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Trichogramma are 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 there are several groups of egg parasitoids commonly employed for biological control throughout the world, Trichogramma have been the most extensively studied. There have been more than a thousand papers published on Trichogramma and they are the most used biological control agents in the world. Trichogramma are unique in approaching the size limit of how small an insect can be, which would be determined by how few neurons they can fit in their central nervous system, yet exhibiting a complex behavior to sustain their life. Trichogramma have less than 10,000 neurons, which is a hundred times fewer than the next smallest insect.
To locate host eggs, adult females use chemical and visual signals, such as egg shape and colour. After she finds a suitable egg, an experienced female will attempt to determine if the egg has previously been parasitized, using her ovipositor and antennal drumming (tapping on the egg surface). Females also use antennal drumming to determine the size and quality of the target egg, which determines the number of eggs the female will insert. A single female can parasitize one to ten host eggs a day.
Trichogramma are small and very uniform in structure, which causes difficulty in identifying the separate species. As females are all relatively similar, taxonomists rely upon examination of males to tell the different species apart, using features of their antennae and genitalia.
The first description of a Trichogramma species was in North America in 1871, by Charles V. Riley. He described the tiny wasps that emerged from eggs of the Viceroy butterfly as Trichogramma minutum. In taxonomy, original specimens are very important as they are the basis of reference for subsequent descriptions of species. The original specimens, however, were lost. Riley also described a second species in 1879 as Trichogramma pretiosum, but these specimens were also lost. To correct these errors, entomologists returned to the areas where Riley originally found the species and obtained neotype specimens of T. minutum and T. pretiosum. These specimens are now preserved properly in the United States National Museum. Currently the number of Trichogramma species is over 200, but as of 1960 only some 40 species of Trichogramma had been described.
Wolbachia in Trichogramma
Wolbachia is a widespread bacterium that infects insect's 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 who 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, T. atopovirilia; however, there are limitations to transmission. In-vitro successful horizontal transfer is uncommon within Trichogramma, which suggests that the density of Wolbachia must be relatively high inside of the host’s 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.
Trichogramma have been used for control of lepidopteran pests for many years. They can be considered the Drosophila of the parasitoid world, as they have been used for inundative releases and much of our understanding today comes from experiments with these wasps.
Entomologists in the early 1900s began to rear Trichogramma for biological control. Trichogramma minutum is one of the most commonly found species in Europe and was first mass reared in 1926 on eggs of Sitotroga cerealella.
Nine species of Trichogramma are produced commercially in insectaries around the world, with 30 countries releasing them. Trichogramma are used for control on numerous crops and plants; these include cotton, sugarcane, vegetables, sugar beets, orchards and forests. Some of the pests that are controlled include Cotton bollworm (Helicoverpa armigera), Codling moth (Cydia pomonella), Lightbrown apple moth (Epiphyas postvittana), and European corn borer (Ostrinia nubilalis).
Trichogramma species vary in their host specificity. This can lead to non-target hosts being parasitized. This in turn can cause problems by reducing the amount of parasitism of the target host; and, depending on the rate of parasitism, non-target effects could be significant on non-target host populations.
Trichogramma pretiosum is the most widely distributed Trichogramma 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 (ORIA) IPM scheme.
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, there has not been much work 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 are the stage that causes 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 like 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 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 20-50 on the upper surfaces of basal leaves in grapevines.
- Trichogramma aomoriense
- Trichogramma atopovirilia
- Trichogramma brassicae
- Trichogramma brevicapillum
- Trichogramma carverae
- Trichogramma chilonis
- Trichogramma deion
- Trichogramma dendrolimi
- Trichogramma evanescens
- Trichogramma exiguum
- Trichogramma falx
- Trichogramma fuentesi
- Trichogramma funiculatum
- Trichogramma japonicum
- Trichogramma maori
- Trichogramma minutum
- Trichogramma nubilale
- Trichogramma papilonis
- Trichogramma platneri
- Trichogramma pretiosum
- Trichogramma siddiqi
- Trichogramma thalense
- Trichogramma valentinei
- Trichogramma yawarae
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