Tospovirus and thrips: an alliance that terrifies plants

by Piter Kehoma Boll

I recently presented a thrips in the Friday Fellow section, in that case a thrips that infects mostly fig trees. This group of insects, which make up the insect order Thysanoptera, is poorly known by the general public, but is certainly known by gardeners and farmers, as they can be a serious nuisance for many plant types.

We could imagine thrips as being kind of the mosquitoes of plants. They pierce the surface of plants and suck their juices just like mosquitoes do with vertebrates. And we all know that a mosquito bite may lead to much more than a small blood loss and local irritation of the skin. Many parasites use mosquitoes as vectors to travel from host to host, including protists such as Plasmodium falciparum, which causes malaria, and many types of virus, such as those of the genus Flavivirus, which cause the yellow, dengue and zika fevers.

A similar thing happens in the association of thrips with plants. A special genus of virus, called Tospovirus, infects many plant species and uses thrips as a vector. Inside the thrips bodies, the viruses reproduce after infecting the epithelial cells of the gut and, from there, travel via blood to the salivary glands and, when a thrips perforates a plant, the virus is injected in it. The cycle is basically the same used by Flavivirus in mosquitoes and ticks to infect vertebrates. Isn’t it amazing how a virus such as Tospovirus can infect both an animal and a plant? But what exactly is the disease caused by these viruses?

Basil leaf infected with the tomato spotted wilt virus. Photo by Scot Nelson.**

One of the most common Tospovirus is the so-called Tomato spotted wilt virus (TSWV), which is considered one of the most economically devastating plant viruses in the world. It can infect many crops, such as tomato, tobacco, bellpepper, peanut and basil. The symptoms vary from plant to plant, but usually include stunting, poorly developed fruits, commonly with ring spots on the surface, and necrosis of the leaves. It is transmitted to plants by thrips of the genus Frankliniella, mainly the western flower thrips Frankliniella occidentalis. Although the virus usually needs several hours to be able to reinfect a plant after infecting a thrips, in ideal conditions the time can e as short as five minutes.

The western flower thrips Frankliniella occidentalis. Photo by Dave Kirkeby.*

But why would a thrips feed on an obviously sick plant, all ugly and full of spots? They would certainly prefer a healthy plant, but that would prevent the virus to spread. As a result, the virus developed several strategies to attract the thrips. The TSWV is able to increase the amount of free aminoacids in infected plants, and these are essential nutrients for egg production in thrips. As a consequence, infected plants become more nutritious and attract more thrips. Feeding on infected plants, the thrips will certainly get infected and at the same time ingest more nutrients than non-infected thrips. Thus, a sick thrips actually has an increased fitness and usually lays more eggs. The plants would certainly get effing scared if they were able to have emotions.

The Soybean vein necrosis virus (SVNV) is another Tospovirus of economic concern. As it names suggests, it attacks mainly soy plants, and its main vector is the soybean thrips Neohydatothrips variabilis. Infected soybean thrips produce significantly more offspring than non-infected ones, although heavily infected individuals lay few viable eggs. How do thrips bypass this problem? It’s simple! Once they are infected, they stop feeding on infected plants and prefer non-infected ones, which increases their reproductive success by avoiding becoming heavily infected and at the same time they spread the virus further to non-infected plants. A nightmare for the plants once more.

Soybean thrips Hydatothrips variabilis. Photo by Even Dankowicz.***

A recent study investigated the relationship of another Tospovirus-thrips pair, this time of the iris yellow spot virus (IYSV), which commonly attacks garlic and onion plants, and its main vector, the onion thrips, Thrips tabaci. Infected thrips did not show an increased daily fecundity but had an increased lifespan, allowing them to lay more eggs simply because they lived longer.

Iris yellow spot virus lesion on an onion leaft. Extracted from https://vegetableguide.usu.edu/diseases/onion/iris-yellow-spot-virus

But the effect of Tospovirus on thrips can go further. For example, although plants infected by the TSWV release more aminoacids that attract and increase the fecundity of thrips, the infections still seems to have some deleterious effects on the insect. Infected males of Frankliniella occidentalis increase their consumption of food juices and increase the transmission of the virus. Females, on the other hand, seem to need nutrients that cannot be found in plants. As a result, they increase the consumption of eggs of the two-spotted spider mite Tetranychus urticae, with which they often coexist. Although primarily herbivorous as most thrips, the western flower thrips eventually feeds on mite eggs, and being infected by TSWV makes females become more eager to eat eggs. This is certainly not a strategy of the virus itself as the other ones, since a female that is feeding on mite eggs does not contribute for the virus’ reproductive success. Nevertheless, this is an interesting phenomenon that show us how the interactions in a trophic web can be dynamic, changing, for example, due to an uninentional side effect of a virus trying to survive.

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References:

Keough S, Han J, Shuman T, Wise K, Nachappa P (2016) Effects of Soybean Vein Necrosis Virus on Life History and Host Preference of Its Vector, Neohydatothrips variabilis , and Evaluation of Vector Status of Frankliniella tritici and Frankliniella fusca. Journal of Economic Entomology 109(5): 1979–1987. doi: 10.1093/jee/tow145

Leach A, Fuchs M, Harding R, Nault BA (2019) Iris Yellow Spot Virus Prolongs the Adult Lifespan of Its Primary Vector, Onion Thrips (Thrips tabaci) (Thysanoptera: Thripidae). Journal of Insect Science 19(3): 8. doi: 10.1093/jisesa/iez041

Shrestha A, Srinivasan R, Riley DG, Culbreath AK (2012) Direct and indirect effects of a thrips‐transmitted Tospovirus on the preference and fitness of its vector, Frankliniella fusca. Entomologia Experimentalis et Applicata 145(3): 260–271. doi: 10.1111/eea.12011

Stafford-Banks CA, Yang LH, McMunn MS, Ullman DE (2014) Virus infection alters the predatory behavior of an omnivorous vector. Oikos 123(11): 1384–1390. doi: 10.1111/oik.01148

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*Creative Commons License This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

** Creative Commons License This work is licensed under a Creative Commons Attribution 2.0 Generic License.

***Creative Commons License This work is licensed under a Creative Commons Attribution 4.0 International License.

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Filed under Behavior, Entomology

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