Delia Radicum Research Articles

Geographical origin of an introduced pest species, Delia radicum (Diptera: Anthomyiidae), determined by RAPD analysis and egg micromorphology

The origin of introduction of the cabbage root fly, Delia radicum Linnaeus to the north-eastern coast of North America in the 19th century has been assumed to be from Europe. From that point of introduction, D. radicum gradually spread westward to occupy available ecological niches. DNA fingerprinting and egg micromorphology were used to determine the most likely geographical origin of the North American populations of this species. Forty-five informative RAPD loci obtained from ten primers and three criteria for egg micromorphology were studied. These characters indicated a common origin for the North American populations and a high similarity between populations from North America and north-western Europe. The results suggest a single entrance point of D. radicum into North America, probably via the north-eastern coast (New York area) from north-western Europe. The implications of this study in assisting selection of natural enemies of this important agricultural pest are discussed.

Natural Coprevalence of Strongwellsea castrans, Cystosporogenes deliaradicae, and Bacillus thuringiensis in the Host, Delia radicum

Adult cabbage root flies (Delia radicum) from three Danish localities were diagnosed microscopically for the natural prevalence of Strongwellsea castrans, Cystosporogenes deliaradicae, and Bacillus thuringiensis. C. deliaradicae was significantly coprevalent with S. castrans. B. thuringiensis sporangia were diagnosed in the hemolymph in two D. radicum which were also infected with S. castrans and proved to belong to serovar aizawai and serovar balearica. The biological characterization of S. castrans proved that at 17.5°C flies developed an abdominal hole 7.9 days (mean) after infection and that 5.7 days (mean) passed from the emergence of the hole to the death of the infected host. No mortality effect among D. radicum subjected to B. thuringiensis serovar aizawai, balearica, and kurstaki isolates was detected. RAPD with DNA proved that six B. thuringiensis serovar balearica isolates (all from the same fly) were indistinguisable. This indicates that proliferation of B. thuringiensis in the abdomen of an S. castrans-infected D. radicum may be due to just one genotype. The profiles of one isolated aizawai strain did not correspond to the profiles of other serovar aizawai strains used for comparison. The biological significance of the interaction between the involved pathogens is discussed.

Fences Excluding Cabbage Maggot Flies and Tiger Flies (Diptera: Anthomyiidae) from Large Plantings of Radish

The influence of a specially constructed insect exclusion fence on the number of cabbage maggot flies, Delia radicum (L.), and tiger flies Coenosia tigrina (F.) (a predator of cabbage maggot flies) entering large plots of radish was determined. Plots with or without the exclusion fence or with an exclusion fence surrounded by a trap crop of radish were evaluated in a single growing season in 1995. Overall during the growing season, the mean cabbage fly catch per trap in the completely fenced plots was 90% fewer than in the unfenced control plots. Fewer cabbage flies were caught inside the completely fenced plots than inside the fence surrounded by a trap crop, suggesting that trap crops do not ultimately impede the movement of cabbage flies into fenced enclosures. The general pattern of trap catch of tiger flies in the various treatments was similar to that observed for cabbage flies. During the heaviest infestation of cabbage maggot flies, infestation was highest in the control plots (84.5%), followed by the fence + trap crop plots with 39.8% infested radishes, and the completely fenced plots with 24.7% infestation. The potential of exclusion fences for use in pest management programs for Brassica crops is discussed.

The influence of odour on the oviposition behaviour of the cabbage root fly

Host plant odours are known to be important in long-range host location by the cabbage root fly, whereas at short distances orientation is mainly visual. We show that olfaction also plays a significant role after a fly lands on a plant and before it moves down onto the soil to oviposit. Host plant acceptance by the cabbage root fly seems to result from a synergistic response to simultaneously perceived olfactory and contact chemostimulation.

