The hidden side of the Allee effect: correlated demographic traits and extinction risk in experimental populations

Because Allee effects have major impacts on the dynamics of small populations, they are routinely included in demographic models for the evaluation of extinction risks. However, the structure of most common models implies that other demographic parameters (like the maximum growth rate) are modified by the inclusion of an Allee effect, which also affects in return the extinction risk of the population. Whether such correlations between demographic traits occur in natural populations or merely reflect a practical constraint related to model formalism is of primary importance to understand better the dynamics of small populations. We investigated this question using 20 populations of Trichogramma wasps raised under similar conditions, of which 3 were subject to an Allee effect. We showed that these 3 populations were also characterized by lower maximum growth rate and lower carrying capacity, and that their extinction probability was higher than for non-Allee populations. These results provide the first empirical demonstration of a correlation between the presence of positive density-dependence and impaired demographic performance, which increases the extinction risk of population, especially during the establishment phase

Life-history traits, pace of life and dispersal among and within five species of Trichogramma wasps: a comparative analysis

Major traits defining the life history of organisms are often not independent from each other, with most of their variation aligning along key axes such as the pace-of-life axis. We can define a pace-of-life axis structuring reproduction and development time as a continuum from less-fecund, longer-developing ″slow″ types to more-fecund, shorter-developing ″fast″ types. Such axes, along with their potential associations or syndromes with other traits such as dispersal, are however not universal; in particular, support for their presence may be taxon and taxonomic scale-dependent. Knowing about such life-history strategies may be especially important for understanding eco-evolutionary dynamics, as these trait syndromes may constrain trait variation or be correlated with other traits. To understand how life-history traits and effective dispersal covary, we measured these traits in controlled conditions for 28 lines from five species of Trichogramma, which are small endoparasitoid wasps frequently used as a biological model in experimental evolution but also in biocontrol against Lepidoptera pests. We found partial evidence of a pace-of-life axis at the interspecific level: species with higher fecundity also had faster development time. However, faster-developing species also were more likely to delay egg-laying, a trait that is usually interpreted as ″slow″. There was no support for similar covariation patterns at the within-species line level. There was limited variation in effective dispersal between species and lines, and accordingly, we did not detect any correlation between effective dispersal probability and life-history traits. We discuss how expanding our experimental design by accounting for the density-dependence of both the pace of life and dispersal might improve our understanding of those traits and how they interact with each other. Overall, our results highlight the importance of exploring covariation at the ″right″ taxonomic scale, or multiple taxonomic scales, to understand the (co)evolution of life-history traits. They also suggest that optimizing both reproductive and development traits to maximize the efficiency of biocontrol may be difficult in programs using only one species

The hidden side of the Allee effect: correlated demographic traits and extinction risk in experimental populations

Because Allee effects have major impacts on the dynamics of small populations, they are routinely included in demographic models for the evaluation of extinction risks. However, the structure of most common models implies that other demographic parameters (like the maximum growth rate) are modified by the inclusion of an Allee effect, which also affects in return the extinction risk of the population. Whether such correlations between demographic traits occur in natural populations or merely reflect a practical constraint related to model formalism is of primary importance to understand better the dynamics of small populations. We investigated this question using 20 populations of Trichogramma wasps raised under similar conditions, of which 3 were subject to an Allee effect. We showed that these 3 populations were also characterized by lower maximum growth rate and lower carrying capacity, and that their extinction probability was higher than for non-Allee populations. These results provide the first empirical demonstration of a correlation between the presence of positive density-dependence and impaired demographic performance, which increases the extinction risk of population, especially during the establishment phase

Recherche d’espèces du genre Trichogramma sur Cucurbitacées cultivées et plantes associées en Martinique en vue du développement de méthodes de lutte biologique (Hymenoptera, Trichogrammatidae)

Sampling of Trichogramma species on Cucurbitaceae and associated plants in Martinique : a first step towards biological control (Hymenoptera, Trichogrammatidae). Cucurbitaceae represent an important part of the vegetable production in Martinique. One of the main pest on these crops is the melonworm, Diaphana hyalinata. With the final objective of defining sustainable strategies for the management of this pest with low quantities of pesticides, an inventory of the Trichogramma egg parasitoids was conducted in 2014 on the island. This survey was implemented using two methods, the use of sentinel eggs of Ephestia kuehniella and collections of natural egg clutches. Based on molecular information, two species were finally observed, Trichogramma pretiosum Riley, 1879, and T. chilonis Ishii, 1941. Based on this survey, a strain of T. pretiosum was mass-reared using Corcyra cephalonica and used for first promising inundative releases.Les Cucurbitacées représentent une part importante de la production maraîchère de Martinique. Un des principaux ravageurs de ces cultures est la Pyrale des cucurbitacées, Diaphania hyalinata. Avec pour objectif final la définition de stratégies de gestion économe en produits phytosanitaires, un inventaire des parasitoïdes oophages du genre Trichogramma a été réalisé en novembre 2014 sur l’île de la Martinique. Cet échantillonnage a été réalisé selon deux méthodes, l’utilisation d’oeufs-sentinelles d’Ephestia kuehniella et la recherche de pontes naturelles. Au final, deux espèces ont été identifiées sur une base moléculaire, Trichogramma pretiosum Riley, 1879, et T. chilonis Ishii, 1941. Suite à cet échantillonnage, T. pretiosum a été mis en élevage sur un hôte de substitution, Corcyra cephalonica, et utilisé pour des premiers lâchers inondatifs prometteurs.Dumbardon-Martial Thierry, Lucas Pierre-Damien, Warot Sylvie, Ris Nicolas, Groussier Géraldine. Recherche d’espèces du genre Trichogramma sur Cucurbitacées cultivées et plantes associées en Martinique en vue du développement de méthodes de lutte biologique (Hymenoptera, Trichogrammatidae). In: Bulletin de la Société entomologique de France, volume 123 (4),2018. pp. 401-410

