Cultivated and wild host plants supporting populations of the cabbage root fly

To determine the relative importance of different potential host plants for supporting populations of the cabbage root fly, wild and cultivated crucifers were sampled for pupae at four locations during 1971-3. In addition, eighty-three species of Cruciferae native to or naturalized in the British Isles were inoculated with cabbage root fly eggs in a glasshouse to determine which species could support the larvae. In the field, most pupae (28-7/plant) were collected from untreated cauliflowers. Fewer pupae (i-7–8-6/plant) were obtained from untreated crops of Brussels sprout, cabbage and swede. Applications of chlorfenvinphos reduced populations to two or less pupae per plant on all crops. Of five common weed species sampled, only Raphanus raphanistrum produced as many pupae as certain of the untreated brassica crops. Pupae did not occur in samples from Capsella bursa-pastoris but Sisymbrium officinale, Thlaspi arvense and Sinapis arvensis usually supported low numbers. In the glasshouse, only forty-four of the eighty-three cruciferous species tested supported larval development. Most pupae were obtained from 12-wk-old plants of Barbarea intermedia. B. stricta, Brassica napus, Cochlearia officinalis and R. raphanistrum and from 24-wk-old plants of Brassica rapa, Erysimum aureum, Cochlearia anglica and C. officinalis. Plant age considerably affected pupal production. Plants within a genus often gave similar results, pupae not being recovered from any of the Diplotaxis or Arabis species tested, or from young plants of Erysimum spp. In other families, Reseda lutea and R. luteola supported larval development, but the widely-separated Plantago major did not. Arguments for and against the removal of cruciferous weeds from the vicinity of cruciferous crops are discussed.     

Insect-crop interactions in a diversified cropping system: parasitism by Aleochara bilineata and Trybliographa rapae of the cabbage root fly, Delia radicum, on cabbage in the presence of white clover

Parasitism of the cabbage root fly, Delia radicum (L.) by the staphylinid Aleochara bilineata Gyllenhal and the cynipid Trybliographa rapae Westwood was examined in a cabbage monoculture and a mixed stand of cabbage undersown with white clover. Number of overwintering cabbage root fly pupae per plant was consistently reduced in the mixed stand, and the incidence of plants attacked by cabbage root fly was either reduced or not different in the mixed stand compared to cabbage monoculture. For both parasitoids, the probability of D. radicum attacked plants having at least one parasitized pupa increased with density of cabbage root fly pupae around the plant. For A. bilineata, this positive relation between presence of parasitism and host density was consistently stronger in cabbage monoculture than in cabbage undersown with clover. Location of a host plant by T. rapae was not consistently affected by the presence of clover. D. radicum attacked plants situated in the cabbage and clover mixture were found by T. rapae as easily as in cabbage monoculture. Overall, the total risk of parasitism for a cabbage root fly pupa by A. bilineata was reduced in the mixed stand compared to the cabbage monoculture, whereas the risk of parasitism by T. rapae was not consistently affected by clover. For both parasitoids, intensity of parasitism showed a variable relationship with host density on individual plants attacked by the cabbage root fly. Overall, in spite of consistently lower total density of pupae in the mixed cabbage—clover than in cabbage monoculture, the density of unparasitized pupae was reduced by the presence of non-host plants only in two of the four experiments. The results emphasize the need to include not only herbivore and crop, but also other plant species as well as natural enemies when evaluating management methods.     

The fate, distribution and biological performance of insecticide residues in vegetable crops following seedling treatment

Carrot, cauliflower and radish seedlings raised in sand culture and treated with aqueous suspensions containing chlorfenvinphos, diazinon, iodofenphos or triazophos were planted into the field in 1982 and 1983 to compare the performance against cabbage root fly and carrot fly of these treatments with that of bow-wave applications of granular formulations at sowing. Initial residue concentrations were up to 4100 mg kg^-1 in the seedlings at planting. Thereafter the amounts of insecticides per root system declined. Residue concentrations in mature carrot and radish roots were smaller when plants had been treated as seedlings rather than by bow-wave application. Chlorfenvinphos was the most effective insecticide against both pests. Amounts of insecticide applied in transplanted seedlings were < 5% of the amounts applied by the bow-wave method but short-term control of cabbage root fly on radish was similar. Carrot fly damage was reduced by 60% by some of the seedling treatments after 16 wk in 1982 but in 1983 they were less effective. Pot experiments with chlorfenvinphos showed that most of the insecticide in the seedlings was transferred rapidly into the surrounding soil and it was concluded that the bioactivity resulted mainly from uniform distribution of the released insecticide between and around individual plants.     

