The Outsized Influence of a Tiny Virus: The Central Role of PDVs in Parasitoid Wasp Reproduction

In natural ecosystems, organisms are linked through intricate and often surprising biological relationships. One particularly fascinating example is the symbiosis between parasitoid wasps and a group of viruses known as polydnaviruses (PDVs). These viruses infect insects and are intimately associated with certain lineages of parasitoid wasps. To date, more than fifty PDV types have been identified, primarily linked to parasitoid wasps belonging to groups such as Microgastrinae, Campopleginae, Cheloninae, and Cardiochilinae. PDVs replicate in a specialized region of the female wasp's ovary known as the calyx, yet they cause no disease in the wasp itself. Instead, they form a mutually beneficial partnership with their host.

Cotesia melanoscelus（Image source：James Lindsey's Ecology of Commanster Site， CC BY-SA 3.0 ）

The parasitoid wasp deposits its egg inside the caterpillar; ovipositor; hemocyte; wasp egg; polydnavirus (PDV)（Image source：Ikehiker， CC BY-SA 4.0 ）

Parasitoid wasps are hymenopteran insects that lay their eggs inside the larvae of butterflies or moths. When a female wasp deposits her eggs into a caterpillar, she simultaneously injects venom and PDVs along with them. Once inside the caterpillar, the viruses begin infecting host tissues. Their primary function is to suppress the caterpillar's immune system and alter its physiology. By weakening immune defenses, PDVs ensure that the parasitoid eggs can hatch and the developing wasp larvae can grow safely inside the caterpillar without being attacked by the host's immune cells. This process provides the larvae with a protected developmental environment until they mature and eventually kill the host. In addition, the venom injected by the wasp can enhance the expression of viral genes, strengthening the virus's biological effects. For some parasitoid species, PDVs are therefore essential for successful reproduction. If a parasitoid wasp lacks these viruses, its eggs are rapidly attacked and removed by the caterpillar's hemocytes, severely reducing the chances of survival for the next generation.

Life cycle of the parasitoid wasp（Courtesy of The University of Waikato Te Whare Wānanga o Waikato）

After infection by PDVs, caterpillars experience more than immune suppression. Their growth rate can increase, allowing them to accumulate additional nutrients that ultimately benefit the developing parasitoid larvae. Research suggests that this phenomenon involves an indirect interaction with the plants the caterpillars feed on. When parasitized caterpillars chew plant tissue, their saliva—now containing viral influences—comes into contact with the plant. Normally, plants respond to herbivory by activating defensive strategies. These responses often include the release of herbivore-induced plant volatiles (HIPVs) or the production of toxic compounds such as alkaloids. These defenses can repel herbivores, attract predators or parasitoids of the herbivore, or directly harm the feeding insect. However, when the attacking caterpillar carries PDVs, the plant's defensive system becomes weakened. Not only are defense responses suppressed, but the chemical composition of emitted volatiles may also change. As a result, the plant becomes less resistant, enabling the caterpillar to grow more efficiently and providing greater nutritional resources for the parasitoid larvae developing within it.

Yet even this system has another layer of complexity. Parasitoid larvae themselves can become hosts for another group of insects known as hyperparasitoids. These “parasites of parasites” lay their eggs inside the larvae or pupae of parasitoid wasps. Examples include species such as Lysibia nana and Baryscapus galactopus. These hyperparasitoids face an obvious challenge: their hosts are hidden within the bodies of caterpillars. How do they find them?

Research has revealed that the answer lies again in the ecological ripple effects of PDV infection. When caterpillars carrying PDVs feed on plants, the resulting changes in plant volatile emissions create a chemical signal. Hyperparasitoids can detect these altered plant odors and use them to identify plants that are likely hosting caterpillars already parasitized by wasps. In other words, the chemical dialogue between plant and herbivore inadvertently reveals the presence of the parasitoid larvae hidden inside the caterpillar. Hyperparasitoids are attracted to these signals and can then locate the infected caterpillar.

Furthermore, hyperparasitoids appear capable of extracting even more detailed information from these cues. By sensing differences in plant volatiles and the odor emitted by the caterpillar itself, they can estimate the developmental stage of the parasitoid larvae inside the host. This allows them to choose the optimal moment to lay their own eggs, ensuring that their offspring will develop successfully within the parasitoid.

Taken together, these interactions reveal an astonishingly complex ecological network involving plants, herbivorous caterpillars, parasitoid wasps, hyperparasitoids, and viruses. Each participant influences the others through multiple biological pathways, creating a cascade of interactions across several trophic levels. Within this web, PDVs occupy a particularly intriguing position. Although microscopic, they act as hidden orchestrators, shaping the physiological state of the caterpillar, altering plant defenses, and indirectly influencing the behavior of other insects in the ecosystem.

Such a system illustrates how seemingly insignificant biological agents can exert profound ecological influence. A virus too small to see can reshape immune responses, manipulate plant chemistry, and even determine the success or failure of multiple species linked within a food web. In this sense, PDVs demonstrate that ecological dominance does not always belong to the largest organisms. Sometimes, the most powerful forces in nature are also the smallest.

Author: Shui-Ye You

References:

1. Tan CW, et al. (2018). Symbiotic polydnavirus of a parasite manipulates caterpillar and plant immunity. Proc Natl Acad Sci USA.

2. Zhu F, et al. (2018). Symbiotic polydnavirus and venom reveal parasitoid to its hyperparasitoids. Proc Natl Acad Sci USA.

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