Inter- and Intrafield Distribution of Cereal Leaf Beetle Species (Coleoptera: Chrysomelidae) in Belgian Winter Wheat.

Cereal leaf beetles (CLBs), a group of chrysomelid beetles of the genus Oulema (Coleoptera: Chrysomelidae), are well-known pest insects of small-grain cereals in many countries of the Northern hemisphere. Due to the small differences in morphology of species within this genus, classification up to species level remains a challenging task. Since an accurate view of species composition is important for developing targeted control strategies, the goal of this study was to unravel the Oulema species composition in Flanders' wheat fields. During three subsequent years at a series of different fields, Oulema species were collected and classified up to species level (2016: 28 fields, 2017: 30 fields, and 2018: 23 fields). This study reveals that the population consists of four different species: Oulema melanopus, Oulema duftschmidi, and Oulema obscura were most frequently encountered, while Oulema rufocyanea was only marginally present. Furthermore, the population was highly dynamic, as the population share of each species varied between different growing seasons and between the various sampling events within each season. The distance from the field edge had a minor influence on the species composition, but the abundance of beetles increased with the distance to the field edge. A discriminant analysis revealed that based on the measurements of various body parts, an accurate classification up to species level is possible. In conclusion, we observed that the population densities fluctuated within and between years, resulting in variable incidence of CLB in winter wheat fields in the Flanders region.

Potentials and Limitations of a Growing Degree Day Approach to Predict the Phenology of Cereal Leaf Beetles.

Cereal leaf beetles (CLBs) are described as an invasive pest of small grain cereals in many regions worldwide. Prediction models aimed to prevent yield losses caused by these feeding insects have been developed by researchers all over the world. As a foundation for many of these prediction models, it is known that a specific number of heat units, or growing degree days (GDDs), is required for an insect to complete a certain physiological process. In this paper, we overview the existing GDD models for CLBs. Furthermore, we used our Belgian input data to compare model predictions with our own observations. Though, the existing models were not able to predict the seasonal trends present in our data: the occurrence of various life stages were monitored earlier then the model predicted. Hence, a weighted GDD model was tested on the data as well: the accumulated GDDs during certain periods were balanced according to the significance of this period for the insect. Rainfall and/or relative humidity were included as well. Based on these selected variables, multiple linear regression models, ridge regression models, and regression trees were fitted. This approach performed considerably better compared to the simple accumulation of GDD. However, based on cross-year cross-location validation method, to gain insight in the future performance of the models, the accuracy was still too low to serve as an accurate warning tool.

Improving the biocontrol potential of Steinernema feltiae against Delia radicum through dosage, application technique and timing.

BACKGROUND: The potential of the entomopathogenic nematode (EPN) Steinernema feltiae Filipjev as a biocontrol agent against the cabbage maggot Delia radicum (L.), was assessed in three field tests, focusing on EPN dosage, application technique and timing. RESULTS: Spraying cabbage plant trays with different doses of infective juveniles (IJs) (50,000, 100,000 and 200,000 per plant) generated a similar reduction of plant mortality. Spraying plant trays with 200,000 IJs of Steinernema feltiae per plant temporarily reduced the number of maggots around the plants' roots, while neither spraying a lower dose (50,000 IJs/plant) nor soil drenching with 200,000 or 50,000 IJs/plant) reduced maggot numbers. When applied as a plant tray spray, IJs of S. feltiae took 1-2 weeks to spread through the soil surrounding the roots. The pathogenicity of the EPNs, as evaluated by a Galleria mellonella bait test, was highest (up to 100% mortality) until up to five weeks after application, and declined to control levels after 4-7 weeks. Follow-up drench applications with EPNs, applied one and/or two weeks after the first EPN application, did not influence control of Delia radicum. CONCLUSION: Plant tray spraying provides better placement of Steinernema feltiae than soil drench treatments for control of Delia radicum. Plant mortality was not dose-dependent in the presented trials, unlike the reduction of maggot numbers. Further research into timing and application technique of follow-up treatments with S. feltiae is required to increase efficacy to commercial standards.

Improving the biocontrol potential of entomopathogenic nematodes against Mamestra brassicae: effect of spray application technique, adjuvants and an attractant.