The potential of soil amendment with insect exuviae and frass to control the cabbage root fly

Reliable options to control the cabbage root fly, Delia radicum L., are lacking in many countries as restrictions on insecticide use have tightened due to environmental concerns. Although microbial control agents are often considered as a sustainable alternative, their application in agriculture is constrained by inconsistent efficacy owing to low field persistence. To stimulate naturally occurring beneficial microbes, soil amendment with the residual streams of insect production has been suggested as an alternative to synthetic fertilization and a new approach to microbial crop protection. In a set of greenhouse experiments, exuviae and frass of black soldier fly larvae, Hermetia illucens L., house crickets, Acheta domesticus L. and exuviae of mealworms, Tenebrio molitor L., were added to soil from an organically managed field. Exuviae and frass treatments were compared to treatments with synthetic fertilizer. Brussels sprouts, Brassica oleracea L., plants were grown in amended soil for 5 weeks before being infested with cabbage root fly larvae. Insect and plant performance were assessed by recording cabbage root fly survival, biomass and eclosion time and seed germination and plant biomass, respectively. Whereas soil amendment with black soldier fly frass or exuviae reduced cabbage root fly survival and biomass, respectively, amendment with house cricket or mealworm residual streams did not negatively affect root fly performance. Furthermore, seed germination was reduced in soil amended with house cricket exuviae, while amendment with either residual stream derived from black soldier fly larvae or house crickets resulted in lower plant shoot biomass compared with the synthetic fertilizer treatment. Amending soil with black soldier fly residual streams could become a novel and low-cost tool to be integrated in cabbage root fly management programmes, especially where methods currently available are insufficient. Therefore, the mechanisms underlying the effects of insect-derived soil amendments described here should be the focus of future research.     

A seed treatment for the control of Pythium damping-off diseases and Peronospora parasitica on brassicas

In soil inoculated with Pythium ultimum or Pythium irregulare, seed treatment with either Apron 70 (=1 g metalaxyl and 1 g captan/kg seed) or thiram gave control of pre-emergence damping-off of Brussels sprout and cabbage seedlings. On cauliflower, Apron 70 was significantly more effective than thiram. No post-emergence damping-off occurred in either of these crops or in oil-seed rape following seed treatment with Apron 70 whilst post-emergence losses from untreated seed ranged from 10·2–19·4% and from thiram treated seed from 5·7-7·4%. Apron 70 gave complete control of Peronospora parasitica on cauliflower inoculated 10 days after sowing; thiram was ineffective. Following seed treatment with Apron 70, metalaxyl was detected in the cotyledons, true leaves and roots of cabbage seedlings up to 4 wk from sowing.     

Host plant susceptibility to the swede midge (Diptera: Cecidomyiidae)

The relative resistance and susceptibility of various cruciferous plants to swede midge, Contarinia nasturtii (Kieffer) (Diptera: Cecidomyiidae), damage was investigated to provide growers with planting recommendations and to identify potential sources of resistance to the swede midge. Broccoli cultivars experienced more severe damage than cabbage, cauliflower, and Brussels sprouts. The broccoli 'Paragon', 'Eureka', and 'Packman' are highly susceptible to the swede midge, whereas 'Triathlon' and 'Regal' showed reduced susceptibility to damage and slower development of damage symptoms. No differences were found between normal and red cultivars of cabbage and cauliflower in damage severity and progression of damage symptoms. Four new plant species (Brassica juncea Integlifolia group, Erucastrum gallicum (Willd.) O. E. Shulz., Lepidium campestre (L.) R.Br., and Capsella bursa-pastoris (L.) Medic.) are reported as hosts of the swede midge. The weed species Descurainia sophia (L.) Webb, Camelina microcarpa Andrz. ex Dc., and Erysimum cheiranthoides L. exhibited no damage symptoms, and they seem to be nonhost crucifers for the swede midge.     