Factors affecting ovipositor probing in Trybliographa rapae, a parasitoid of the cabbage root fly

AbstractThe effect of allelochemicals from its host, the larva of the cabbage root fly, Delia radicum (L.) (Diptera: Anthomyiidae), and the host‘s food plant on the ovipositor probing response of the parasitoid Trybliographa rapae (Westw.) (Hymenoptera: Cynipidae) were investigated. Trybliographa rapae probed both cabbage root fly infested and uninfested swede (Brassica napus var. napobrassica), although significantly more wasps responded to infested swede. Antennal sensilla are likely to be the mediators of this response. The synomones and kairomones involved are extractable in water, diethyl ether and methanol. No response was observed to washed, starved cabbage root fly larvae. Wasps spent significantly longer searching infested swede than uninfested, although probing frequency remained constant. It is suggested that the initiation of probing in T. rapae is dependent on a threshold concentration of general synomones or host related synomones and kairomones, whereas time spent searching a particular area is dependent on the environment perceived by sensilla on the ovipositor.

Screening of field performance of entomopathogenic fungi and nematodes against cabbage root flies (Delia radicum l. and d. floralis (fall.); Diptera, Anthomyiidae)

Finnish isolates of Beauveria bassiana (8 isolates), Metarhizium anisopliae (7), Tolypocladium sp. (2), Paecilomyces farinosus (2), P. fumosoroseus (1), Steinernema feltiae (3) and Bacillus thuringiensis ('Muscabac') were tested for efficacy against mixed populations of Delia radicum and D. floralis under field conditions in 1986–90. All pathogens were applied preventatively, the fungi as aqueous conidial or mycelial suspensions, dry conidia or dry mycelial powder. In only two of the nine experiments did B. bassiana or M. anisopliae give some control. In 1986, B. bassiana SF85–2 and Tolypocladium sp. SF85–4 (both at rate 1.2 x 109 conidia plant‐ 1), and ‘Muscabac’ (25 g 1−1, 1 dl plant−1) reduced the number of pupae by 80%, 60% and 50%, respectively, as compared with untreated and chemical (isophenphos) controls. In 1990, M. anisopliae SF86–39 at rate 1.6 x 109conidia plant−1 and 1.5 x 108 CFU plant−1 and S. feltiae SFS‐22 (35000 plant−1), increased the yield of cauliflower 2.2, 1.8, and 2.3‐fold, respectively, as compared with the untreated control, but these yields were only 19%, 15% and 19% of those of the chemical (diazinon) control. Paecilomyces isolates were ineffective in the 1986 experiment in which they were included. Our results suggest that it is difficult to find efficient control agents among the fungal and nematode species tested for use as biopesticides against cabbage root flies, but that the potential of M. anisopliae against these pests deserves further study.

The relationship between cabbage root fly (Delia radicum) larval feeding and the freeze‐dried matter and sugar content of Brassica roots

AbstractFive genotypes of swede (Brassica napus var. napobrassica), two genotypes of kale (B. oleracea var. acephala), and two genotypes of rape (B. napus var. napus) were each inoculated at the 8–10 true leaf stage with five cabbage root fly (Delia radicum) eggs. The percentage pupation after larval feeding on individual plant genotypes ranged from 45 to 78%, and the mean pupal weight from 6.5 to 13.0 mg. After 5 weeks, larval feeding damage had reduced root weight by up to 47%, compared with uninoculated plants. The dry matter content of undamaged roots was higher in the kales and rapes than in the swedes. Whilst the dry matter content of the rapes and swedes were not changed by D. radicum damage, that of the kales was elevated. The ethanol‐soluble sugar content of the root was reduced in all cases by D. radicum larval damage. However, the effect of D. radicum damage on the concentrations of individual sugars (glucose, fructose and sucrose) was crop‐ and genotype‐dependent. In the roots of kales and rapes, the glucose and fructose concentrations were either very low or unaffected by D. radicum damage, whilst both glucose and fructose were generally reduced in swede roots by D. radicum damage. The root sucrose concentration was either reduced or not significantly affected by D. radicum damage in all of the crop types tested. The percentage pupation and the mean pupal weight of D. radicum were inversely correlated to root freeze‐dried matter content. D. radicum pupal weight was positively correlated with root fructose, glucose and ethanol‐soluble sugar contents.

