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.Consoli FL, Parra JRP, Zucchi RA (2010) 'Egg Parasitoids in Agroecosystems with Emphasis on Trichogramma.' (Springer).Knutson A (2005) 'The ''Trichogramma'' Manual: A guide to the use of ''Trichogramma'' for Biological Control with Special Reference to Augmentative Releases for Control of Bollworm and Budworm in Cotton.' (Texas Agricultural Extension Service).

Although several groups of egg parasitoids are commonly employed for biological control throughout the world, ''Trichogramma'' spp. have been the most extensively studied.Upadhyay RK, Mukerji KG, Chamola BP (2001) 'Biocontrol potential and its Exploitation in Sustainable Agriculture: Insect Pests.' (Kluwer Academic/ Plenum Publishers). More than a thousand papers have been published on ''Trichogramma'' species, and they are the most used biological control agents in the world.Knutson A (2005) 'The Trichogramma Manual: A guide to the use of Trichogramma for Biological Control with Special Reference to Augmentative Releases for Control of bollworm and Budworm in Cotton.' (Texas Agricultural Extension Service).

''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.

Sensation

''Trichogramma'' have highly developed chemosensory organs due to their need to discriminate host from nonhost in a crowded environment. Zhang et al. 1979 finds 13 sensilla types on the antennae, eyes, mouthparts, wing, leg, and external genitalia of '' T. dendrolimi''. This is considered to generalize to the entire genus, and there may be more still undiscovered.

Parasitism

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 attempts 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 up to 10 host eggs a day.

Identification

''Trichogramma'' wasps 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 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.

Biological control

''Trichogramma'' spp. 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 understanding today comes from experiments with these wasps. .

Entomologists in the early 1900s began to rear ''Trichogramma'' spp. for biological control. ''T. minutum'' is one of the most commonly found species in Europe and was first mass reared in 1926 on eggs of '' Sitotroga cerealella''. ''T. minutum'' has been investigated as a method of biological control of the ''Choristoneura fumiferana'', a major pest of spruce and fir forests.Smith, S.M.; Hubbes, M.; Carrow, J.R. 1986
Factors affecting inundative releases of ''Trichogramma minutum'' Ril. against the spruce budworm
J. Appl. Entomol. 101(1):29–39.

Nine species of ''Trichogramma'' are produced commercially in insectaries around the world, with 30 countries releasing them. ''Trichogramma'' wasps are used for control on numerous crops and plants; these include cotton, sugarcane, vegetables, sugarbeets, orchards, and forests. Some of the pests 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 nontarget 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, nontarget effects could be significant on nontarget host populations. Research is being done on the use of ''Trichogramma'' wasps to control populations of spruce bud moth ('' Zeiraphera canadensis)'', which damages white spruce trees.

''Trichogramma'' began to be seriously used in the 1990s in China. Since then some applications have fallen out of use due to the rise of ''Bt'' crops because ''Bt'' is also toxic to the parasitoid. Future expansion of ''Bt'' in China is expected, and this threatens some uses of ''Trichogramma'', however for some crops/pests it remains the better option and so is expected to continue instead of expanded ''Bt'' in those applications. ''Trichogramma'' will be especially necessary for resistance management if ''Bt'' maize/''Bt'' corn is widely adopted.

In 2021 the National Trust in England embarked on a trial of using '' Trichogramma evanescens'', which parasitises clothes moth eggs, in conjunction with pheromones to control common clothes moths, which cause serious damage to carpets, furniture, clothing and other wool and silk objects in historic buildings. The trial was abandoned in 2023; while the microwasps performed well at reducing moth populations in combination with pheromones, they were no better than pheromones alone. It was suspected that the high interiors may not have been suitable and the Trust may continue to use the wasps in smaller stores or where they can be sited close to a known infestation source.

Species used

The most commonly used species for biological control are '' T. atopovirilia'', '' T. brevicapillum'', '' T. deion'', '' T. exiguum'', '' T. fuentesi'', '' T. minutum'', '' T. nubilale'', '' T. platneri'', '' T. pretiosum'', and '' T. thalense''.

''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.Llewellyn R (2002) ''The good bug book: beneficial organisms commercially available in Australia and New Zealand for biological pest control.' (Integrated Pest Management Pty Ltd). 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

References

External links

Biocontrol-oriented ''Trichogramma'' Manual



Trichogramma dropping by drones

{{Taxonbar, from=Q2662992

Trichogrammatidae
Insects used as insect pest control agents
Biological pest control wasps
Hymenoptera genera