Preference for plants damaged by conspecific larvae in ovipositing cabbage root flies: influence of stimuli from leaf surface and roots

In laboratory dual-choice assays females of the cabbage root fly, Delia radicum, prefer for oviposition plants with roots damaged by conspecific larvae to undamaged controls. Cauliflower and kale plants were inoculated with root fly eggs (25 per plant) and the hatching larvae were allowed to feed on the roots for various periods of time (1–17 days). After 4 (cauliflower) or 5 (kale) days of larval feeding the oviposition preference was most pronounced and flies laid between 64% and 68% of their eggs near plants with damaged roots. Later, with increasing damage but fewer surviving, and thus actively feeding, larvae, the magnitude of the preference declined. The preference for plants already damaged by conspecific larvae may contribute to the previously observed aggregated distribution of D. radicum eggs in Brassica crop fields.Further experiments revealed that the sensory cues inducing this oviposition preference originate from the complex consisting of the damaged roots, the surrounding substrate (soil) and associated microbes, rather than from the aerial plant parts. In choice assays using the root-substrate complex of damaged and control plants (aerial parts removed), the observed preference for damaged roots was similar to that found for the entire plant but was more pronounced. The damaged roots alone, compared to control roots, received up to 72% (cauliflower) and 75% (kale) of the eggs. By contrast, surrogate leaves sprayed with methanolic leaf surface extracts from the most preferred plants which had been damaged were not discriminated from surrogate leaved sprayed with extracts of the respective control plants. Analysis of glucosinolate levels in methanolic leaf surface extracts revealed that root damage resulted in enhanced concentrations of indole-glucosinolates on the leaf surface in kale but not in cauliflower. Although indole-glucosinolates are oviposition stimulants for the cabbage root fly, the induced changes were apparently too small to influence oviposition behaviour.     

Tolerance and antibiosis resistance to cabbage root fly in vegetable Brassica species

Four accessions of the wild species Brassica fruticulosa Cyrillo (Brassicaceae) were studied in order to identify its tolerance and antibiosis resistance to the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae), in comparison to a widely cultivated cauliflower cultivar and a rapid cycling Brassica oleracea L. line. Antibiosis was prominent, as the insects reared on resistant accessions showed reduced individual pupal weight, total pupal weight, adult dry weight, and the longest average fly eclosion time. Host plant resistance, however, did not affect the sex ratio of adult flies. A study of the root architecture of plants with and without root fly inoculation revealed differences in the structure within B. oleracea accessions. A long main root and a high number of lateral roots appeared to be important characteristics for a Brassica type, with a higher tolerance level to cabbage root fly attack.     

Studies on broccoli necrotic yellows virus

Electron microscopy of infected D. stramonium cells showed that the virus particles occurred in the cytoplasm. Particles were mostly bacilliform and measured 297 ± 18 times 64 ± 4 nm. In negatively stained leaf homogenates, particles were mostly disrupted; intact particles measured 267 ± 20 times 69 ± 6 nm. In brussels-sprout cells containing BNYV and CIMV, BNYV particles were rarely found compared with those of CIMV, and they occurred within and near CIMV inclusion bodies. BNYV particles were also found in extracts of virus-carrying Brevicoryne brassicae. Broccoli necrotic yellows (BNYV) and cauliflower mosaic (CIMV) viruses occurred together in naturally infected Brussels sprout plants, which showed conspicuous symptoms, and in cauliflower. BNYV was transmitted to and maintained in Datura stramonium and Hyoscyamus niger. It was partially purified from D. stramonium. Using these preparations, from which inhibitor had been removed, BNYV was manually transmitted to cauliflower, causing mild symptoms, and to Brussels sprout, causing a symptomless infection. BNYV also infected Sinapis alba but not cabbage, lettuce, Sonchus oleraceus or Plantago major. BNYV was transmitted by Brevicoryne brassicae but not by Myzus persicae, Hyperomyzus lactucae or Aleyrodes proletella.     