Oviposition stimulants for the cabbage root fly: isolation from cabbage leaves

Abstract Two compounds present on the surface of Brassica oleracea cv. botrytis leaves have been isolated and identified which stimulate very effectively oviposition in the cabbage root fly, Delia radicum and which are perceived by a specific receptor neuron in the tarsal sensillum C5 of the female fly. Activity of extracts and chromatographic fractions were bioassayed, using oviposition experiments and mainly electrophysiological recordings from the C5 tarsal contact chemoreceptor sensillum of female flies. Spectroscopic data indicate that the main compound is 1,2-dihydro-3-thia-4,10,10b-triaza-cyclopenta[.a.]fluorene-1-carboxylic acid, a novel compound related to Brassica phytoalexins like brassicanal C. It is accompanied by its glycine conjugate.

Carabid beetles in sustainable agriculture: a review on pest control efficacy, cultivation impacts and enhancement

This review article on carabids in sustainable agro-ecosystems of the temperate Northern hemisphere presents a compilation of the available knowledge on the significance of carabids for natural pest control and the effects of cultivation methods (except pesticides) and landscape structural elements. Field carabids are species rich and abundant in arable sites, but are affected by intensive agricultural cultivation. For sampling, fenced pitfall trapping or pitfall trapping is recommended according to the type of study. Many of the assumed beneficial pest control activities of carabids are still based on laboratory feeding records. In the field, carabids have been demonstrated to reduce cereal and sugar beet aphid populations in their early colonization phase, mainly by foraging on aphids that have fallen from the vegetation. Egg predation on Dipteran eggs, e.g. the cabbage root fly, has been overestimated in earlier literature. Scattered data indicate carabidforaging on certain coleopteran pest larvae. In North America, some evidence has been found for control of pest lepidopterans. Larger carabids, e.g. Abax parallelepipedus, can effectively control slugs in greenhouses. Because of their spermophagous feeding habits, certain species of Harpalus and Amara could have some potential for biological weed control. As a result of their sensitive reaction to anthropogenic changes in habitat quality, carabids are considered of bioindicative value for cultivation impacts. Carabids seem to be negatively affected by deep ploughing and enhanced by reduced tillage systems. No negative effects have been found for mechanical weed control and flaming. Carabid recruitment is enhanced by proper organic fertilization and green manuring. Intensive nitrogen amendment might indirectly affect carabids by altering crop density and microclimate. Field carabid assemblages are not bound to a certain crop type, but shift in dominance according to the crop-specific rhythmicity of cultivation measures and changes in crop phenology and microclimate. Crop rotation effects could also be influenced by field-size dependent recolonization capability of carabids. They are enhanced by crop diversification in terms of monocrop heterogeneity and weediness as well as by intercropping and the presence of field boundaries, although corresponding increases in their pest reduction efficacy have not yet been evidenced.

Damage Thresholds for Cabbage Root Fly [Delia radicum (L.)] in Cauliflower Assessed from Pot Experiments

The influence of cabbage root fly [Delia radicum (L.)] on cauliflower dry matter production was investigated in three experiments during 1992 and 1993. Cauliflower plants grown outdoors in pots were inoculated with different numbers of cabbage root fly eggs, 1, 2, 3 or 4 weeks after transplanting as seedlings. A non-destructive method was used to estimate dry matter production during the growing season. Regression analyses of plant dry matter against egg number demonstrated that control measures against cabbage root fly larvae are needed to protect young seedlings during the first 3-4 weeks after planting, whereas 100 eggs or more were required to cause a 5% reduction in mean plant dry matter when plants were inoculated 4 weeks after transplanting. Egg and larval mortality varied with means in the range from 47 to 61% and density-dependent larval mortality was demonstrated in two out of four combinations of experiment and inoculation time.

