Bernd Ulber

Parasitoids of Oilseed Rape Pests in Europe: Key Species for Conservation Biocontrol

The six most important pests of oilseed rape are host to at least 80 species of parasitoid, mostly parasitic Hymenoptera, particularly braconids, chalcids and ichneumonids. Most of them attack the egg or larval stages of their hosts. Based on reviews of the literature and extensive sampling programmes during the EU project MASTER (QLK5-CT-2001-01447), 12 species have been identified as the key parasitoid species of these pests in winter oilseed rape, and, with little divergence, also in spring rape in nearly all European countries where their hosts occur. Some key species have been recorded for the first time in individual partner countries. They are sufficiently widespread and abundant across Europe to be of potential economic importance for conservation biological control of the target pests. Their incidence and abundance in European countries were associated with the occurrence of their hosts, thereby indicating close host-parasitoid-relationships.

Forecasting migration of cereal aphids (Hemiptera: Aphididae) in autumn and spring.

The migration of cereal aphids and the time of their arrival on winter cereal crops in autumn and spring are of particular importance for plant disease (e.g. barley yellow dwarf virus infection) and related yield losses. In order to identify days with migration potentials in autumn and spring, suction trap data from 29 and 45 case studies (locations and years), respectively, were set‐off against meteorological parameters, focusing on the early immigration periods in autumn (22 September to 1 November) and spring (1 May to 9 June). The number of cereal aphids caught in a suction trap increased with increasing temperature, global radiation and duration of sunshine and decreased with increasing precipitation, relative humidity and wind speed. According to linear regression analyses, the temperature, global radiation and wind speed were most frequently and significantly associated with migration, suggesting that they have a major impact on flight activity. For subsequent model development, suction trap catches from different case studies were pooled and binarily classified as days with or without migration as defined by a certain number of migrating cereal aphids. Linear discriminant analyses of several predictor variables (assessed during light hours of a given day) were then performed based on the binary response variables. Three models were used to predict days with suction trap catches ≥1, ≥4 or ≥10 migrating cereal aphids in autumn. Due to the predominance of Rhopalosiphum padi individuals (99.3% of total cereal aphid catch), no distinction between species (R. padi and Sitobion avenae) was made in autumn. As the suction trap catches were lower and species dominance changed in spring, three further models were developed for analysis of all cereal aphid species, R. padi only, and Metopolophium dirhodum and S. avenae combined in spring. The empirical, cross‐classification and receiver operating characteristic analyses performed for model validation showed different levels of prediction accuracy. Additional datasets selected at random before model construction and parameterization showed that predictions by the six migration models were 33–81% correct. The models are useful for determining when to start field evaluations. Furthermore, they provide information on the size of the migrating aphid population and, thus, on the importance of immigration for early aphid population development in cereal crops in a given season.

The proPlant Decision Support System: Phenological Models for the Major Pests of Oilseed Rape and Their Key Parasitoids in Europe

The computer-based DSS proPlant incorporates weather-based phenological models for the six major pests of oilseed rape in Europe: the cabbage stem flea beetle, the rape stem weevil, the cabbage stem weevil, the pollen beetle, the cabbage seed weevil and the brassica pod midge. The models have now been validated for several European countries. The system predicts the start and course of pest infestation and provides site-specific crop protection treatment decisions, a selection of suitable chemicals, computes optimum dates and rates for application and evaluates the efficacy of past applications. Phenological models for migration of some key parasitoids of the pests have also been constructed recently; their use in finding spray windows to conserve parasitoids is described. Further work is needed to validate them before they can be integrated into a pest control strategy for farmers.

Interactions between the stem-mining weevils Ceutorhynchus napi Gyll. and Ceutorhynchus pallidactylus (Marsh.) (Coleoptera: Curculionidae) in oilseed rape

Abstract  1 The rape stem weevil Ceutorhynchus napi Gyll. and the cabbage stem weevil Ceutorhynchus pallidactylus (Marsh.) share the same habitat and food resource within the stems of oilseed rape, Brassica napus L. var. oleifera. Interactions occurring between these two sympatric species on this host were studied under both field and laboratory conditions.