Bromide residues in transplanted brassicas following methyl bromide treatment of seedbed and field

Methyl bromide applied to brassica seedbeds resulted in bromide concentrations in cabbage, cauliflower and Brussels sprout transplants of 389-4-2082-3 mg/kg fresh tissue. When the transplants were grown to maturity on untreated field plots bromide concentrations in the plants fell to levels similar to those occurring naturally. Treatment with methyl bromide of both seedbeds and field plots resulted in plant bromide concentrations in cabbage plants of 211-4 mg/kg, a level in excess of the proposed maximum residue limit of 100 mg/kg fresh matter. Bromide concentrations in the edible portions of cauliflower were between 24-8 and 96-0 mg/kg and those in Brussels sprouts between 49-3 and 121-3 mg/kg.     

Cultivar‐specific plant odour preferences of a generalist aphid parasitoid Aphidius colemani and a possible mechanism for maternal priming of resistance to toxic plant chemistry

Aphidius colemani Viereck, emerging from Myzus persicae (Sulzer) mummies on the Brussels sprout cultivar ‘Bedford Winter Harvest’ (BWH), responds positively in the olfactometer to the odour of that cultivar in comparison with air. Responses to the odours of other sprout cultivars, cabbage and broad bean could be explained by the humidity from plant leaves. In a choice between BWH and other sprout cultivars, the BWH odour is preferred, or that of cv. ‘Red Delicious’ (RD) if the parasitoids are reared on RD. This confirms previous work showing that the secondary chemistry of a cultivar is learnt from the mummy cuticle during emergence. Adults emerging from pupae excised from the mummy show a similar but less pronounced preference. Parasitoids developing in aphids on an artificial diet do not discriminate between the odours of BWH and RD, unless allowed contact with a mummy from the same cultivar that the mother develops on. This suggests a cultivar‐specific maternal cue. This cue is speculated to consist of a small amount of the secondary chemistry (probably glucosinolates in the present study) that are left in or on the egg at oviposition, which subsequently induces enzymes that detoxify plant‐derived toxins in the aphid host. Indeed, when parasitoids emerging from diet‐reared aphids are released on aphid‐infested sprout plants, fewer mummies are produced than by parasitoids emerging from mummies of plant‐reared aphids or from excised pupae. Only parasitoids that emerge from mummies of plant‐reared aphids prefer the cultivar of origin as shown by the number of mummified hosts.     

Field and laboratory selection of brassicaceous plants that differentially affect infestation levels by Delia radicum

Several plant traits control plant–insect interactions and shape host range of herbivorous insects according to their degree of dietary specialization. Understanding how plant species diversity influences herbivore infestations is of interest for the development of alternative crop protection strategies. In a pest management context, an appropriate selection of plants can modify pest distribution at the field scale. To develop a ‘push–pull’ strategy against the cabbage root fly, Delia radicum, we conducted a field study to both determine which plants exhibit contrasted pest infestation levels and to evaluate their influence on egg predation activity. Our field experiment reveals that infestation levels of brassicaceous plants by the cabbage root fly in the field can vary considerably according to plant genotype and species, while the number of predated eggs is only slightly affected by plant species. Olfactometry studies carried out under laboratory conditions revealed that plants harbouring the highest number of eggs in the field were also highly attractive, suggesting that olfactory stimuli are responsible, at least partially, for the differential infestation levels observed in the field. In a ‘push–pull’ context, this study demonstrates that different brassicaceous plants could be used to redistribute cabbage root flies in broccoli crops without compromising herbivore control by natural enemies. In addition, the importance of plant volatiles for infestation levels suggests a potential for developing a semiochemically assisted ‘push–pull’ system in which trap plants would be enhanced by synthetic release of attractive compounds.     

Effects of three different cultivars of cruciferous plants on the age‐stage,two‐sex life table traits of Plutella xylostella (L.) (Lepidoptera: Plutellidae)

