Effects of land use on infestation and parasitism rates of cabbage seed weevil in oilseed rape

How do outside-hosts-overwintering parasitoids, at the adult stage, cope with cold?

INTRODUCTION

When overwintering, most endoparasitoids are protected from the cold inside their hosts. However, some endoparasitoids, along with ectoparasitoids, fall into the category called outside-hosts-overwintering parasitoids (OHOP) at immature or adult stages. We compared the cold-hardiness capacity and strategy between adult OHOP and their hosts (HOST) by examining their supercooling points (SCP), with acclimation periods and acclimation temperatures, and their lower lethal temperatures at 50% mortality (LLT50). We hypothesized that OHOP are more cold-hardy than their HOST, with lower SCP and LLT50.

MATERIALS AND METHODS

Throughout the summers of 2020, 2021, and 2022, adult cabbage seedpod weevils (HOST) were sampled with a sweep net at the canola pod stage, and thousands of pods were collected and placed in emergence boxes to retrieve the adult OHOP Trichomalus perfectus. Regarding SCP measures, OHOP and HOST were separated according to various treatments. Each treatment considered a target exposure temperature (5, 10, or 20 °C) or a target exposure period (5, 15 or 25 days) at 5 °C. Regarding LLT measures, OHOP and HOST were categorized into five treatments, each corresponding to a specific exposure temperature (-5, -10, -15, -20 or -25 °C).

RESULTS AND CONCLUSION

Acclimations to a lower temperature (5 °C) and a longer period (25 days) led to a significantly lower SCP of OHOP than HOST. Regarding OHOP, the average SCP was -19.71 °C when the acclimation temperature was 20 °C and significantly decreased to -23.20 °C when it was 5 °C. The average SCP was -18.82 °C when the acclimation period was five days and significantly decreased to -23.20 °C when it was 25 days. Conversely, the average SCP for HOST was never below -20 °C. At 20 °C acclimation temperature, HOST exhibited a significantly higher SCP of -14.64 °C compared to acclimations at 5 °C (-19.19 °C) and 10 °C (-20.00 °C), but there were no significant differences between 5 and 10 °C nor between acclimation periods. Therefore, the adult OHOP is more cold-hardy than its HOST. OHOP also exhibited a lower LLT50 than HOST, with -19.20 °C versus -17.59 °C. Finally, OHOP and HOST employ the same freeze-avoidance strategy, as evidenced by their SCP values (-19.57 °C versus -16.80 °C) which closely align with their respective LLT50. Adult OHOP better survive winter than their HOST in cold environments.

Spatiotemporal distancing of crops reduces pest pressure while maintaining conservation biocontrol in oilseed rape

BACKGROUND

Agricultural landscapes provide resources for arthropod pests as well as their natural enemies. To develop integrated pest management (IPM) practices, it is important to understand how spatiotemporal location influences crop colonization and damage severity. We performed a 3-year (2016-2018) field experiment in winter oilseed rape (OSR, Brassica napus) fields in Estonia, where half of the fields were within 500 m of the location of the previous year's winter OSR field and half were outside this zone. We investigated how distance from the previous year's OSR crop influences the infestation and parasitism rates of two of its most important pests: the pollen beetle (Brassicogethes aeneus) and the cabbage seed weevil (Ceutorhynchus obstrictus).

RESULTS

When the distance from the previous year's OSR crop was >500 m, we recorded significantly reduced pest pressure by both B. aeneus and C. obstrictus in the study fields. Biocontrol of both pests, provided by parasitic wasps, was high in each study year and commonly not affected by distance. Mean parasitism rates of B. aeneus were >31%, occasionally reaching >70%; for C. obstrictus, mean parasitism was >46%, reaching up to 79%, thereby providing effective biocontrol for both pest species.

CONCLUSION

Spatiotemporal separation of OSR fields can reduce pest pressure without resulting in reduced parasitism of OSR pests. This supports a spatiotemporal field separation concept as an effective and sustainable technique for IPM in OSR. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Parasitoids of the cabbage seed weevil deliver high and consistent parasitism in variable landscapes: A showcase of conservation biocontrol

BACKGROUND

Insect pest resistance to insecticides and societal pressure to reduce pesticide load has increased oilseed rape (OSR) farmers' motivation to protect and exploit parasitoids for pest control. However, parasitoid presence and parasitism must be made visible to influence farmers' spraying decisions. Parasitism of cabbage seed weevil (CSW) (Ceutorhynchus obstrictus (Marsham)) reduces damage to OSR immediately, making them a good case for demonstrating conservation biocontrol to farmers. We assessed the occurrence and activity of CSW parasitoids in 84 OSR fields over 2 years and identified the impact of associated local landscape factors.

