Outbreeding of isofemale lines as a candidate approach to improve the quality of thelytokous Wolbachia-infected line of the egg parasitoid, Trichogramma dendrolimi Matsumura (Hymenoptera: Trichogrammatidae

The egg parasitoids, Trichogramma spp. are important biological control agents targeting various lepidopteran pests in agriculture. However, laboratory rearing often reduces genetic diversity, and potentially decreased fitness of Trichogramma wasps. Besides, infection of parthenogenesis-inducing Wolbachia allow Trichogramma females to produce nearly 100 % females, but may decrease fitness of infected females. This study investigated whether outbreeding the Wolbachia-infected thelytokous line (Td-W) with the high-fecundity bisexual line (Td-H) could improve its fitness. Furthermore, a bisexual Wolbachia-free line (Td), sharing the same genetic background as Td-W, and a line with less favorable traits (Td-L) were involved in cross-breeding experiments. The results showed that Td-W females could mate and be fertilized by bisexual males. Short Tandem Repeat (STR) analysis revealed no significant difference in the proportion of heterozygous offspring between mated Td-W and uninfected bisexual females. Outbreeding significantly increased the fecundity and body size of infected and uninfected offspring. Heterozygous offspring exhibited heterosis in fecundity and body size, regardless of Wolbachia infection, and displayed heterosis in emergence rates in Td-H♂×Td♀, Td♂×Td-L♀, and Td-L♂×Td♀ crosses. However, the emergence rate of Wolbachia-infected heterozygous offspring was lower than that of their Td-H or Td-L parents. Additionally, the adult longevity of heterozygous offspring did not differ significantly from that of their parents, irrespective of Wolbachia infection. Our findings suggest that outbreeding enhances fecundity and body size of Wolbachia-infected T. dendrolimi, regardless of the particular bisexual isofemale line used. This study proposes a strategy to improve the quality of Wolbachia-infected Trichogramma by outbreeding different isofemale lines before field release in biological control programs.