Impact of Insecticides on Parasitoids of Oilseed Rape Pests

Insecticide application for control of insect pests on oilseed rape may impact parasitoid activity and the levels of parasitism. Field and laboratory studies show that the insecticide product, the dose rate and the timing of application can affect parasitoid abundance and levels of parasitism differentially. Insecticides applied during flowering have most potential to threaten parasitoid populations. This knowledge will help in the future to provide recommendations to the agricultural industry on the selection of less harmful insecticides and the optimization of the time of application, in order to minimize detrimental side effects on the beneficial entomofauna.

Screening of oilseed rape and other brassicaceous genotypes for susceptibility to Ceutorhynchus pallidactylus (Mrsh.)

Production of oilseed rape, Brassica napus L., is affected by various insect pests. The cabbage stem weevil, Ceutorhynchus pallidactylus (Mrsh.) (Col.: Curculionidae), is one of the most damaging pests in Northern and Central Europe that requires regular control measures. Host plant resistance is a key factor in integrated pest management systems. To evaluate a large number of genotypes for their susceptibility to infestation by C. pallidactylus, new screening techniques were developed for testing both, the amount of feeding and the number of eggs deposited by adult C. pallidactylus on accessions of Brassicaceae under controlled conditions. In no‐choice screening tests, the leaf area consumed by adult cabbage stem weevil was quantified on a wide spectrum of 107 brassicaceous genotypes (B. napus, Brassica rapa L. and Brassica oleracea L. cultivars, breeding lines, resynthesized rapeseed lines and wild Brassicaceae). In comparison to feeding on the standard cultivar ‘Express’, the average leaf area consumed by C. pallidactylus on nine oilseed rape cultivars, four resynthesized rapeseed lines and five other accessions [B. oleracea, Camelina alyssum (Mill.) and Lunaria annua L.] was significantly reduced by 44–90%. In dual‐choice screening tests for the evaluation of oviposition preferences on 42 genotypes, female C. pallidactylus laid significantly fewer eggs into plants of two oilseed rape cultivars, five resynthesized rapeseeds and three accessions of B. oleracea and Brassica fruticulosa Cyrillo, respectively, than into plants of the standard cv ‘Express’. Results of both laboratory screening tests were confirmed by results of additional field testing.

Improving phenological forecasting models for rape stem weevil, Ceutorhynchus napi Gyll., based on long-term multisite datasets

New threshold‐based models to predict the start of invasion by the stem‐boring pest, the rape stem weevil (Ceutorhynchus napi Gyll.) of winter oilseed rape (Brassica napus L.), were developed and compared to published models using long‐term datasets on weather and weevil phenology from experimental locations in Germany and Luxembourg. Threshold values for daily records of maximum air temperature, mean soil temperature, sunshine duration and total precipitation were adjusted to local conditions on the date of first weevil migration in spring. Mean error and the root mean squared error were used to assess model quality, where the error is defined as the number of days between predicted and observed arrival of weevils on the crop (regardless of sign). Best model results predicted first crop invasion by rape stem weevil when the thresholds of daily maximum air temperature ≥7.8°C, mean soil temperature ≥6.6°C, daily total precipitation ≤1.0 mm and sunshine duration ≥1 h were matched. This model takes into account meteorological variables likely to influence conditions at the overwintering site of the weevils in the soil, as well as variables that may limit weevil flight. Adjusted air temperature threshold values were consistently lower for Luxembourg sites than for those optimized for Germany. A simple model relating the date of first weevil invasion to accumulated daily maximum air temperature above 0°C (from 1 January) was also evaluated. This proved less suitable for forecasting crop invasion by C. napi. We suggest that phenological models using locally adjusted meteorological‐based thresholds have the potential to offer sufficiently accurate forecasts of first immigration flights by C. napi for appropriate timing of insecticide application. In addition, the developed models are suitable tools to be used in climate change impact studies.

