napus and B. campestris) and related weed species was determined by hand pollination under controlled greenhouse conditions. Canola was used as both male and female parent in crosses to the related weed species collected in the Inland Northwest region of the United States. Weed species used included: field mustard (B. rapa), wild mustard (S. arvensis) and black mustard (B. nigra). Biological and cytological aspects necessary for successful hybrid seed production were investigated including: pollen germination on the stigma; pollen tube growth down the style; attraction of pollen tubes to the ovule; ovule fertilisation; embryo and endosperm developmental stages. Pollen germination was observed in all 25 hybrid combinations. Pollen tubes were found in the ovary of over 80% of combinations. About 30% of the hybrid combinations developed to the heart stage of embryo development or further. In an additional study involving transgenic glufosinate herbicide resistant B. napus and field mustard it was found that hybrids occurred with relatively high frequency, hybrids exhibited glufosinate herbicide resistance and a small proportion of hybrids produced self fertile seeds. These fertile plants were found to backcross to either canola or weed parent.     

Interactions between an eucoilid [Hymenoptera] and a staphylinid [Coleoptera] parasitoid of the cabbage root fly

The interaction betweenTrybliographa rapae andAleochara bilineata, 2 parasitoids of the cabbage root fly, is discussed. Larvae ofA. bilineata could not differentiate between cabbage root fly pupae containingT. rapae in its endoparasitic state and unparasitized pupae but could recognize pupae containingT. rapae once the latter had reached its ectoparasitic state. Attack byA. bilineata whileT. rapae was still in its endoparasitic state usually resulted in the staphylinid killing the eucoilid. IfT. rapae had reached the ectoparasitic state before the host pupa was attacked byA. bilineata larvae the eucoilid survived attack by the beetle larva. Multiparasitism, however, resulted in increased levels of mortality of both parasitoid populations.      

Life history comparison between two competitive Aleochara species in the cabbage root fly, Delia radicum: implications for their use in biological control

This study evaluates the efficacy of Aleochara bilineata Gyll and Aleochara bipustulata L. (Coleoptera: Staphylinidae) as biological control agents against the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae). Biological and demographic characters are documented and comparisons made between the two species. The following results were obtained:(1) The reproductive potential and longevity of A. bipustulata females are significantly higher than those of A. bilineata. These differences are correlated with the biology and ecology of the hosts of both species.(2) Under laboratory conditions the rate of increase (r_m) of both staphylinids species is significantly higher than that of the pest and their major competitor Trybliographa rapae West. (Hymenoptera:Figitidae). This may indicate that A. bilineata and A. bipustulata can be effective biological control agents.(3) Aleochara bilineata appears to be the most interesting biological control agent against D. radicum, because of its higher rate of increase, higher host specificity and host acceptance, and a development time which is well synchronised with that of its host. With its higher competitive ability, a population of A. bilineata may increase even if competition occurs. However, A. bipustulata presents interesting biocontrol characteristics, but the best strategy may be only to use the potential of its natural populations.     

The relative attractiveness of Brassica napus,B. rapa,B. juncea and Sinapis alba to pollen beetles

It is often suggested that weeds from the same family as the crop plant may increase insect pest damages by providing shelter and additional oviposition opportunities. We compared the relative attractiveness of Brassica rapa L., B. juncea L., Sinapis alba L. and B. napus L. (Capparales: Brassicaceae) to the pollen beetle and its hymenopteran parasitoids in field conditions. Our results revealed that none of the investigated plants increased the pest abundance on B. napus plants. On the contrary, B. juncea and S. alba lured beetles away from B. napus during its damage-susceptible stage. The parasitism rate of pollen beetle larvae was the highest on B. juncea plants, indicating that cruciferous weeds could improve the natural control of the pollen beetle by providing additional hosts for parasitoids. Therefore, close relatives of oilseed rape might be used to trap pollen beetle adults, but also to support populations of natural enemies that could decrease the number of beetles.     

Hybridisation between oilseed rape (Brassica napus) and tetraploid Brassica rapa under field conditions

