Developing a decision support tool to forecast the abundance of the cabbage stem weevil in winter oilseed rape

Reducing the use of pesticides in agricultural systems is a prerequisite for sustainable agriculture and, therefore, knowledge on the factors that influence the regional insect pest densities is necessary. Based on multi-site and multi-annual observations of the cabbage stem weevil [Ceutorhynchus pallidactylus (Marsham, 1802)] in winter oilseed rape (Brassica napus Linnaeus) and the corresponding meteorological measurements, a statistical relationship for forecasting the abundance was derived. The model explains 84% of the variation of the data set. The remaining 16% might be explained by the landscape effects and agricultural practices, such as crop protection. Based on the statistical relationship between the mean winter air temperature and the abundance of the cabbage stem weevil in the winter oilseed rape, risk maps were derived as a forecast tool for practical farming.

Control of cabbage stem weevil and pollen beetle with one insecticide application

Over the course of three years (2016–2018), the effects of insecticides on stem-mining weevils [(Ceutorhynchus pallidactylus (Marsham, 1802), Ceutorhynchus napi (Gyllenhal, 1837)] were assessed under field conditions. The dates for spraying were determined on the basis of the recorded percentages of weevil females carrying mature eggs in their ovaries (timing I: the first females with mature eggs present in yellow water traps; timing II: more than 50% of the females with mature eggs present). Delaying the first spring insecticide application till timing II made it possible to combine the control of the stem weevil along with the control of the pollen beetle, Brassicogethes aeneus (Fabricius, 1775). However, the poor effectiveness of the tested insecticides on the stem-mining weevils, regardless of the date they were sprayed, indicates it is impossible to successfully control the insect pests with one insecticide application during the seasons with prolonged egg-laying periods.

Phenology and suppression of stem weevils (Ceutorhynchus napi gyllenhal and Ceutorhynchus pallidactylus marsham) in oilseed rape in northern Serbia

Stem weevils (Ceutorhynchus pallidactylus and Ceutorhynchus napi) are significant pests of oilseed rape that farmers regularly suppress by insecticide treatments intended against Brassicogethes aeneus. Recent studies have shown that in some regions of northern Serbia C. napi is dominant, while C. pallidactylus prevails in others. Increased damage from stem weevils has elicited an exploration of the situation and a study of their phenology and effects of insecticides in three differently managed fields: conventional, organic and integrated. New findings regarding the phenology of stem weevils show that both species were sporadically present in our experimental oilseed rape fields as early as during autumn (from the end of October 2010, BBCH 17-18). The main period of stem weevil settlement is the end of winter and early spring. Maximum flight of both weevil species was recorded on March 23 at the beginning of oilseed rape (OSR) stem elongation (BBCH 22-25), and one week later in the organic field. There were no significant differences in the number of settled weevils among the three differently managed fields. The insecticide treatment against C. napi was applied two weeks earlier than the usual treatment against B. aeneus and C. pallidactylus. Thus, it is clear that insecticide treatment directed against B. aeneus and C. pallidactylus is not effective against C. napi. In this way, such a practice can contribute to an increase in OSR damage, and density of the next generation of C. napi. A new generation of C. pallidactylus emerged from OSR fields in June 2011, while the majority of C. napi emerged in March of the following year.

Spatio-temporal Distributions and Associations of Cabbage Stem Weevil (Ceutorhynchus Pallidactylus Marsham, 1802) and Pollen Beetle (Brassicogethes Aeneus Fabricius, 1775) in winter oilseed rape

From 2013 to 2015 the within-field spatio-temporal distributions and interactions between Ceutorhynchus pallidactylus and Brassicogethes aeneus in winter oilseed rape crops were assessed using SADIE analysis (Spatial Analysis by Distance IndicEs) and Quick association analysis. A significant tendency for aggregation was recorded in C. pallidactylus males in the crop in both 2013 and 2015. Females showed a high tendency towards aggregation only in 2013. Both sexes were significantly associated in crops in 2013 and 2015. B. aeneus adults showed a high tendency towards aggregation in all three years. From the three seasons the cumulative distributions (based on the sums of insect catches in traps for the individual dates) of males and females of C. pallidactylus were significantly associated with the cumulative distribution of B. aeneus adults only in 2013. Contrary to this, an almost significant dissociation between cumulative distributions of C. pallidactylus males and B. aeneus adults was recorded in 2014.