First record of Telenomus kolbei (Hymenoptera, Scelionidae) in France, parasitizing the eggs of Nymphalis antiopa and Aglais io (Lepidoptera, Nymphalidae)

Telenomus kolbei is newly reported in France, parasitizing two new host species: Aglais io and Nymphalis antiopa. Oviposition by the parasitoid into eggs of both species was observed. Films were made, to which links are given. DNA sequences were obtained and deposited in Genbank

Up and to the light: intra- and interspecific variability of photo- and geo-tactic oviposition preferences in genus Trichogramma

International audienceTrichogramma are parasitic microwasps much used as biological control agents. The genusis known to harbor tremendous diversity, at both inter- and intra-specific levels. The successful selection of Trichogramma strains for biocontrol depends on characterizing the existing diversity in this group, especially regarding oviposition performance and behavior.Oviposition preferences in relation to environmental cues such as light and gravity havereceived little attention so far, although they are suspected to play an important role inmicrohabitat selection and parasitism patterns. The extent of their variability, and theirpotential correlated variations, is virtually unknown. Here we use a novel experimental approach relying on automatic image analysis to characterize the oviposition preferences inrelation to light and gravity, as well as their interaction, in 25 populations of Trichogrammafrom five species. We show that most Trichogramma populations and species harbour preferences for light and preferences for elevated parts. However, the two traits harbor significant inter and intraspecific variation. The effects of light and gravity on oviposition patterns were found to be almost perfectly additive overall, with two exceptions. Ovipositionpreference patterns were not static but very plastic in time: preferences tended to relaxover consecutive days, and the strongest preferences relaxed the fastest, presumably because of the density-dependent effect of resource depletion. A correlation of ovipositionpatterns with the vegetation stratum at which populations were sampled suggests thatdifferent species/populations may be associated with different strata with correspondingdifferentiation in light- and gravity-related oviposition preferences

Heavy metals used as fungicide may positively affect Trichogramma species used as biocontrol agents in IPM programs

The use of biological control agents in Integrated Pest Management programs has increased in the last decades, but may be affected by antagonistic effects generated by the accumulation of some pesticides and other chemicals. The objective of this study was to evaluate the performance of Trichogramma cordubensis (Hymenoptera: Trichogrammatidae), a candidate agent to control grapevine moths, when these beneficial insects are exposed to copper treatments through trophic accumulation. A generation of the host Lobesia botrana (Lepidoptera: Tortricidae) was reared on a diet with different concentrations of copper from Bordeaux mixture incorporated into the diet. The host eggs laid by these females were then exposed to female parasitoids. Our results showed that copper consumed by the host parental generation had no effects on the parasitism rate but had positive effects on the emergence rate and size of emerging parasitoids at the highest copper concentration. These effects of copper on parasitoids may be due to stress effects of copper on the host parental generation, linked with a trade-off between development and defenses leading to a reduced immunity in their host eggs. Another hypothesis may be that copper has been transferred into the host eggs at potentially beneficial concentrations for parasitoids due to detoxification mechanisms of the hosts' mothers facing a chemical stress. These positive effects of copper pesticides on parasitoids have been observed at the highest copper concentration, consistent with real exposure conditions in the field. This study thus highlights potential synergetic effects between pesticides and natural enemies

Landscape connectivity alters the evolution of density-dependent dispersal during pushed range expansions

A bstract As human influence reshapes communities worldwide, many species expand or shift their ranges as a result, with extensive consequences across levels of biological organization. Range expansions can be ranked on a continuum going from pulled dynamics, in which low-density edge populations provide the “fuel” for the advance, to pushed dynamics in which high-density rear populations “push” the expansion forward. While theory suggests that evolution during range expansions could lead pushed expansions to become pulled with time, empirical comparisons of phenotypic divergence in pushed vs. pulled contexts are lacking. In a previous experiment using Trichogramma brassicae wasps as a model, we showed that expansions were more pushed when connectivity was lower. Here we used descendants from these experimental landscapes to look at how the range expansion process and connectivity interact to shape phenotypic evolution. Interestingly, we found no clear and consistent phenotypic shifts, whether along expansion gradients or between reference and low connectivity replicates, when we focused on low-density trait expression. However, we found evidence of changes in density-dependence, in particular regarding dispersal: populations went from positive to negative density-dependent dispersal at the expansion edge, but only when connectivity was high. As positive density-dependent dispersal leads to pushed expansions, our results confirm predictions that evolution during range expansions may lead pushed expansions to become pulled, but add nuance by showing landscape conditions may slow down or cancel this process. This shows we need to jointly consider evolution and landscape context to accurately predict range expansion dynamics and their consequences