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Inbreeding and outbreeding in biological control agents: Do they actually matter?

It has been suggested for nearly 40 years that understanding and controlling population genetics of biological control agents is an important tool for improvement of the efficiency of biocontrol. Be it in industrial mass-rearing or field conditions, population may indeed suffer from inbreeding depression or benefit from higher genetic diversity. Hybridization of populations may also create heterosis effects or outbreeding depressions. Although theory is clear about the processes at play, little is known about their actual importance in concrete industrial productive conditions.

The mixed INRA-Biobest R&D team studied the impact of inbreeding and outbreeding depressions on several biocontrol agents in mass-rearing conditions. They used Allotropa burrelli (Hymenoptera Platygastridae), Macrolophus pygmaeus (Hemiptera Miridae), Chrysoperla externa (Neuroptera Chrysopidae) and Cryptolaemus montrouzieri (Coleoptera Coccinellidae) to study the effects of inbreeding on fitness-related phenotypic traits. In all species, they compared traits between individuals generated by random or sibling crosses.

They observed different levels of inbreeding depression suggesting that it has to be taken into consideration although the effect range depends on each organism. Moreover, they observed strong variations of fitness (from hybrid vigor to outbreeding depression) depending on genetic distance between M. pygmaeus populations. 

Potential negative effects of inbreeding within populations of E. kuehniella maintained in mass production conditions

Augmentative biological control relies on mass productions of biocontrol agents as well as their related hosts/preys and artificial media. Among the mass–reared hosts/preys, the Mediterranean flour moth, Ephestia kuehniella Zeller 1879 (Lepidoptera: Pyralidae) is of utmost importance for numerous biocontrol agents (oophagous parasitoids and predatory insects). Nevertheless, potential issues (e.g. inbreeding depression) related to the long-term rearing as well as opportunities for its genetic improvement have not yet been precisely addressed.

Within the frame of the COLBICS project, the mixed INRA-IAS-PUC R&D team, firstly, molecularly characterized various (lab- or wild) E. kuehniella strains, and, secondly, investigated intraspecific variability for relevant phenotypic traits using a dialllel cross involving four strains.

Taken as a whole, their results evidenced no strong molecular divergence between the available strains but an intraspecific variability for phenotypic traits. Additionally, the current “industrial strains” evidenced the best performances under standardized conditions and no inbreeding depression. These results however do not prevent investigating the possible long-term benefits of its introgression by wild strains.