Plutella xylostella is an important pest of cruciferous crops worldwide. However, information regarding the age‐stage, two‐sex life parameters of P. xylostella, which is vital for designing more effective control methods, is currently lacking. The present study reports age‐stage, two‐sex life table parameters for P. xylostella on napa cabbage (Brassica oleracea var. napa), white cabbage (B. oleracea var. capitata), and cauliflower (B. oleracea var. botrytis) under laboratory conditions at 25 ± 2°C, 50–60% relative humidity, and a 16‐h light : 8‐h dark photoperiod. The time for development from an egg to a male or female adult P. xylostella on white cabbage (mean [± SE] 41.15 ± 0.54 and 39.50 ± 0.54 days, respectively) was significantly longer than that on cauliflower and napa cabbage. Furthermore, P. xylostella fecundity on cauliflower (261.90 ± 4.53 eggs female) was significantly highest than on napa cabbage and white cabbage. Intrinsic rate of increase (r) and finite rate of increase (λ) were highest on cauliflower 0.182 day^?1 and 1.199 day^?1 respectively as comparison to napa cabbage and white cabbage. The highest gross reproductive rate (GRR) and net reproductive rates (R₀) of P. xylostella 65.87 and 52.58 respectively on cauliflower then those of other hosts. The findings of the present study indicate that cauliflower is the most suitable cultivar (host) for the development of P. xylostella. Based on these findings, crops like cauliflower can be used as trap crops when napa cabbage and white cabbage are the main crops.     

Evaluation of entomopathogenic fungus Metarhizium anisopliae against soil-dwelling stages of cabbage maggot (Diptera: Anthomyiidae) in glasshouse and field experiments and effect of fungicides on fungal activity

The effect of two isolates of the entomopathogenic fungus Metarhizium anisopliae (Metchnikoff) Sorokin (389.93 and 392.93) on root-feeding stages of cabbage root fly, Delia radicum (L.), was studied under glasshouse and field conditions. In glasshouse studies, the effect of drenching a suspension of conidia (concentration 1 x 10(8) m(-1), 40 ml per plant, applied on four occasions) onto the base of cabbage plants infested with D. radicum eggs was compared with mixing conidial suspension into compost modules (concentration 1 x 10(8) ml(-1), 25 ml per plant) used to raise seedlings. Drench application reduced the mean number of larvae and pupae recovered per plant by up to 90%, but the compost module treatment had no statistically significant effect. Both application methods reduced the emergence of adult flies from pupae by up to 92%. Most conidia applied as a drench application remained in the top 10-cm layer of compost. Applications of the fungicides iprodione and tebuconazole, which are used routinely on brassica crops, were compatible with using M. anisopliae 389.93 against D. radicum under glasshouse conditions, even though these fungicides were inhibitory to fungal growth on SDA medium. In a field experiment, drench applications of M. anisopliae 389.93 to the base of cauliflower plants at concentrations of 1 x 10(6) to 1 x 10(8) conidia ml(-1) did not control D. radicum populations, although up to 30% of larval cadavers recovered supported sporulating mycelium. Drench applications often exhibited considerable lateral movement on the soil surface before penetrating the ground, which may have reduced the amount of inoculum in contact with D. radicum larvae.     

Can imitation companion planting interfere with host selection by Brassica pest insects?

• 1 Companion planting with nonhosts may offer a non‐insecticidal means of controlling pests, although the results of studies can be variable and species‐dependent.
• 2 The effect of companion planting on two pests of Brassica crops, Plutella xylostella (L.) and Brevicoryne brassicae (L.), was examined using Brussels sprout as the host plant and imitation cereal plants made from green plastic as the nonhost. For P. xylostella, the effect of nonhost density was also investigated.
• 3 Oviposition (P. xylostella) and abundance (B. brassicae) were lower on Brussels sprout plants presented on a background of high‐density imitation cereal plants (reductions of 59% and 85%, respectively).
• 4 The results are discussed in the context of host location by pest insects and the selection of nonhost companion plants for pest management.
• 5 It is concluded that nonhost plants interfere with pest host selection through disruption to visual host location processes.

     

Host-plant susceptibility to the carrot fly, Psila rosae. 2. Suitability of various host species for larval development

Several apiaceous and two asteraceous species were tested for their suitability to support larval development of the carrot fly. Plants grown in pots or transplanted from seed beds into pots, were inoculated with a specific number of eggs. Pupae and non-pupated larvae were collected 6–7 wk after inoculation. Both the number and weights of pupae produced varied widely among the species. Cultivated carrots Daucus carota sativus often gave rise to only moderate numbers of pupae, but these invariably attained the highest weights. Pimpinella major was the only apiaceous plant tested that did not yield any carrot flies. The two asteraceous plants Cichorium intybus and Tanacetum vulgare failed to support larval development. Total carrot fly biomass produced per plant was influenced by both the host species and the root weight. Emergence rates of adult flies were positively correlated with pupal weights. Small individuals tended to have a longer total developmental time from egg to adult fly.     