The Influence of Synthetic Insecticides on the Dynamics of Cabbage Stem Weevil (Ceutorhynchus pallidactylus Marsh.) and Cabbage Pod Weevil (Ceutorhynchus obstrictus Marsh.) in Winter Oilseed Rape

Oilseed rape is affected by a complex of different crucifer pests. Cruciferous stem weevils (Ceuthorhynchus spp.) are relatively new oilseed rape (Brassica napus L.) pests in Latvia. Although currently brassica pest control is performed according to the appearance of first specimens of Ceutorhynchus or other brassica pests, for a large number of insecticide treatments a positive biological efficiency is observed (however, within a large range – 8–98%). The current application of brassica pest control may be described as preventive, which is not permitted in the integrated pest management system. The present research estimates biological efficiency and application time of synthetic pyrethroid (PIR) and combined pyrethroid+neonicotonoid (PIR+NNI) to control winter oilseed rape pest species, as well as stimulates discussion about their use. Evaluation of the monitoring data of Ceutorhynchus spp. showed that C. typhae and C. sulcicollis emerge first, but damaging species C. pallidactylus – a week later. Consequently it is not only necessary to monitor the weevils but also to identify their species in order to decide on the control measures. The combined PIR+NNI insecticide proved to have the highest biological efficiency for cabbage stem weevil control. Both classes of synthetic insecticides revealed a considerable biological efficiency for weevil control. The research suggests that effectiveness of insecticide is affected by many factors: plant development stage during the spray, agroclimatic conditions, spray quality, field location and size, distribution of pests, and crop rotation.

Ensemble-based analysis of regional climate change effects on the cabbage stem weevil (Ceutorhynchus pallidactylus (Mrsh.)) in winter oilseed rape (Brassica napus L.)

SUMMARYThe impact of projected regional climate change on the migration of cabbage stem weevil (Ceutorhynchus pallidactylus) to oilseed rape crops in the Grand Duchy of Luxembourg is evaluated for past and future time spans. Several threshold-based statistical models for the emergence and the main migration of C. pallidactylus were chosen from the literature and combined with selected regional climate change projections of the EU ENSEMBLES project. Additionally, a simple degree-day based model was used to assess the plant development under expected climate change conditions. An earlier onset as well as a prolongation of the possible emergence times and the main migration periods was detected. The onset of stem elongation of oilseed rape was predicted to occur 3·0 days earlier per decade, while emergence of C. pallidactylus was expected to occur between 3·0 and 3·3 days earlier per decade. The main migration period of the weevil to the field may start 2·0 days earlier per decade under future climate conditions. Additionally, the time span of possible migration is prolonged for about 30 days under projected future climate conditions.

Resynthesized lines and cultivars of Brassica napus L. provide sources of resistance to the cabbage stem weevil (Ceutorhynchus pallidactylus (Mrsh.))

