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Psylliodes chrysocephala is among the main pests of oilseed rape in Europe and is mainly managed with insecticides. Increasing insecticide resistance and health and environmental concerns have highlighted the need for alternatives to chemical control, such as conservation biological control. Conservation biological control can be enhanced by (1) providing resources to beneficial organisms in and around crop fields, like providing parasitoids with resources through sowing flower strips, and (2) by redesigning cropping systems to promote bottom-up and top-down processes contributing to natural pest control.
Using an observational approach based on a network of farmers' fields, we assessed the effects of cropping systems and flower strips on P. chrysocephala and their regulation throughout their development cycle. We selected fields in the Paris Basin grown under organic farming, under soil conservation practices, and conventionally, with many intermediary practices. Some of the fields contained a perennial flower strip sown in 2018. Observations on the autumn phase of P. chrysocephala cycle were carried out in 2020 and 2021, and those on the spring part of the cycle in 2019 and 2020, with 20 to 30 fields monitored each time.
The damage caused by P. chrysocephala is mainly depending on the number of larvae present in the oilseed rape plants in winter. An initial hypothesis was related to the presence of residues on the soil surface, favoured in no-till farming, which could hinder the movement of P. chrysocephala on the soil surface. At the field level, we observed direct effects of certain practices, such as a negative effect of insecticide treatments (halving the number of larvae) and a higher presence of larvae with the occurrence of ploughing. However, we did not observe any effect of residue covering the soil surface on the amount of pest larvae. The percentage of soil covered by total vegetation (crop, companion plants and weeds) or by companion plants and weeds alone had a negative effect on the number of pest larvae, with a threefold reduction of their number when the total vegetation cover increases from 10 to 100%). Finally, of the predators present on the soil surface (carabid beetles, harvestmen), only spider activity-density had a slight negative effect on the number of pest larvae per plant. At the landscape scale, inter-annual variation in oilseed rape area within a radius of 1000 m influenced the number of P. chrysocephala per plant, with an increase in larvae quantities when the oilseed rape area decreased between the previous and the current year.
At the end of winter, P. chrysocephala larvae were on average 30% parasitised by microhymenoptera parasitoids (Tersilochus sp.), with considerable variation between fields. Parasitism was slightly higher (but not significantly) in insecticide-untreated fields with a flower strip than in insecticide-treated fields without a flower strip. In addition, flowering and nectar-providing weeds in the field had a positive effect on parasitism levels.
At the end of their development in oilseed rape stems and leaves, larvae fall to the soil surface before pupating underground, where they are highly vulnerable to predators. Using sentinel prey similar to P. chrysocephala larvae, we observed that approximately half of the prey was consumed within 24 hours, with no difference between cropping systems. Camera observations coupled with sentinel prey revealed that larvae were mainly consumed by carabids (64% of predation events) and secondarily by rove beetles, ants and chilopods. These are generalist predators that are functionally redundant, resulting in homogeneous predation rates between cropping systems, despite observed differences in the composition of ground-dwelling predator communities between cropping systems.
In conclusion, P. chrysocephala populations can be limited throughout their life cycle. Growing oilseed rape with service plants in the autumn and the presence of floral resources in the spring, for example in flower strips, are two ways of promoting this regulation. We have also verified and validated the fact that P. chrysocephala spend the summer in forests and that flower strips are not a favourable habitat for these pests during their summer diapause. The biological control by predators and parasitoids occurs after the damage has been done to the crop. It does contribute to reduce pest populations only in the following year, which argues in favour of multi-annual and territorial management of P. chrysocephala populations.
| Keywords | cropping system; stem flea beetle; conservation biological control; flower strip; agroecological pest management |
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