Attempts to control cabbage root flies Delia radicum L. and Delia floralis (Fall.) (Dipt., Anthomyiidae) with entomopathogenic fungi: laboratory and greenhouse tests

One laboratory and three greenhouse experiments were conducted to study the pathogenicity and efficacy of Finnish isolates of entomopathogenic hyphemycetous fungi against cabbage root flies. In Petri dishes, exposure to 1.5 × 1010 spores of Metarhizium anisopliae per dish caused 40–50% mortality of undifferentiated second‐ and third‐stage larvae of Delia floralis, and 1 × 107 spores per dish caused 40–50% mortality of Delia radicum larvae. In one greenhouse test, 1 × 108 and 1 × 109 spores of M. anisopliae and Paecilomyces fumosoroseus reduced the root damage of head cabbage by 20–70% compared with untreated controls, although this was not accompanied by significant reductions in the number of pupae. Only M. anisopliae consistently grew out of larvae and pupae of D. floralis during incubation that followed their recovery from the soil at the end of an experiment testing different formulations of M. anisopliae and Beauveria bassiana, but the frequency of the latent infections of the pest by M. anisopliae was not associated with reduced severity of damage to seedlings of head cabbage.

The decision to reject an oviposition site: sequential analysis of the post‐alighting behaviour ofDelia floralis

Abstract.The post‐alighting behaviour of gravid female turnip root fly, Delia floralis (Fallén) (Diptera: Anthomyiidae), was observed in the laboratory, using four plant genotypes differing in antixenotic resistance, to stimulate a range of oviposition‐related behavioural events. Analysis of the behavioural sequences of individual female flies suggested that during the post‐alighting behaviour of D. floralis, the decision to reject a highly resistant plant was predominantly based on plant cues received during a stationary period immediately after landing on the leaf (the leaf contact phase). Female flies that rejected a plant immediately after the leaf contact phase did so after spending approximately twice as long in the leaf contact phase as those flies that continued to explore the plant. On a plant genotype of intermediate susceptibility, D. floralis females on the stem of the plants increased proboscis extension. The mechanisms involved in early host plant rejection decisions and the possible ecological advantages of such behaviour patterns to female flies seeking suitable oviposition sites are explored. The potential advantages of using detailed studies of individual behaviour sequences to focus studies seeking to elucidate chemical oviposition stimuli on the plant’s surface are also discussed.

Evaluation of felt traps to estimate egg numbers of cabbage root fly, Delia radicum, and turnip root fly, Delia floralis in commercial crops

AbstractThe use of felt traps to estimate oviposition by the cabbage root fly, Delia radicum (L.), and turnip root fly, Delia floralis (Fall.), was compared with soil sampling at seven localities between 1992 and 1994 in Denmark and Norway. In all, 281 comparisons were made, based on 6800 samples. In 4.6% of these comparisons no eggs were found by either method. In 16% of the comparisons, 2.0±0.41 (±S.E.) eggs were found per soil sample and no eggs were found in felt traps, whereas in 0.7% of the comparisons 0.10±0.03 eggs were found per felt trap and no eggs in soil samples. When eggs were found using both methods, the ratio between soil sampling and felt traps varied from 13.1±3.2 when the egg laying rate was very low to 1.8±0.2 at high egg laying rates. Regression analysis showed significant correlation between felt trap catches and soil sampling (P<0.001), whereas locality did not influence felt trap efficiency (P=0.096). The influence of the physical conditions of felt traps, i.e., whether they were clean and dry, were analysed in a single year. Tentative action thresholds for control measures against cabbage root flies and turnip root flies were developed for cauliflower, Brassica oleracea, convar. botrytis (L.) Alef., var. botrytis, based on data from the literature and the present results.

Infochemicals mediating the foraging behaviour of Aleochara bilineata Gyllenhal adults: sources of attractants

Abstract Aleochara bilineata Gyll. (Coleoptera: Staphylinidae) adults feed on eggs and larvae of cabbage maggot, Delia radicum (L.) (Diptera: Anthomyiidae), and might be used to control this pest. We undertook tests in a Y‐tube olfactometer to determine if infochemicals are involved in the food foraging behaviour of A. bilineata adults and to precisely determine the potential sources of stimuli. A. bilineata adults oriented toward the stimuli from both the food‐plant of the prey (rutabaga: Brassica napus var. napobrassica (L.) Reichb.) and the prey‐plant complex (rutabaga infested by cabbage maggot larvae), but adults significantly preferred the effluvia of infested to uninfested rutabaga in a choice test. The sources of infochemicals in the infested rutabaga were the cabbage maggot larvae themselves and their frass. However, in a choice test adults preferred the effluvia of the larvae to that of the damaged rutabaga from which those larvae were obtained. It is not expected that a generalist predator uses precise herbivore‐derived signals in food foraging activity, but the preference of A. bilineata adults for the larval volatiles over frass volatiles may reflect the dependence of this species on dipteran pupae for reproduction.