A maternal influence on the conditioning to plant cues of Aphidius colemani Viereck, parasitizing the aphid Myzus persicae Sulzer

Abstract. The offspring of parasitoids, Aphidius colemani Viereck, reared on Brussels sprouts and emerging from Myzus persicae Sulzer on a fully defined artificial diet, show no preferences in a four-way olfactometer, either for the odour of the diet, the odour of Brussels sprouts, or the odour of two other crucifers (cabbage and Chinese cabbage). A similar lack of odour preferences is shown when the host aphids are exposed for parasitization (for 48 h) on cabbage, Chinese cabbage or wheat. However, if parasitization occurs on Brussels sprouts, a weak but statistically highly significant response to Brussels sprout odour is observed. Although as many as 30–35% of the parasitoids show no response to any odour, another 35% respond positively to the odour of Brussels sprout compared with responses to the odours of cabbage, Chinese cabbage or wheat of only approximately 10%. An analagous result is obtained when the parent parasitoids are reared on cabbage. In this case, significant positive responses of their offspring to cabbage odour occur only if the 48-h parasitization has occurred also on cabbage. However, with parasitoids from Brussels sprouts parasitizing the aphids for 48 h also on Brussels sprouts, the offspring subsequently emerging from pupae excised from the mummies show no preference for Brussels sprout odour. Thus, although the Brussels sprout cue had been experienced early in the development of the parasitoids, they only become conditioned to it when emerging from the mummy. Both male and female parasitoids respond very similarly in all experiments. It is proposed that the chemical cue (probably glucosinolates in these experiments) is most likely in the silk surrounding the parasitoid pupa, and that the mother may leave the chemical in or around the egg at oviposition, inducing chemical defences in her offspring to the secondary plant compounds that the offspring are likely to encounter.     

Companion planting – do aromatic plants disrupt host‐plant finding by the cabbage root fly and the onion fly more effectively than non‐aromatic plants?

Brassica and Allium host‐plants were each surrounded by four non‐host plants to determine how background plants affected host‐plant finding by the cabbage root fly (Delia radicum L.) and the onion fly [Delia antiqua (Meig.)] (Diptera: Anthomyiidae), respectively. The 24 non‐host plants tested in field‐cage experiments included garden ‘bedding’ plants, weeds, aromatic plants, companion plants, and one vegetable plant. Of the 20 non‐host plants that disrupted host‐plant finding by the cabbage root fly, fewest eggs (18% of check total) were laid on host plants surrounded by the weed Chenopodium album L., and most (64% of check total) on those surrounded by the weed Fumaria officinalis L. Of the 15 plants that disrupted host‐plant finding in the preliminary tests involving the onion fly, the most disruptive (8% of check total) was a green‐leaved variant of the bedding plant Pelargonium × hortorum L.H. Bail and the least disruptive (57% of check total) was the aromatic plant Mentha piperita × citrata (Ehrh.) Briq. Plant cultivars of Dahlia variabilis (Willd.) Desf. and Pelargonium×hortorum, selected for their reddish foliage, were less disruptive than comparable cultivars with green foliage. The only surrounding plants that did not disrupt oviposition by the cabbage root fly were the low‐growing scrambling plant Sallopia convolvulus L., the grey‐foliage plant Cineraria maritima L., and two plants, Lobularia maritima (L.) Desv. and Lobelia erinus L. which, from their profuse covering of small flowers, appeared to be white and blue, respectively. The leaf on which the fly landed had a considerable effect on subsequent behaviour. Flies that landed on a host plant searched the leaf surface in an excited manner, whereas those that landed on a non‐host plant remained more or less motionless. Before taking off again, the flies stayed 2–5 times as long on the leaf of a non‐host plant as on the leaf of a host plant. Host‐plant finding was affected by the size (weight, leaf area, height) of the surrounding non‐host plants. ‘Companion plants’ and aromatic plants were no more disruptive to either species of fly than the other plants tested. Disruption by all plants resulted from their green leaves, and not from their odours and/or tastes.