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

Abstract

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.

Entomophagous insects: Predators and parasitoids that shape insect communities and offer valuable tools for insect pest management

“…Both, biotic and abiotic factors conditioning the development of parasitoids play a crucial role in shaping their life-history traits and population dynamics. Martel, Régnière, et al (2024) investigate the competitive interactions between two parasitoid species, Tranosema rostrale (Brishke) (Hymenoptera: Ichneumonidae) and Elachertus cacoeciae (Howard) (Hymenoptera: Eulophidae), targeting the spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae). Through field experiments conducted over 35 years, the authors 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, indicating adaptive strategies to minimize direct competition and maximize their respective performance.…”