Parasitoid Wasps

In the movie Alien, the “facehugger” creature that implants an embryo into its host is horrifying to imagine, yet it is based on a very real biological strategy. In nature, parasitoid wasps use a similar approach. These insects lay their eggs inside or on the surface of another animal, most often other insects. When the eggs hatch, the larvae feed on the host’s tissues, ultimately killing it. Parasitoids are different from regular parasites because they always kill their hosts, while parasites typically feed on a host without causing death. This important feature makes parasitoids unique, as they are considered predators in terms of ecological roles (Godfray, 2019). In ecosystems and agriculture, parasitoid wasps also play an important role as natural pest control agents (Miller et al., 2021). They help regulate populations of herbivorous insects that can damage crops, forests, and native plants (Miller et al., 2021). Because of this, many species are used in biological control programs to reduce the use of chemical pesticides.

About 80% of parasitoid species belong to the insect order Hymenoptera, which also includes ants, bees, and sawflies (Quicke, 2009). Species within Hymenoptera are characterized by two pairs of membranous wings, chewing mouthparts, and a narrow connection between the thorax and abdomen called a petiole (Johnson, 2013). Hymenoptera have a wide range of social and reproductive behaviors, but parasitoid wasps are among the most diverse and ecologically important of all. Unlike many other animals, they are found in nearly every terrestrial ecosystem across the globe. Many species are so small that they are rarely noticed by humans, yet they play a critical role in regulating insect populations.

Giant ichneumon wasp in its natural environment. Image credit: Boris Hrasovec, Faculty of Forestry, Bugwood.org.

Parasitoid behavior first appeared in the Jurassic period, over 150 million years ago (Johnson, 2013). Evidence from molecular and fossil data suggests that parasitoidism evolved multiple times independently within Hymenoptera, this is known as convergent evolution. This repeated evolution of similar traits in unrelated groups highlights how effective the parasitoid lifestyle is. One of the most important adaptations that made parasitoidism possible is their modified ovipositor, a specialized egg-laying structure found in female wasps (Quicke, 2009). Ovipositors have many variations depending on the species, but their purpose of depositing eggs remains the same. The ovipositor in parasitoid wasps is composed of three valves derived from abdominal appendages that can move in a drilling motion to penetrate the host (Quicke, 2009). In many parasitoid species, the ovipositor has also evolved into a stinger capable of injecting venom to paralyze the host and prepare it for their egg deposit (Quicke, 2009).

The wasp’s ovipositor in action, used to deposit eggs into the host. Image credit: Encyclopædia Britannica, Inc.

Once inside the host, the parasitoid larvae can develop using two different growth strategies. Idiobiont parasitoids permanently paralyze or kill their host immediately, preventing further development or movement (Quicke, 2009). On the other hand, Koinobiont parasitoids allow the host to continue feeding and growing while the parasitoid larvae develop inside it (Quicke, 2009). This difference influences how the parasitoid interacts with its environment and what kinds of hosts it can use. Just like all other wasps, parasitoid wasp larvae undergo complete metamorphosis, passing through the stages of egg, larva, pupa, and adult. During development, the larvae typically consume the non-essential tissues first, therefore if the host isn't already dead,  this can keep the host alive as long as possible to increase the nourishment provided to the parasitoid. Eventually, the host dies as the parasitoid matures and emerges from within.

Life cycle of a parasitoid wasp showing its complete metamorphosis and interaction with its host. Image credit: The University of Waikato Te Whare Wānanga o Waikato.

Many parasitoid wasps have evolved very complex reproductive systems. One common strategy is parthenogenesis, where females can produce offspring without mating. In arrhenotokous parthenogenesis, unfertilized eggs develop into males, while fertilized eggs produce females. This system, called haplodiploidy, allows females to control the sex ratio of their offspring, which can maximize reproductive success under varying environmental conditions (Belshaw and Quicke, 2003). Some parasitoid species also reproduce through thelytokous parthenogenesis, where females are produced from unfertilized eggs, allowing their population to rapidly increase when mates are scarce (Belshaw and Quicke, 2003). These different reproductive strategies give parasitoid wasps important flexibility when presented with ecological pressures.

An adult wasp emerging from the host, while larvae are still attached. Image credit: David Cappaert, Bugwood.org.

One of the most interesting features of parasitoid wasp biology is their symbiotic relationship with viruses. Symbiotic means a close, long-term relationship between two different organisms where at least one benefits. When both partners benefit, this symbiotic relationship is called mutualism. In this case, the wasps gain protection for their eggs and successful reproduction, while the virus persists and is passed on to the next generation. Certain groups of parasitoid wasps, such as those in the families Braconidae and Ichneumonidae, carry viral DNA known as polydnaviruses (PDVs) (Shelby and Webb, 1999). These viruses are passed from generation to generation within the wasp and cannot survive on their own outside of this relationship. When a female wasp lays eggs inside a host, the virus is injected along with the eggs and venom. The virus suppresses the host’s immune system, preventing it from attacking the parasitoid eggs and halting the host’s normal development. There are two main types of PDVs. Bracoviruses are found in braconid wasps and originated from an ancient nudivirus infection about 100 million years ago (Strand, 2000). Ichnoviruses are present in ichneumonid wasps and evolved independently from bracoviruses (Strand, 2000). A third group, the banchoviruses, is related to PDVs but is less studied. This mutualism is essential for successful parasitoidism because neither the wasp nor virus can complete their life cycles without the other, representing an important example of coevolution in nature.

Parasitoid wasps include a wide variety of families and species. Braconid and ichneumonid wasps are among the most studied due to their diversity and ecological importance. Other groups such as fairyflies and aphidiine wasps are known for their small size and specialized host relationships. Other organisms have also evolved a parasitoid lifestyle including some flies, nematodes, gordian worms, and even certain beetles, further supporting the adaptive success of this ecological strategy (Strand, 2000).

Parasitoid wasps are a great example of how complex species interactions can influence the natural world. Understanding their behavior and life cycles provides insight to their role in evolution and keeping environments stable.

If you want to learn more about similar species’ relationships, check out these related OCH blogs:

• Species Relationships

• Animal-Parasite Relationships

• Native Parasitic Species in Orange County

References:

• Belshaw, R.; Quicke, D. L. J. The cytogenetics of thelytoky in a predominantly asexual parasitoid wasp with covert sex. Genome 2003, 46, 170–173. 

• Godfray, H. C. J. Parasitoids: Behavioral and Evolutionary Ecology; Princeton University Press: Princeton, NJ, 2019. 

• Heimpel, G. E.; de Boer, J. G. Sex determination in the hymenoptera. Annual Review of Entomology 2008, 53, 209–230. 

• Johnson, N. F. Hymenoptera. Encyclopedia of Biodiversity 2013, 177–184. 

• Miller, K. E.; Polaszek, A.; Evans, D. M. A dearth of data: Fitting parasitoids into Ecological Networks. Trends in Parasitology 2021, 37, 863–874. 

• Quicke, D. L. J. Hymenoptera. Encyclopedia of Insects 2009, 473–484. 

• Shelby, K. S.; Webb, B. A. Polydnavirus-mediated suppression of insect immunity. Journal of Insect Physiology 1999, 45, 507–514. 

• Strand, M. R. Ten: Developmental Traits and life-history evolution in Parasitoids. Parasitoid Population Biology 2000, 139–162.