Identification of high levels of resistance to cabbage root fly, Delia radicum, in wild Brassica species

Wild Brassica species and associated breeding lines were evaluated for their resistance to Delia radicum, the cabbage root fly, in the field in 1993 and in the field, glasshouse and laboratory in 1996. High levels of antibiosis resistance were discovered in the field in 1993 in Brassica fruticulosa, B. incana, B. villosa and B. spinescens and confirmed in the field in 1996 while two B. oleracea accessions and the susceptible control Brussels sprouts variety, ‘Oliver’, were highly susceptible. No D. radicum pupae were found in the soil around the roots of B. fruticulosa and B. spinescens at the end of the season. All the Brassica species were attractive to egg-laying by D. radicum in the field and in a laboratory experiment and therefore lacked antixenosis resistance. In a series of glasshouse experiments, Brassica species and breeding lines were inoculated with D. radicum eggs supplied from a laboratory culture and the effects of larval feeding on plants recorded. Brassica incana, B. fruticulosa and B. spinescens were highly resistant, most plants surviving, whilst B. macrocarpa and B. villosa were moderately resistant as were two F1 lines bred from a cross between B. macrocarpa and B. oleracea.

Phytoalexins from Brassica (Cruciferae) as oviposition stimulants for the cabbage root fly, Delia radicum

Bacteria species known to induce the biosynthesis of crucifer-specific phytoalexins have earlier been shown to be associated with Delia flies. Eleven crucifer-specific phytoalexins and related synthetic compounds were applied on surrogate paper leaves and offered to cabbage root flies in oviposition assays. Since three of these compounds (methoxybrassinin, cyclobrassinin, brassitin) proved to be significantly stimulatory whereas the remaining metabolites had no effect, the reaction of the fly appears to be structure-specific. Inactive phytoalexins had no inhibitory effect on oviposition. 100 μg of methoxybrassinin per surrogate leaf was as stimulatory as 0.05 gle (gram leaf equivalent) of a methanolic host-leaf (Brassica oleracea) extract. Thus the three active phytoalexins can explain only part of the activity of host-pant extracts but might induce a preference for infected plants.

Tillage regime, row spacing, and seeding rate influence infestations of root maggots (Delia spp.) (Diptera: Anthomyiidae) in canola

Infestations of root maggots (Delia spp.) (Diptera: Anthomyiidae) were assessed in Brassica rapa L. and Brassica napus L. grown under conventional and zero tillage regimes, at three row spacings (10, 20 and 30 cm) and three seeding rates (5.0, 7.5 and 10.0 kg ha−1 or 120, 180 and 240 plants m−2). The studies were conducted during two growing seasons (1995 and 1996) at each of two sites in central Alberta. Root maggot infestations were assessed by determining the numbers of eggs laid per plant during the growing season and by larval feeding damage to canola taproots assessed at the end of the season. Seed yields of the treatment plots also were determined. Plants of B. rapa were significantly more susceptible to root maggot infestations than were plants of B. napus. Root maggot egg populations and root damage were generally greater with zero tillage than with conventional tillage. Plants grown at higher seeding rates (7.5 and 10.0 kg ha−1) usually had less root damage than plants grown at the lowest (5.0 kg ha−1) seeding rate, and canola grown at wider row spacings (20 and 30 cm) had less root damage and higher yields than canola grown at the narrowest spacing (10 cm). Response surface regression analyses determined that deposition of fewest root maggot eggs per plant, least root damage and maximum yields occurred at seeding rates ranging from 7 to 11 kg ha−1 and at row spacings ranging from 17 to 25 cm. Even though canola grown in zero tillage had greater root maggot infestations than canola grown in conventional tillage, higher yields still occurred with zero tillage. Zero tillage is therefore an appropriate agronomic practice in areas infested by high populations of root maggots. Sowing B. napus rather than B. rapa, increasing seeding rates and widening row spacings are also appropriate cultural control practices for reducing crop damage from these pests. Key words: Delia radicum, Delia floralis, tillage, cultural control, canola