RESULTS

Mean cabbage seed weevil infestation rates were 11% and 10% in 2020 and 2021, and parasitism rates were high in both years (75% and 74%, respectively). Temporal and spatial dynamics of OSR in the landscape surrounding the focal fields were important for both CSW and parasitoid numbers, suggesting a dilution effect for increasing OSR area since the previous year. A multimodel inference analysis showed that OSR-related factors were important predictors for both the infestation rate of CSW and the number of parasitoids. For parasitoids, protected nature areas and hedgerows were important. Parasitism rates were high and largely unaffected by landscape factors.

CONCLUSION

CSW and its parasitoids respond similarly to interannual changes in the OSR resource; in addition, parasitoids benefit from uncropped areas in the surrounding landscape. The complexity of the pest and parasitoid response to landscape factors limits the prospect of designing landscapes for improved pest control by the parasitoids. Parasitoids of CSW may be present as local populations in agricultural landscapes with the potential for consistent and substantial parasitism. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Landscape complexity effects on Brassicogethes aeneus abundance and larval parasitism rate: a two-year field study

Global biodiversity has suffered a decline primarily attributed to landscape simplification and intensified agricultural practices. Agricultural environments, characterized by homogeneity and frequent disturbances, are often suboptimal habitats for various insect species. While agricultural fields do favour pests, they generally fail to provide suitable habitats for natural enemies. The inclusion of diverse supporting habitats, such as semi-natural habitats, grassy and woody field margins etc. surrounding agricultural fields, play a crucial role in fostering effective biodiversity conservation. Moreover, determining the influence of different adjacent habitat types is essential in elucidating their influence on pest abundance and parasitism rates. Our two-year field study focused on assessing the abundance of Brassicogethes aeneus and its parasitism rate. The findings revealed that the adjacent habitat type did not significantly increase pest abundance and the parasitism rate of B. aeneus larvae consistently stayed over the threshold for effective biological control throughout the fields. This was attributed to the high proportion (35 and 38% in the 2 study years) of semi-natural habitats within most of the 1 km radius study areas. While our study did not identify any specific adjacent habitat type or habitat within a 1 km radius that directly impacted B. aeneus abundance, it emphasises the intricate interplay between the pests, parasitism and the surrounding environment because the interactive effect of distance from the crop edge and habitat type had a significant influence on B. aeneus infestation levels but not on parasitism. Decision tree analysis suggests that > 18% semi-natural habitat is needed to ensure sufficient levels of parasitism for effective biological control. A comprehensive understanding of habitats that influence not only B. aeneus but also other pests is critical for the successful implementation of IPM strategies and conservation initiatives within the agricultural sector.

Factors influencing short-term parasitoid establishment and efficacy for the biological control of Halyomorpha halys with the samurai wasp Trissolcus japonicus

BACKGROUND

Classical biological control has been identified as the most promising approach to limit the impact of the invasive pest species Halyomorpha halys (Heteroptera: Pentatomidae). This study investigated the parasitism rate at sites where the biocontrol agent Trissolcus japonicus (Hymenoptera: Scelionidae) was released and where its unintentional introduction took place, in the Trentino-South Tyrol region. The effect of land-use composition was studied to understand which factors favor the establishment of hosts and parasitoids, including native and exotic species.

RESULTS

The released T. japonicus were detected a year after the start of the program, with a significant parasitoid impact and discovery, compared to control sites. Trissolcus japonicus was the most abundant H. halys parasitoid, and Trissolcus mitsukurii and Anastatus bifasciatus were recorded also. The efficacy of T. mitsukurii was lower in sites where T. japonicus was successfully established, suggesting a possible competitive interaction. Parasitism level by T. japonicus at the release sites was 12.5% in 2020 and 16.4% in 2021. The combined effect of predation and parasitization increased H. halys mortality up to 50% at the release sites. Landscape composition analysis showed that both H. halys and T. japonicus were more likely to be found at sites with lower altitude and with permanent crops, whereas other hosts and parasitoids preferred different conditions.

CONCLUSION

Trissolcus japonicus showed a promising impact on H. halys, at release and adventive sites, with minor nontarget effects, mediated by landscape heterogeneity. The prevalence of T. japonicus in landscapes with permanent crops could support IPM in the future. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Recent advances in agroecological research for increasing scope of areawide pest management of arthropods in cropping systems

We propose that a growing research base considering pest management as an ecosystem service can be leveraged to expand areawide pest management (AWPM) to an agroecological-oriented framework when addressing pest arthropods in cropping systems. This AWPM framework centers on the innate capacity of the agroecosystem to suppress pests and is supported with strategic insertion of AWPM tactics. Recent studies on agroecological pest management are valuable to identify AWPM candidates. The estimation and predictability of AWPM outcomes may be improved by measuring effects of interactions of pest and pest suppression agents, and mediating factors such as landscape and weather. This knowledge helps formulate selection and strategic insertion of AWPM tactics into the system to support innate pest suppression. Advances in biotechnology and agricultural engineering have increased effectiveness of AWPM tactics, further improving positive AWPM outcomes. Furthermore, multifunctional agricultural, environmental, and economic benefits may be seen when adopting this framework.

