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.

Sensation

Trichogramma have highly developed chemosensory organs due to their need to discriminate host from nonhost in a crowded environment. 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.

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. These manipulations include male killing (increasing ratio of infected females that can reproduce), feminization (males become fertile females), parthenogenesis, and cytoplasmic incompatibility. 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. Many generations of reproductive isolation of these different groups may result in speciation.

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.

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. 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

References

  • Biocontrol-oriented Trichogramma Manual
  • Trichogramma Article
  • Trichogramma dropping by drones