The potential of resynthesized lines to provide resistance traits against rape stem weevil in oilseed rape

The rape stem weevil, Ceutorhynchus napi Gyll., is a serious pest of winter oilseed rape (Brassica napus L.) crops in Europe that causes stem distortion, severe disruption of stem growth, and significant yield loss. No resistance has been identified in current B. napus breeding material. Resynthesized lines of oilseed rape can be used to introduce genetic material from the parent species Brassica oleracea L. and B. rapa L. The susceptibility to C. napi of five resynthesized lines and three cultivars of oilseed rape was compared in a multi-choice field plot experiment. Morphological stem traits and stem levels of glucosinolates, carbon, and nitrogen were assessed as potential mediators of resistance. Eggs and larvae of C. napi in stems were counted weekly from the beginning of oviposition to larval maturation and stem pith degeneration in response to oviposition was assessed. Egg counts per stem were negatively dependent on stem length and positively dependent on the levels of five glucosinolates, glucobrassicanapin, glucoalyssin, glucobrassicin, neoglucobrassicin, and gluconasturtiin. The resynthesized line S30 was significantly more resistant to C. napi than the commercial cultivar Sollux, having fewer eggs and larvae. This resistance appeared to be largely antixenotic, due to the combination of a long stem and a paucity of the five key glucosinolates. We propose that resynthesized lines such as S30 should be used to introduce genes determining traits such as long stem length and glucosinolate profile into breeding programs to enhance resistance against C. napi in oilseed rape.

Assessing climate change impacts on the rape stem weevil, Ceutorhynchus napi Gyll., based on bias- and non-bias-corrected regional climate change projections

Agricultural production is directly affected by projected increases in air temperature and changes in precipitation. A multi-model ensemble of regional climate change projections indicated shifts towards higher air temperatures and changing precipitation patterns during the summer and winter seasons up to the year 2100 for the region of Goettingen (Lower Saxony, Germany). A second major controlling factor of the agricultural production is the infestation level by pests. Based on long-term field surveys and meteorological observations, a calibration of an existing model describing the migration of the pest insect Ceutorhynchus napi was possible. To assess the impacts of climate on pests under projected changing environmental conditions, we combined the results of regional climate models with the phenological model to describe the crop invasion of this species. In order to reduce systematic differences between the output of the regional climate models and observational data sets, two different bias correction methods were applied: a linear correction for air temperature and a quantile mapping approach for precipitation. Only the results derived from the bias-corrected output of the regional climate models showed satisfying results. An earlier onset, as well as a prolongation of the possible time window for the immigration of Ceutorhynchus napi, was projected by the majority of the ensemble members.

Evaluation of nine genotypes of oilseed rape (Brassica napus L.) for larval infestation and performance of rape stem weevil (Ceutorhynchus napi Gyll.)

The rape stem weevil, Ceutorhynchus napi Gyll., is a serious pest of winter oilseed rape (Brassica napus L.) crops in Europe causing severe yield loss. In currently used oilseed rape cultivars no resistance to C. napi has been identified. Resynthesized lines of B. napus have potential to broaden the genetic variability and may improve resistance to insect pests. In this study, the susceptibility to C. napi of three cultivars, one breeding line and five resynthesized lines of oilseed rape was compared in a semi-field plot experiment under multi-choice conditions. Plant acceptance for oviposition was estimated by counting the number of C. napi larvae in stems. The larval instar index and the dry body mass were assessed as indicators of larval performance. The extent of larval feeding within stems was determined by the stem injury coefficient. Morphological stem traits and stem contents of glucosinolates were assessed as potential mediators of resistance. The resynthesized line S30 had significantly fewer larvae than the cultivars Express617 and Visby and the resynthesized lines L122 and L16. The low level of larval infestation in S30 was associated with a low larval instar and stem injury index. Low numbers of larvae were not correlated with the length or diameter of stems, and the level of stem glucosinolates. As indicated by the low larval infestation and slow larval development the resistance of S30 to C. napi is based on both antixenotic and antibiotic properties of the genotypes. The resynthesized line S30 should therefore be introduced into B. napus breeding programs to enhance resistance against C. napi.