Of all the things that Charles Darwin witnessed on his expeditions around the world, the mildly famous naturalist came to question the very existence of God because of Ichneumon wasps. In a letter to renowned botanist Asa Gray he wrote: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice.” Ichneumon or Darwin wasps are just one family in the diverse and fascinating world of parasitoid wasps. The term “parasitoid” importantly highlights the type of parasitic relationship these wasps share with their hosts — ending with the host’s death. After a female parasitoid wasp has successfully mated, she will search for a host (insects, spiders, or their eggs) for her newly fertilised eggs. Once she has found a suitable host, she uses her ovipositor (syringe-like protrusion from the wasp’s rear-end) to “inject” her eggs into the host. Some wasps like Ganaspis sp. have been found to inject their target host (Drosophila larvae) with a venom that weakens its immune system, allowing the eggs to thrive inside the host comfortably. Within the next few days (typically 2-3), the eggs deposited inside the host develop into hungry larvae that begin to feed on their host. As the host dies from being eaten from the inside, the larvae initiate the pupa stage of their metamorphosis. After a brief period of development inside their cocoons, adult wasps emerge to take on the world and someday find a living home for the next generation. This process may seem unnervingly gruesome to many of us viewing these interactions with “morality” tinted glasses. Nature, however, is a complex theatre of interactions that lead to some forms of life evolving remarkable traits to survive and others evolving remarkable traits to eat the former. These are the vital processes that shape ecosystems at all scales, from tiny gardens to the entire earth.
Many families of wasps have evolved to target specific groups of hosts within these ecosystems. Cockroach wasps (Ampulicidae), for instance, lay their eggs inside unsuspecting cockroaches. Ripan Biswas, an accomplished nature photographer, found respite when Covid travel restrictions were relaxed and went straight to the village of Buxa (famous for its tiger reserve) looking for cockroach wasps. Cockroach wasps are adept aerial acrobats that can quite easily outmanoeuvre a cockroach in the open. On the trunk of a tree, however, cockroaches may find adequate shelter in the tree’s nooks and crannies. After a week of patiently watching cockroach wasps visit the tree with little success, Ripan saw a cockroach wasp in action. The wasp delivered multiple stings of venom to target the nervous system and incapacitate (but not kill) its target (top left). The wasp then dragged the paralysed cockroach into a cosy crevice where it would deposit eggs into the cockroach (top right). Some Ichneumonid wasps employ an incisive approach to finding hosts hiding in tree trunks. Their ovipositors are capable of drilling through solid wood to deposit their eggs in or on wood-boring hosts that live in the trunk (above). Photos: Ripan Biswas
Much like the Ichneumonid wasp, another parasitoid family, Leucospidae, use their ovipositor to investigate small gaps and crevices in tree trunks to find a route to their host — other wasps and bees. Interestingly, this family of wasps are “ectoparasitoids”, i.e. they lay their eggs “on” the host and not “in” them. (Top) Wasps like Leucospis sp. search tree trunks and fallen logs for signs of their host and then introduce their eggs onto the host through any small openings they can find. Many families of wasps are ectoparasitoids that target caterpillars of moths and butterflies. (Above left) The infected caterpillars remain alive with the eggs safely attached to them. When the eggs hatch, larvae emerge and slowly begin consuming their host. Subsequently, the larvae pupate on the host and emerge as adults (above right). The end result for the host, unfortunately, is death. Photos: Ripan Biswas (top and above right), Jithesh Pai (above left)
Most adult parasitoid wasps feed on nectar and play a vital role as pollinators despite eating their hosts to death at the larval stage. Additionally, the vast diversity of wasps is also prey for other forms of life. (Top) A parasitoid wasp hoping to lay her eggs amid aphids on this China rose leaf is caught off guard by a Rhene sp. jumping spider with fatal consequences. (Above) In a strange sense of what some might call poetic justice, parasitoid wasp larvae can fall prey to hungry predators like lynx spiders. Parasitoid wasps act as controllers of arthropod populations, pollinators of plants, and sources of nutrition for predators — they are invaluable parts of ecosystems at all scales. And so, I cannot help but wonder if Darwin, while disturbed by their lifecycle processes, still saw beauty in these fascinating creatures and how they contribute to the somewhat vicious but beautiful web of life. Photos: Ripan Biswas (top), Jithesh Pai (above)
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Samuel John
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