The cabbage stem weevil (Ceutorhynchus pallidactylus (Mrsh.)) (Col., Curculionidae) is a serious pest of winter oilseed rape (Brassica napus L. var. oleifera Metzg.) in central and northern Europe. Although host-plant resistance is a key tool in integrated pest management systems, resistant genotypes are not yet available for this species. Resynthesized rapeseed lines (B. oleracea L.×B. rapa L.) are broadening the genetic diversity and might have potential as sources of resistance to pest insects. The host quality, of nine resynthesized rapeseed lines and six genotypes of B. napus to cabbage stem weevil, was evaluated in laboratory screening tests and in a semi-field experiment. In dual-choice oviposition tests, female C. pallidactylus laid significantly fewer eggs on five resyntheses and on swede cv ‘Devon Champion’ than on the moderately susceptible oilseed rape cv ‘Express’, indicating a lower host quality of these genotypes. Results of laboratory screenings were confirmed in a semi-field experiment, in which twelve genotypes were exposed to C. pallidactylus females. The number of larvae was significantly lower in two resyntheses and in cv ‘Devon Champion’ than in oilseed rape cv WVB 9. The total, as well as individual, glucosinolate (GSL) content in the leaves differed substantially among the genotypes tested. The amount of feeding by larvae of C. pallidactylus, as measured by a stem-injury coefficient, was positively correlated with the indolyl GSL compounds 3-indolylmethyl and 4-methoxy-3-indolylmethyl, and with the aromatic GSL 2-phenylethyl, whereas it was negatively correlated with 4-hydroxy-3-indolylmethyl. Thus, the composition and concentration of GSL compounds within the plant tissue might be a key factor in breeding for pest resistance in oilseed rape.

The importance of monitoring the Ceutorhynchus pallidactylus female flight activity for the timing of insecticidal treatment

The effects of two pyrethroids and one combination of organophosphate and pyrethroid (alpha-cypermethrin, etofenprox, chlorpyrifos + cypermethrin) on <I>Ceutorhynchus pallidactylus</I> (Marsham, 1802) (<I>Coleoptera: Curculionidae</I>) were tested under field conditions in the Czech Republic in 2006–2008. Significant differences in the effects of the compared insecticides on <I>C. pallidactylus</I> were recorded in the particular years (2006, 2007, 2008). It was less important and somewhat less complicated to establish the most suitable time for spraying in the case of the chlorpyrifos + cypermethrin combination in comparison with the pyrethroids applied separately. The effectiveness of the tested insecticides was markedly influenced by the time of spraying. The effects of the pyrethroids applied singly achieved results comparable to those of the chlorpyrifos + cypermethrin combination only at the optimal spraying time. The most suitable time for spraying varied from the point when the first females appeared in yellow water traps in somewhat higher quantities to the time when a substantial proportion of caught females was able to lay eggs.

The separation of two hymenopteran parasitoids, Tersilochus obscurator and Tersilochus microgaster (Ichneumonidae), of stem-mining pests of winter oilseed rape using DNA, morphometric and ecological data

Tersilochus obscurator Aubert and Tersilochus microgaster (Szépligeti) are larval endoparasitoids of economically-important stem-mining pests of winter oilseed rape (Brassica napus L.) in Europe. They are difficult to separate morphologically. Their hosts are Ceutorhynchus pallidactylus (Marsham) and Psylliodes chrysocephala Linnaeus, respectively. The parasitoids' taxonomic status, identification, host range and phenology were studied using genetic, morphometric and ecological data. The study used 527 female parasitoids from the UK and Germany, either field-collected in emergence traps or reared from field-collected host larvae. Two morphometric characters, the ovipositor sheath to first metasomal tergite ratio and the percentage of the mesopleuron spanned by the sternaulus, were measured. A 440 bp section of mitochondrial DNA cytochrome oxidase subunit I (COI) gene was sequenced from 35 parasitoids reared from C. pallidactylus, 20 reared from P. chrysocephala and individuals from two outgroups, Tersilochus heterocerus Thomson and Phradis interstitialis Thomson. Distinct and invariable COI sequences corresponded exclusively to each parasitoid group, confirming that T. obscurator and T. microgaster are discrete species. Measurements of host-reared and COI-sequenced specimens indicated that the ranges of both morphometric characters overlapped between species. Using these ranges as criteria, all but 3.6% of UK specimens and 2% of German specimens were identifiable to species without reference to host or phenology. There were differences in emergence phenology in the UK, adult T. microgaster emerging from winter diapause by 29 March 2000, T. obscurator emerging between 12 April and 24 May 2000. The value of molecular techniques in the identification of closely-related parasitoid species is discussed.