Density‐dependence and competition between Tranosema rostrale and Elachertus cacoeciae, two parasitoids of the spruce budworm

• Published in: Entomologia experimentalis et applicata 2024-06, Vol.172 (6), p.533-543
• Main Authors: Martel, Véronique, Régnière, Jacques, Therrien, Pierre
• Format: Article
• Language: English
• Subjects: Choristoneura fumiferana; Competition; density dependence; Eulophidae; forest; Host-parasite interactions; Hymenoptera; Ichneumonidae; idiobiont; Insects; koinobiont; Larvae; Lepidoptera; multiple parasitism; outbreak; Parasitoids; Pest outbreaks; Population density; Population regulation; Tortricidae
• ISSN: 0013-8703; 1570-7458
• DOI: 10.1111/eea.13426

Competition between parasitoids for available hosts is common in insects. These interactions are often negative and are believed to reduce the overall impact of top‐down population regulation. In the spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae), two parasitoid species are very common and often interact during the period of low‐density populations between outbreaks: the koinobiont endoparasitoid Tranosema rostrale (Brishke) (Hymenoptera: Ichneumonidae) and the idiobiont ectoparasitoid Elachertus cacoeciae (Howard) (Hymenoptera: Eulophidae). Data collected from field‐implanted larvae of spruce budworm exposed in summer during a 35‐year period between 1987 and 2022 were used to estimate the response of these two parasitoids to the density of their spruce budworm host, and the nature and importance of competitive interactions between them. It was found that both parasitoids are most effective at very low host population density, and that a strong, density‐dependent, and negative correlation of frequencies exists between them. This negative correlation is evident both among and within years and indicates that the parasitoids have developed seasonal history and host choice behaviors that tend to minimize the negative impact of direct competition on their respective performance. It appears that the koinobiont T. rostrale manages, through those adaptive strategies, to avoid direct competition inside a larval host with the competitively superior idiobiont E. cacoeciae. Résumé [Different language – online only] La compétition entre les parasitoïdes pour les hôtes disponibles est fréquente. Ces interactions sont souvent négatives et considérées réduire l'impact global de la régulation par le haut des populations. Chez la tordeuse des bourgeons de l'épinette, Choristoneura fumiferana (Clemens), durant les périodes de basse densité de populations entre les épidémies, deux espèces de parasitoïdes sont très communes et interagissent souvent: l'endoparasitoïde koïnobionte, Tranosema rostrale (Brishke), et l'ectoparasitoïde idiobionte, Elachertus cacoeciae (Howard). Des données obtenues à partir de larves de tordeuse des bourgeons de l'épinette implantées sur le terrain durant l'été pendant une période de 35 ans entre 1987 et 2022 ont permis d'estimer la réponse de ces deux parasitoïdes à la densité de leur hôte, la tordeuse des bourgeons de l'épinette, et la nature et l'importance de ces interactions entre eux. Les deux espèces sont plus e