Cross-compatible relatives of crop species contribute to the uncertainty regarding the potential risk of transgene escape from genetically modified varieties. The most successful crossing partner of oilseed rape (Brassica napus L.) is diploid Brassica rapa L. Variation of ploidy level among B. rapa cultivars has, until recently, been neglected in the context of gene flow and hybridisation with oilseed rape. We estimated the extent of hybridisation between autotetraploid B. rapa varieties (female) and B. napus (pollen donor) under experimental field conditions. Morphology, variation of relative DNA amount, and microsatellite markers were used to distinguish between intraspecific offspring of tetraploid B. rapa and interspecific hybrids with B. napus. Of 517 seed progenies of tetraploid B. rapa, 45 juvenile plants showed species specific morphological traits of oilseed rape. The detection of putative hybrids based on variation in relative DNA amounts was problematic due to the occurrence of aneuploidy. In total, 84 offspring showed relative DNA amounts deviating from tetraploid B. rapa, four of which were hexaploids. Of the 205 offspring analysed at three microsatellite loci, 67 had oilseed rape alleles. Based on molecular evidence a minimum hybridisation rate of 13.0% was estimated. A few mother plants accounted for the majority of hybrids. The mean pollen viability of hybrids between B. napus and tetraploid B. rapa (80.6%) was high in comparison with mean pollen viability of triploid hybrids between B. napus and diploid B. rapa. Therefore, the occurrence of tetraploid B. rapa should be taken into consideration when estimating the likelihood of gene flow from oilseed rape to close relatives at the landscape level. Tetraploid B. rapa is a common component of several seed mixtures and establishes feral populations in northwest Germany. Assuming a similar abundance of diploid and tetraploid B. rapa, gene flow from B. napus to tetraploid may be more likely than gene flow to diploid B. rapa.     

Reduced Susceptibility of Brassica napus to Pratylenchus neglectus in Plants with Elevated Root Levels of 2-Phenylethyl Glucosinolate

The effect of canola (Brassica napus) as a crop suppressive to Pratylenchus neglectus is in part due to the release of nematicidal isothiocyanates, particularly 2-phenylethyl isothiocyanate, from degrading root tissues. However, many cultivars of canola are relatively susceptible to P. neglectus and will fail to reduce soil populations of the nematode. A survey of B. napus accessions and closely related species revealed limited scope to decrease the susceptibility of canola through conventional intercrossing. Susceptibility to P. neglectus was not related to the total glucosinolate levels, but there were signifi- cant, negative correlations (r = -0.619, -0.517; P < 0.001) between root levels of 2-phenylethyl glucosinolate (isothiocyanate precursor) and plant susceptibility to P. neglectus: plants containing more than a certain threshold level of 2-phenylethyl glucosinolate showed reduced susceptibility to the nematode. Selection for high root levels of 2-phenylethyl glucosinolate should reduce the susceptibility of the plants during the growing season while also increasing the nematicidal impact of the degrading root tissues, thereby improving the suppressive benefits of the crop when used in rotation with cereals.     

Host range testing of a prospective classical biological control agent against cabbage maggot,Delia radicum,in Canada

The introduction of a European natural enemy, Aleochara bipustulata L. (Coleoptera: Staphylinidae), is being considered for control of cabbage maggot, Delia radicum (L.) (Diptera: Anthomyiidae) in canola in Canada, and the host specificity of this pupal parasitoid must first be studied. Contemporary guidelines were used to select 18 species of Diptera to represent non-target species taxonomically related or ecologically similar to reported hosts of A. bipustulata, or beneficial species. No-choice tests were used to determine which of the 18 species are within A. bipustulata’s fundamental host range, and whether puparial structure or mass or duration of pupal development influence their acceptability and suitability as hosts. Five species were consistently suitable as hosts, and these were either relatively small or were taxonomically closely related to the target host. The probability that a puparium would be entered by a parasitoid larva was greatest for small puparia, but was unaffected by pupal duration. The probability of completing parasitoid development once a puparium was entered was influenced by both puparial mass and duration of pupal development. Pitfall traps to assess habitat associations caught A. bipustulata adults in a variety of crop habitats but none were caught in forests. Host range and habitat data are used to argue that there is little risk of parasitism to beneficial taxa. Non-target species taxonomically closely related to the target D. radicum or with small puparia may fall within the fundamental host range of A. bipustulata. However, risk to many of these species may be minimal because of the habitat preferences of the parasitoid and its cues for host-finding and recognition.     

Additive transgene expression and genetic introgression in multiple green-fluorescent protein transgenic crop × weed hybrid generations