Spectral sensitivity and visual conditioning in the parasitoid wasp Trybliographa rapae (Hymenoptera: Cynipidae)

AbstractSpectral sensitivity of the wasp Trybliographa rapae (Westwood), a parasitoid of cabbage root fly larvae, was measured by the electroretinogram (ERG) technique and by a complimentary behavioural technique, to 15 selected wavelength bands from 340 to 670 nm. Peaks of electroretinogram sensitivity were found in the ultraviolet, blue and green-yellow regions of the spectrum. This corresponds to known classes of photoreceptor present in the Hymenoptera. Behavioural peaks of phototactic attraction were found in the ultraviolet and green-yellow regions, but not in the blue. No differences were observed between the sexes. We suggest that ultraviolet and green-yellow wavelengths initiated ‘escape’ and ‘foliage’ orientated behaviour respectively. Blue wavelengths appear to have a less specific function but may be important for colour discrimination. Blue wavelengths did, however, become significantly more attractive to female wasps after a pre-experimental conditioning period involving exposure to blue wavelengths in combination with swede infested with cabbage root fly larvae. Thus T. rapae has been demonstrated to show plasticity in its visually motivated behaviour. This may prove advantageous in the development of an artificial egg collecting technique for this species.

Expression of early and late‐emerging phenotypes in both diapausing and non‐diapausing Delia radicum L. pupae

AbstractThe presence of early and late phenotypes of Delia radicum L. (Diptera: Anthomyiidae) has been attributed to the existence of differences in development after diapause. The criteria for the early phenotype is emergence in less than 20 days at 20 °C (320 DD above 4 °C) and more than 30 days at 20 °C (280 DD above 7 °C + 320 DD above 4 °C) for the late phenotype. The emergence pattern of 10008 non‐diapausing D. radicum pupae reared at 20 °C, L16:D8 and 60% r.h. showed an asymmetrical bimodal distribution similar to the emergence pattern of 477 (1994) and 326 (1995) diapausing pupae from the field. When reared individuals were segregated based on their emergence pattern and reared separately, the proportion of early phenotype increased from 61 to 96% in three generations, while the proportion of late phenotype stayed at 78 to 80% during three generations. These results indicate clearly that diapause is not necessary for the expression of the early and late phenotypes. The presence of these two phenotypes probably enables D. radicum to optimize the use of the available resources, to ensure that a sizeable proportion of the larval population enters diapause and to maintain its distribution throughout the holarctic region (35 to 68 °N).

THE EFFECT OF EXCLUSION FENCES ON THE COLONIZATION OF RUTABAGAS BY CABBAGE FLIES (DIPTERA: ANTHOMYIIDAE)

AbstractA specially constructed fence was developed to exclude cabbage flies, Delia radicum (L.), from plantings of rutabaga. The number of first-flight female D. radicum caught on traps inside fenced enclosures declined linearly with fence height from 0 to 90 cm. Females caught in plots surrounded by a 90 cm high fence were 80.6 and 82.8% fewer than in open check plots in 1991 and 1992, respectively. The percentage of transplanted rutabagas killed by cabbage maggot in the 90 cm high enclosures was 1.4% in 1991 and 25.5% in 1992, compared with 11.8 and 84.5% in the open check plots, respectively. The mean damage index rating for rutabagas was severe in the open check plots but slight in the 90 cm high enclosures in 1991. Only 1.2% of rutabagas in the open check plots would have been of marketable grade in 1991, compared with 54% in the 90-cm enclosures. The mean damage rating was highest in the open check plots in 1992, but damage was also severe in all fenced plots due to the heavy infestation levels that year. The potential of exclusion fences for use in pest-management programs for rutabagas and other brassica crops is discussed.