BACKGROUND: Steinernema carpocapsae Weiser, an entomopathogenic nematode (EPN), is a potential biological control agent for the cabbage moth (Mamestra brassicae L.). This research aimed to identify a suitable spray application technique, and to determine whether yeast extract added to an EPN spray has an attracting and/or a feeding stimulant effect on M. brassicae. The biological control capabilities of EPN against this pest were examined in the field. RESULTS: Good coverage of the underside of cauliflower leaves, the habitat of young instar larvae (L1-L4) of M. brassicae was obtained using different spray boom configurations with vertical extensions that carried underleaf spraying nozzles. One of the configurations was selected for field testing with an EPN spray. Brewer's yeast extract stimulated larval feeding on leaves, and increased the mortality of these larvae when exposed to EPN. The field trial showed that a spray application with S. carpocapsae, Addit and xanthan gum can effectively lower the numbers of cabbage heads damaged by M. brassicae. Brewer's yeast extract did not significantly increase this field performance of EPN. CONCLUSION: Steinernema carpocapsae, applied with an appropriate spray technique, can be used within biological control schemes as part of a resistance management programme for Bt.

Effect of spray volume on the deposition, viability and infectivity of entomopathogenic nematodes in a foliar spray on vegetables.

BACKGROUND: Spray volume can influence the amount of free water on the leaf surface and subsequently the ability of entomopathogenic nematodes (EPNs) to move. In this study, an investigation was made of the effect of spray volume (548, 730 and 1095 L ha(-1) ) on the deposition, viability and infectivity of EPNs against Galleria mellonella on savoy cabbage, cauliflower and leek. RESULTS: Increasing spray volume decreased nematode deposition on 7.1 cm2 leek leaf discs at a 15° angle with the spray nozzle. Although the number of living nematodes observed on leek after 240 min of exposure was not significantly different between the low-volume application (548 L ha(-1) ) and the high-volume application (1095 L ha(-1) ), a greater infectivity was obtained in the latter application. The higher number of droplets deposited on the leek discs in the high-volume application may have stimulated nematode movement. No significant effect of spray volume was observed on the relative deposition of Steinernema carpocapsae on the bottom side of cauliflower and savoy cabbage leaf discs. In spite of the low S. carpocapsae deposition on the bottom side of the savoy cabbage discs, high infectivity was obtained against G. mellonella. Using the lowest spray volume on savoy cabbage, infectivity decreased with increasing exposure time, while infectivity was not affected by exposure time when a spray volume of 730 L ha(-1) or more was used. CONCLUSION: Spray volume is an important application parameter, as it affects nematode infectivity. Future research should investigate the effect of spray volume in the field and its influence on the effect of adjuvants.

Effect of the spray application technique on the deposition of entomopathogenic nematodes in vegetables.

BACKGROUND: The present study compared entomopathogenic nematode delivery at the base of savoy cabbage and cauliflower, at the lower side of savoy cabbage and cauliflower leaves and in leek stems and the ground deposition using a five-nozzle spray boom equipped with an ISO 08 flat fan, an air induction flat fan and Twinjet spray nozzles. Additionally, an air support system and a row application system were evaluated. RESULTS: Approximately 40% of the applied nematodes did not reach the foot of the cabbage plants. The use of an air support system or a row application system improved nematode deposition at the savoy cabbage base. Relative nematode deposition on the lower side of savoy cabbage leaves was 27.20%, while only 2.64% of the applied nematodes reached the lower side of cauliflower leaves. After spraying leek with a standard boom, a low relative nematode deposition (26.64%) was measured in the leek stem. Nozzle type affected the distribution of nematodes in droplet spots. CONCLUSION: Nozzle type has a minor effect on the number of entomopathogenic nematodes delivered on difficult-to-reach targets. The use of modified spray application techniques directing the spray to the target site are necessary to increase the chances of contact of entomopathogenic nematodes with their target.

Development of an efficient application of entomopathogenic nematodes in vegetables.

The withdrawal of approvals for agrochemicals on many food crops within Europe is likely to generate an increasing market for bio-pesticide products, including entomopathogenic nematodes (EPN). However, for the EPN to be a success, a cost-effective and reliable usage should be assured. Therefore, progress in areas like application and formulation technology is needed, next to the development of optimal application strategies. A new research project will focus on optimizing the field application of EPN in vegetables. First, both application and formulation technologies will be optimized. Then, these technologies will be tested under semi-field conditions with special attention for the effect of weather conditions and timing of the application. All the acquired knowledge will be implemented and evaluated in field experiments. In this paper, the first results of the experiments on application technology are discussed.