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Last update: May 2021

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Plantes et Système de cultures Horticoles

Zone de texte éditable et éditée et rééditée

Understanding the structure of trophic networks around pests and the impacts of pest enemies on predation and parasitism rates.

The pest – pest enemy interactions and the resulting parasitism or predation of the pests are key for conservation biological control. We enhanced our investigations on these questions thanks to methodological developments, in particular the development of PCR primers specific of the main apple tree pest species (leafrollers, aphids, anthonoma, sawfly) making it possible to detect them in the guts of potential predators. We also developed primers specific of the main predators or parasitoids of these pests.

Syrphelarve pour axe 3

Our studies on parasitoids of codling moth larvae showed that larvae are predominantly attacked by an ovo-larval parasitoid specializing on the tortricidae: Ascogaster quadridentata (Braconidae). This species represents 50 to 90% of the parasitoid community in non- treated orchards. This species is widespread throughout Europe. A second parasitoid (Pristomerus vulnerator, Ichneumonidae), more generalist, was more rarely present in southern France (5 to 15% of the community). Finally, Perilampus tristis (Perilampidae) is the third species present in larvae. The molecular detection of parasitoids in their host larva enabled us to show that P. tristis is an almost exclusively an hyperparasitoid species (98%), developing on A. quadridentata and P. vulnerator. The three species are present on all generations of the codling moth during the season in untreated orchards. Parasitism is low in commercial apple orchards, either organic (about 5%) or conventional (about 2%). It is also lower when orchards are surrounded by conventional orchards.

Codling moth larvae are prey of ground beetles and spiders in the fall. Aphids, including the rosy apple aphid, are prey of canopy dwelling spiders in the spring. The list of arthropods predators of the rosy apple aphid and the codling moth in orchards is quite large (at least 10 families of predators identified by PCR). An advantage of generalist predators such as spiders is that they can be present early in the season and therefore control the pest before it becomes abundant. On the other hand, generalist predators are likely to cause intra-guild predation that may be detrimental to pest control. PCR analyzes of gut contents or Petri dish observations make it possible to study the predator community of pests but do not provide direct evidence of their control of pests. We thus combine these approaches with two other approaches. The first is the exposure of sentinel prey to measure potential predation rates. The use of codling moth eggs as sentinel prey showed the important role of farmers' phytosanitary practices in the study plots and in their surrounding plots on predation, with predation rates ranging from 18% to 63% in August. It also showed an unexpected negative effect of an abundant grassy ground cover (untreated orchard). The second approach, applicable when it is possible to estimate finely the abundances of pests and pest enemies, consists in correlating these abundances. Through monitoring in commercial apple orchards, we have shown that the presence of pest enemies generally reduces the growth of aphid colonies. Early in the season, canopy dwelling spiders (particularly those of the genus Philodromus) may reduce the number of aphid colonies in organic or untreated orchards. However, spiders are also predators of pest enemies such as syrphid larvae, including in the presence of aphids, which may impair the effectiveness of syrphids later in the season.