Predicting crop injury caused by flea beetles in spring oilseed rape through pest monitoring in the autumn

1. Reliably predicting pest damage would allow farmers to reduce insecticide use without incurring economic losses and thus contribute to agricultural sustainability. However, means to predict pest severity are lacking.2. We assessed whether crop feeding injury caused by flea beetles in spring oilseed rape can be predicted from flea beetle pest densities in the previous season using 22 years of suction trap catches of flea beetles in combination with crop feeding injury data from 293 fields.3. We found a strong positive relationship between the densities of flea beetles of the genus Phyllotreta in the summer and autumn activity period of the previous year and crop feeding injury caused by flea beetles in spring oilseed rape the following year. Autumn weather or the total cover of spring oilseed rape in the study region did not improve the prediction further.4. Pest monitoring using suction traps is thus a promising tool to predict crop feeding injury and can reduce insecticide use in years with low pest pressures

Pest management and yield in spring oilseed rape without neonicotinoid seed treatments

Use of neonicotinoid insecticides as seed treatments has been prohibited in the EU. As a consequence, concerns of lost production have been raised among producers. It remains, however, unclear to what extent the ban has increased pest attacks and crop damage, and reduced yield and farm profit. It is also unclear to what extent alternative, non-chemical options can protect crops. Flea beetles (Chrysomelidae: Alticini) are the main pests targeted by insecticide seed treatments in spring oilseed rape (Brassica napus L.). Over three years, we conducted 23 field experiments in which we compared seeds treated with neonicotinoids with untreated seeds, grown at normal or doubled sowing rates. The experiments were established during a range of sowing times at the same time as the hosting farmer sowed, which also allowed us to assess the impact of sowing date. We measured flea beetle activity density, crop plant density, cotyledon damage, crop yield and relative economic performance. Flea beetle activity density was eight times higher in 2014 than in 2016, with intermediate activity in 2015. Neonicotinoid seed treatment, increased sowing rate and an earlier sowing date all reduced crop damage. Seed treatment decreased crop yield loss by 521 kg ha(-1) and relative profit loss by 144 Euro ha(-1) in 2014, but had no effect on yield or profit in 2015-2016. Increased sowing rate did not affect yield, but decreased profit in 2015 (-138 Eum ha(-1)) and 2016 (-114 Euro ha(-1)), mainly due to higher costs for seed. Earlier sowing date was consistently associated with higher yield and profit. Our results put prophylactic seed treatments in question, as they gave lower yield losses in only one year out of three. Earlier sowing and somewhat higher sowing rate emerge as viable alternative pest management practices. Because management outcomes depended on pest pressure, which varied from year to year, crop damage prognosis tools are needed based on improved understanding of the population ecology of crop pests, to support the growers' decisions and avoid unnecessary use of insecticides

Data from: Interactive effects of pests increase seed yield

Loss in seed yield and therefore decrease in plant fitness due to simultaneous attacks by multiple herbivores is not necessarily additive, as demonstrated in evolutionary studies on wild plants. However, it is not clear how this transfers to crop plants that grow in very different conditions compared to wild plants. Nevertheless, loss in crop seed yield caused by any single pest is most often studied in isolation although crop plants are attacked by many pests that can cause substantial yield losses. This is especially important for crops able to compensate and even overcompensate for the damage. We investigated the interactive impacts on crop yield of four insect pests attacking different plant parts at different times during the cropping season. In 15 oilseed rape fields in Sweden, we estimated the damage caused by seed and stem weevils, pollen beetles, and pod midges. Pest pressure varied drastically among fields with very low correlation among pests, allowing us to explore interactive impacts on yield from attacks by multiple species. The plant damage caused by each pest species individually had, as expected, either no, or a negative impact on seed yield and the strongest negative effect was caused by pollen beetles. However, seed yield increased when plant damage caused by both seed and stem weevils was high, presumably due to the joint plant compensatory reaction to insect attack leading to overcompensation. Hence, attacks by several pests can change the impact on yield of individual pest species. Economic thresholds based on single species, on which pest management decisions currently rely, may therefore result in economically suboptimal choices being made and unnecessary excessive use of insecticides

Pollen beetle mortality is increased by ground-dwelling generalist predators but not landscape complexity