Landscape Effects on the Cabbage Seedpod Weevil, Ceutorhynchus obstrictus (Coleoptera: Curculionidae), and on Its Parasitoid, Trichomalus perfectus (Hymenoptera: Pteromalidae), in Canola

The cabbage seedpod weevil (CSW), Ceutorhynchus obstrictus, an exotic pest accidentally introduced in North America in 1931, spread all over this continent and is now a major pest of canola crops. One of its main natural enemies in Europe, Trichomalus perfectus, was observed in eastern Canada in 2009. This study aimed to evaluate the landscape influence on CSW infestation and abundance and on T. perfectus parasitism in Quebec to understand the optimal conditions to potentially release this parasitoid in the Canadian Prairies. Field research was conducted in 19 to 28 canola fields per year, from 2015 to 2020, among eight Quebec regions. CSW was sampled by sweep net during canola blooming and parasitoids by collecting canola pods kept in emergence boxes until adults emerge. Infestation and parasitism calculations were based on pod emergence holes. For analysis, 20 landscape predictors were considered. Results show that CSW infestation and abundance increased if there were more roads and cereal crops in the landscapes. Meanwhile, T. perfectus parasitism decreased when hedgerows length and distance from water were longer. However, it increased when landscape diversity and average crop perimeter-to-area ratio were higher, and along with more hay/pastures and soybean crops. This study's results highlight that these four landscape predictors could provide more resources and overwintering areas, promoting greater efficiency of T. perfectus to control the CSW.

Pollen beetle offspring is more parasitized under moderate nitrogen fertilization of oilseed rape due to more attractive volatile signal

Biocontrol providing parasitoids can orientate according to volatile organic compounds (VOCs) of their host's plants, the emission of which is potentially dependent on the availability of soil nitrogen (N). This paper aimed at finding the optimal N fertilization rate for oilseed rape (Brassica napus L.) to favor parasitism of pollen beetles (Brassicogethes aeneus Fab. syn. Meligethes aeneus Fab.) in a controlled environment. Pollen beetles preferred to oviposit into buds of plants growing under higher N fertilization, whereas their parasitoids favored moderate N fertilization. As a part of induced defense, the proportion of volatile products of glucosinolate pathway in the total oilseed rape VOC emission blend was increased. Our results suggest that the natural biological control of pollen beetle herbivory is best supported by moderate N fertilization rates.

Plant-Rich Field Margins Influence Natural Predators of Aphids More Than Intercropping in Common Bean

Simple Summary

Field margin plants are important in providing resources for natural enemies (NEs) and improving biological control of crop pests. However, the use of field margin plants for biological control particularly of important common bean pests is understudied in smallholder farming systems of sub-Saharan Africa (SSA). We evaluated the potential of field margin plants with respect to intercropping systems in common bean fields to enhance the population of NEs of common bean pests. We observed a high assemblage of important NEs of common bean pests for some insect taxa with minimal impact of intercropping on NEs. Field margin plants could be managed to provide a wide range of resources to NEs and therefore biological control of common bean pests.

Abstract

Field margins support important ecosystem services including natural pest regulation. We investigated the influence of field margins on the spatial and temporal distribution of natural enemies (NEs) of bean pests in smallholder farming systems. We sampled NEs from high and low plant diversity bean fields using sweep netting and coloured sticky traps, comparing monocropped and intercropped farms. NEs collected from within crops included predatory bugs, lacewings, predatory flies, parasitic flies, parasitic wasps, lady beetles, and a range of other predatory beetles; with the most dominant group being parasitic wasps. Overall, high plant diversity fields had a higher number of NEs than low-diversity fields, regardless of sampling methods. The field margin had a significantly higher number of lacewings, parasitic wasps, predatory bugs, syrphid flies, and other predatory beetles relative to the crop, but beneficial insects were collected throughout the fields. However, we observed marginally higher populations of NEs in intercropping than in monocropping although the effect was not significant in both low and high plant diversity fields. We recommend smallholder farmers protect the field margins for the added benefit of natural pest regulation in their fields.

