Trichogramma

Trichogramma is a genus of minute polyphagous wasps that are endoparasitoids of insect eggs. Trichogramma is one of around 80 genera from the family Trichogrammatidae, with over 200 species worldwide. Although several groups of egg parasitoids are commonly employed for biological control throughout the world, Trichogramma spp. have been the most extensively studied. More than a thousand papers have been published on Trichogramma species, and they are the most used biological control agents in the world. Trichogramma spp. are also of interest in neuroscience research, having fewer than 10,000 neurons, approaching the theoretical lower limit of the size of an insect brain, yet exhibiting complex behaviors to sustain their lives.

Trichogramma is a genus of minute polyphagous wasps that are endoparasitoids of insect eggs. Trichogramma is one of around 80 genera from the family Trichogrammatidae, with over 200 species worldwide. Although several groups of egg parasitoids are commonly employed for biological control throughout the world, Trichogramma spp. have been the most extensively studied. More than a thousand papers have been published on Trichogramma species, and they are the most used biological control agents in the world. Trichogramma spp. are also of interest in neuroscience research, having fewer than 10,000 neurons, approaching the theoretical lower limit of the size of an insect brain, yet exhibiting complex behaviors to sustain their lives.

Wolbachia in Trichogramma

Wolbachia is a widespread bacterial genus that infects insects' organs, most commonly the reproductive organs. Wolbachia has been observed to alter the host's reproductive success upon infection. Through a series of manipulations, Wolbachia-infected hosts transmit this intracellular bacterium to uninfected individuals. These manipulations include male killing (increasing ratio of infected females that can reproduce), feminization (males become fertile females), parthenogenesis, and cytoplasmic incompatibility. Horizontal transfer of parthenogenesis-inducing Wolbachia, which has been observed in Trichogramma wasps, causes infected females to asexually produce fertile females and nonfunctional males. The effects of this include potential speciation of Trichogramma, if Wolbachia is maintained long enough for genetic divergence to occur and for a new species of asexual wasps to become reproductively isolated. Transmission of the bacterium through horizontal transfer has been observed within the same species and among different species of Trichogramma, including T. kaykai, T. deion, T. pretiosum, and T. atopovirilia; however, limitations to transmission exist. In vitro successful horizontal transfer is uncommon within Trichogramma, which suggests that the density of Wolbachia must be relatively high inside of the hosts' ovaries. Cytoplasmic incompatibility of the host and bacterium can also be the source of this unsuccessful transfer in-vitro. These limitations in vitro suggest that in nature, horizontal transfer by parthenogenesis-inducing Wolbachia may be a difficult and rare phenomenon. However, when looking at the Wolbachia-host associations, the Trichogramma-Wolbachia form a monophyletic group based on several Wolbachia-specific genes, which may be explained by horizontal transfer of Wolbachia between different species. Therefore, although interspecific horizontal transfer of Wolbachia is limited in vitro, it is likely to occur quite frequently in nature and is not well understood yet. The effects of Wolbachia in Trichogramma have several evolutionary implications. Commonly, uninfected wasps are unable to breed with infected wasps. Many generations of reproductive isolation of these different groups may result in speciation. In addition, some hosts can evolve with a dependency on Wolbachia for core reproductive functions, such as oogenesis, so that eventually an infection is a requirement for successful reproduction. Finally, Wolbachia can influence gender determination in its hosts so that more females are successfully born. This results in a reversal in sexual selection, where females must compete for male mates, which has evolutionary implications as it exposes different phenotypes to natural selection.

T. pretiosum

T. pretiosum is the most widely distributed species in North America. It is a more generalized parasitoid, able to parasitise a range of different species. It has been the focus of many research studies and has been successfully reared on 18 genera of Lepidoptera. T. pretiosum was introduced into Australia in the 1970s as part of the Ord River Irrigation Area IPM scheme.

T. carverae

Trichogramma carverae is mainly used for light brown apple moth and codling moth control, and is predominately used in orchards. In Australia, T. carverae is used for biological control of light brown apple moth in vineyards. Though Australia has its own native Trichogramma species, not much work has been undertaken to use them commercially for biological control within Australia. Light brown apple moth is common throughout Australia and is polyphagous on more than 80 native and introduced species. The larvae cause the most damage, especially to grape berries, as their feeding provides sites for bunch rot to occur. Losses in the crops can amount up to $2000/ha in one season. It is very predominant in areas such as the Yarra Valley. Insecticide use is not a choice method for most growers, who prefer a more natural means of controlling pests. As a result, Trichogramma wasps were considered a good candidate for biological control, even more so as the moth larvae are difficult to control with insecticide. Moreover, light brown apple moths are relatively vulnerable to egg parasitism, with their eggs being laid in masses of 20 to 50 on the upper surfaces of basal leaves in grapevines.

Species

List of Trichogramma species

Text derived from https://en.wikipedia.org/w/index.php?oldid=1373438755 under CC BY-SA 4.0; modifications have been made.