Biological control of crop pests by naturally occurring arthropods depends on the entire community of natural enemies, but generalist predators and parasitoids are rarely considered in the same study. Also, the level of biological control in the field is affected by both within-field and landscape scale management. A multi-taxa approach that integrates multiple scales of management is needed to understand drivers for pest mortality. We examined local (weed cover and soil characteristics) and landscape (proportions of semi-natural and oilseed rape habitat) effects on natural enemy communities and biological control of pollen beetles in 15 oilseed rape (OSR) fields in Sweden. We found that agricultural intensification at the local (low weed cover) and landscape scale (low proportion of semi-natural area) increased evenness of generalist predators, but had no effect on the densities of pests and their natural enemies. This suggests that the generalist predators in OSR are well adapted to crop lands, at least within the examined gradient. Increasing OSR in the landscape decreased parasitoid densities and increased pest density, indicating a potential loss of pest control services by specialist natural enemies in landscapes with a high proportion of OSR. Finally, pollen beetle mortality increased with ground dwelling generalist predator abundance and soil clay content. Parasitism rates did not affect pest mortality, which is interesting as parasitoids have been considered major control agents in OSR. The hypothesis that increasing semi-natural habitat in the landscape enhances natural enemy abundances and species richness in agricultural landscapes was not supported. Local measures targeting generalist predators appear as a reasonable strategy to maximize pollen beetle control

Data from: Habitat heterogeneity induces rapid changes in the feeding behaviour of generalist arthropod predators

1. The “habitat heterogeneity hypothesis” predicts positive effects of structural complexity on species coexistence. Increasing habitat heterogeneity can change the diversity (number of species, abundances) and the functional roles of communities. The latter, however, is not well understood as species and individuals may respond very differently and dynamically to a changing environment. 2. Here, we experimentally test how habitat heterogeneity affects generalist arthropod predators, including epigaeic spiders, carabid and staphylinid beetles, under natural conditions by assessing their diversity and directly measuring their trophic interactions (which provide a proxy for their functional roles). The experiment was conducted in spring barley fields in Southern Sweden where habitat heterogeneity was manipulated by increasing within-field plant diversity. 3. Increased habitat heterogeneity triggered rapid changes in the feeding behaviour of generalist predators characterized by lower trophic specialization at both network (H2’, degree of interaction specialization in the entire network) and species level (d’, degree of interaction specialization at the species level). We presume that this is because spatial separation resulted in relaxed competition and allowed an increased overlap in resources used among predator species. Predators collected from heterogenous habitats also showed greater individual-level dietary variability which might be ascribed to relaxed intraspecific competition. 4. Our results provide conclusive evidence that habitat heterogeneity can induce rapid behavioural responses independent of changes in diversity, potentially promoting the stability of ecosystem functions

High Redundancy as well as Complementary Prey Choice Characterize Generalist Predator Food Webs in Agroecosystems

Food web structure influences ecosystem functioning and the strength and stability of associated ecosystem services. With their broad diet, generalist predators represent key nodes in the structure of many food webs and they contribute substantially to ecosystem services such as biological pest control. However, until recently it has been difficult to empirically assess food web structure with generalist predators. We utilized DNA-based molecular gut-content analyses to assess the prey use of a set of generalist invertebrate predator species common in temperate agricultural fields. We investigated the degree of specialization of predator-prey food webs at two key stages of the cropping season and analysed the link temperature of different trophic links, to identify non-random predation. We found a low level of specialization in our food webs, and identified warm and cool links which may result from active prey choice or avoidance. We also found a within-season variation in interaction strength between predators and aphid pests which differed among predator species. Our results show a high time-specific functional redundancy of the predator community, but also suggest temporally complementary prey choice due to within-season succession of some predator species

Habitat heterogeneity induces rapid changes in the feeding behaviour of generalist arthropod predators

The "habitat heterogeneity hypothesis" predicts positive effects of structural complexity on species coexistence. Increasing habitat heterogeneity can change the diversity (number of species, abundances) and the functional roles of communities. The latter, however, is not well understood as species and individuals may respond very differently and dynamically to a changing environment. Here, we experimentally test how habitat heterogeneity affects generalist arthropod predators, including epigaeic spiders, carabid and staphylinid beetles, under natural conditions by assessing their diversity and directly measuring their trophic interactions (which provide a proxy for their functional roles). The experiment was conducted in spring barley fields in Southern Sweden where habitat heterogeneity was manipulated by increasing within-field plant diversity. Increased habitat heterogeneity triggered rapid changes in the feeding behaviour of generalist predators characterized by lower trophic specialization at both network (H2', degree of interaction specialization in the entire network) and species level (d', degree of interaction specialization at the species level). We presume that this is because spatial separation resulted in relaxed competition and allowed an increased overlap in resources used among predator species. Predators collected from heterogenous habitats also showed greater individual-level dietary variability which might be ascribed to relaxed intraspecific competition. Our results provide conclusive evidence that habitat heterogeneity can induce rapid behavioural responses independent of changes in diversity, potentially promoting the stability of ecosystem functions. A plain language summary is available for this article

Gagic2016_EcoEvo_data

Seed yield, individual yield components and plant damage dat

foodweb data

foodweb dat