Uniting RNAi Technology and Conservation Biocontrol to Promote Global Food Security and Agrobiodiversity

Habitat loss and fragmentation, and the effects of pesticides, contribute to biodiversity losses and unsustainable food production. Given the United Nation's (UN's) declaration of this decade as the UN Decade on Ecosystem Restoration, we advocate combining conservation biocontrol-enhancing practices with the use of RNA interference (RNAi) pesticide technology, the latter demonstrating remarkable target-specificity via double-stranded (ds)RNA's sequence-specific mode of action. This specificity makes dsRNA a biosafe candidate for integration into the global conservation initiative. Our interdisciplinary perspective conforms to the UN's declaration, and is facilitated by the Earth BioGenome Project, an effort valuable to RNAi development given its utility in providing whole-genome sequences, allowing identification of genetic targets in crop pests, and potentially relevant sequences in non-target organisms. Interdisciplinary studies bringing together biocontrol-enhancing techniques and RNAi are needed, and should be examined for various crop‒pest systems to address this global problem.

RNAi efficacy is enhanced by chronic dsRNA feeding in pollen beetle

Double-stranded RNAs (dsRNAs) represent a promising class of biosafe insecticidal compounds. We examined the ability to induce RNA interference (RNAi) in the pollen beetle Brassicogethes aeneus via anther feeding, and compared short-term (3 d) to chronic (17 d) feeding of various concentrations of dsRNA targeting αCOP (dsαCOP). In short-term dsαCOP feeding, only the highest concentration resulted in significant reductions in B. aeneus survival; whereas in chronic dsαCOP feeding, all three concentrations resulted in significant mortality. Chronic dsαCOP feeding also resulted in significantly greater mortality compared to short-term feeding of equivalent dsαCOP concentrations. Our results have implications for the economics and development of dsRNA spray approaches for managing crop pests, in that multiple lower-concentration dsRNA spray treatments across crop growth stages may result in greater pest management efficacy, compared to single treatments using higher dsRNA concentrations. Furthermore, our results highlight the need for research into the development of RNAi cultivars for oilseed rape protection, given the enhanced RNAi efficacy resulting from chronic, compared to short-term, dsRNA feeding in B. aeneus.

Jonathan Willow et al. use dsαCOP-treated anthers to examine RNAi efficacy against pollen beetle, a major pest of oilseed rape. The authors found that chronic feeding resulted in significantly greater mortality compared to short-term feeding; and significant silencing of αCOP was detected only in beetles chronically exposed to dsαCOP.

Approaches to Identify the Value of Seminatural Habitats for Conservation Biological Control

Invertebrates perform many vital functions in agricultural production, but many taxa are in decline, including pest natural enemies. Action is needed to increase their abundance if more sustainable agricultural systems are to be achieved. Conservation biological control (CBC) is a key component of integrated pest management yet has failed to be widely adopted in mainstream agriculture. Approaches to improving conservation biological control have been largely ad hoc. Two approaches are described to improve this process, one based upon pest natural enemy ecology and resource provision while the other focusses on the ecosystem service delivery using the QuESSA (Quantification of Ecological Services for Sustainable Agriculture) project as an example. In this project, a predictive scoring system was developed to show the potential of five seminatural habitat categories to provide biological control, from which predictive maps were generated for Europe. Actual biological control was measured in a series of case studies using sentinel systems (insect or seed prey), trade-offs between ecosystem services were explored, and heatmaps of biological control were generated. The overall conclusion from the QuESSA project was that results were context specific, indicating that more targeted approaches to CBC are needed. This may include designing new habitats or modifying existing habitats to support the types of natural enemies required for specific crops or pests.

Acute effect of low-dose thiacloprid exposure synergised by tebuconazole in a parasitoid wasp

Agricultural practices often involve tank-mixing and co-application of insecticides with fungicides to control crop pests. However, natural methods relying on biological control agents such as hymenopteran parasitoids have been shown to be highly effective in suppressing crop pest populations. The current body of insecticide risk assessment data accounting for fungicide co-application is very small, the present study being the first to examine this in a parasitoid wasp. Through low-dose exposure to dry residues of the neonicotinoid insecticide thiacloprid, we examined its mortal and knockdown effect on Aphelinus abdominalis when co-applied with increasing doses of the fungicide tebuconazole. Both of these acute effects of thiacloprid were synergised (toxicity increased to a greater-than-additive effect) by tebuconazole, resulting in significant mortality from low-dose co-applications of tebuconazole, and significant knockdown even without co-applied tebuconazole, the effect increasing as tebuconazole concentration increased. We show the highly toxic effect that a low dose of thiacloprid imposes on A. abdominalis populations, and a synergistic toxicity when co-applied with low doses of tebuconazole. Our work suggests a need for updating pesticide risk assessment methods, accounting for pesticide mixtures, in order to make these risk assessments more field relevant.