The level of transgene expression in crop × weed hybrids and the degree to which crop-specific genes are integrated into hybrid populations are important factors in assessing the potential ecological and agricultural risks of gene flow associated with genetic engineering. The average transgene zygosity and genetic structure of transgenic hybrid populations change with the progression of generations, and the green fluorescent protein (GFP) transgene is an ideal marker to quantify transgene expression in advancing populations. The homozygous T₁ single-locus insert GFP/Bacillus thuringiensis (Bt) transgenic canola (Brassica napus, cv Westar) with two copies of the transgene fluoresced twice as much as hemizygous individuals with only one copy of the transgene. These data indicate that the expression of the GFP gene was additive, and fluorescence could be used to determine zygosity status. Several hybrid generations (BC₁F₁, BC₂F₁) were produced by backcrossing various GFP/Bt transgenic canola (B. napus, cv Westar) and birdseed rape (Brassica rapa) hybrid generations onto B. rapa. Intercrossed generations (BC₂F₂ Bulk) were generated by crossing BC₂F₁ individuals in the presence of a pollinating insect (Musca domestica L.). The ploidy of plants in the BC₂F₂ Bulk hybrid generation was identical to the weedy parental species, B. rapa. AFLP analysis was used to quantify the degree of B. napus introgression into multiple backcross hybrid generations with B. rapa. The F₁ hybrid generations contained 95–97% of the B. napus-specific AFLP markers, and each successive backcross generation demonstrated a reduction of markers resulting in the 15–29% presence in the BC₂F₂ Bulk population. Average fluorescence of each successive hybrid generation was analyzed, and homozygous canola lines and hybrid populations that contained individuals homozygous for GFP (BC₂F₂ Bulk) demonstrated significantly higher fluorescence than hemizygous hybrid generations (F₁, BC₁F₁ and BC₂F₁). These data demonstrate that the formation of homozygous individuals within hybrid populations increases the average level of transgene expression as generations progress. This phenomenon must be considered in the development of risk-management strategies.Communicated by J. Dvorak     

Development of a chloroplast DNA marker for monitoring of transgene introgression in Brassica napus L.

Chloroplast molecular markers can provide useful information for high-resolution analysis of inter- and intra-specific variation in Brassicaceae and for differentiation between its species. Combining data generated from nuclear and chloroplast markers enables the study of seed and pollen movement, and assists in the assessment of gene-flow from genetically modified (GM) plants through hybridization studies. To develop chloroplast DNA markers for monitoring of transgene introgression in Brassica napus L., we searched for sequence variations in the chloroplast (cp) genome, and developed a simple cpDNA marker that is reliable, time-saving, and easily discriminates among 4 species (B. napus, B. rapa, Raphanus sativus, and Sinapis alba) based on PCR-product length polymorphism. This marker will be useful to identify maternal lineages and to estimate transgene movement of GM canola.     

Phyllotreta cruciferae and Phyllotreta striolata responses to insecticidal seed treatments with different modes of action

The strategy used most commonly in western North America to protect seedlings of canola (Brassica rapa L. and Brassica napus L.) from attack by adults of the flea beetles Phyllotreta cruciferae (Goeze) and Phyllotreta striolata (Fabricius) (Coleoptera: Chrysomelidae) involves planting seed coated with insecticide for systemic activity. Previous research determined that the two beetle species responded differently to the most commonly used neonicotinoid seed dressings. However, other insecticides that exploit different modes of action have commercial potential for managing infestations of these pests, but no information exists on their efficacies for these flea beetle species. Studies were conducted to compare effects of the neonicotinoid compounds, thiamethoxam and imidacloprid, to spinosyn and fipronil as systemic seed treatments for reducing feeding damage to canola seedlings and increasing mortality of P. cruciferae and P. striolata. Phyllotreta cruciferae experienced greater mortality and caused less feeding damage than P. striolata to seedlings treated with the neonicotinoid compounds. Mortality increased and feeding damage decreased significantly when beetles fed upon seedlings treated with fipronil, indicating its potential usefulness for control of these pests. However, spinosyn seed treatment was relatively ineffective against either beetle species. Higher rates of P. striolata mortality with fipronil than thiamethoxam suggest that fipronil may provide improved flea beetle control over hundreds of thousands of hectares in western North America where flea beetle populations are dominated by P. striolata, and control with thiamethoxam has been suboptimal.     

GM risk assessment: Pollen carriage from Brassica napus to B. rapa varies widely between pollinators

Characterizing insect pollen carriage between closely related plant species is especially challenging where source species possess morphologically identical pollen and share many pollinators in common. Here, we use an SNP-based assay using the plant DNA barcoding locus matK to characterize pollen carriage between cultivated Brassica napus and wild Brassica rapa in three sites across southern England. The assay differentiated B. napus and B. rapa pollen carried by honey bees (Apis melifera), bumblebees (Bombus spp.), mining bees (Andrena spp.) and hoverflies (Syrphidae) captured on B. napus plants 1–2 m from wild B. rapa, and on B. rapa plants at various distances from the crop . Apis individuals foraging on B. rapa and carrying B. napus pollen were rarely found beyond 10 m from the crop. However, Bombus and Andrena individuals captured on B. rapa occasionally carried crop pollen up to 300 m from the source field. Hoverflies carried less pollen overall but featured high proportions of B. napus pollen even at the most distant capture points. We predict that different pollinator species will evoke markedly different patterns of interspecific hybrid formation. We conclude that more exhaustive surveys of this kind will help parameterize future mechanistic models to predict the distribution of hybrids between Genetically Modified B. napus and B. rapa on a landscape scale.