Symbiotic relationships are close, long-term associations between two different biological organisms, each with distinct outcomes. Exploring these diverse interactions reveals the intricate web of life on Earth. The interaction between certain wasps and caterpillars presents a compelling case, highlighting a unique biological strategy.
Types of Symbiotic Relationships
Symbiotic relationships are broadly categorized based on how each organism is affected. Mutualism describes an interaction where both species benefit. For instance, goby fish and shrimp often share a burrow, with the shrimp maintaining the burrow and the fish warning the shrimp of predators. Commensalism occurs when one species benefits, while the other is neither helped nor harmed. Remora fish attaching themselves to sharks for transport and feeding on scraps without affecting the shark is a common example.
Parasitism involves one organism, the parasite, benefiting at the expense of another, the host, which is harmed. Typically, parasites do not immediately kill their hosts, as their survival depends on the host remaining alive. However, a specialized form of parasitism, called parasitoidism, results in the host’s death. This describes the relationship between certain wasps and caterpillars, where the wasp ultimately causes the caterpillar’s demise to complete its life cycle.
The Wasp’s Parasitic Approach
The female parasitoid wasp initiates this interaction by locating a suitable caterpillar host. Wasps use chemical cues to identify the correct species and developmental stage. Once a host is found, the wasp employs an ovipositor to deposit its eggs either into or onto the caterpillar’s body. This structure, which can be long and needle-like, allows the wasp to precisely inject eggs.
Along with the eggs, the wasp injects substances, including venom and a polydnavirus. These polydnaviruses are integrated into the wasp’s genome and are essential for the wasp larvae’s survival. Inside the caterpillar, the polydnavirus suppresses the host’s immune system, preventing it from destroying the foreign wasp eggs. This viral injection also manipulates the caterpillar’s physiology, ensuring it remains a suitable environment for the developing wasp larvae.
The Caterpillar’s Unfortunate Role
Following the wasp’s oviposition, the caterpillar’s fate is sealed. The wasp larvae hatch inside the caterpillar and begin to feed on its internal tissues. They strategically consume non-vital organs first, allowing the caterpillar to remain alive and continue growing, thereby providing more sustenance for the developing larvae. Parasitized caterpillars might even consume more food than their unparasitized counterparts, inadvertently benefiting the wasp larvae by accumulating more nutrients.
As the wasp larvae mature, they can induce significant behavioral changes in their caterpillar host. For certain wasp species, once the larvae are ready to emerge, they release chemicals that cause the caterpillar to cease feeding and become immobile. The caterpillar may even exhibit a “bodyguard” behavior, protecting the newly emerged wasp cocoons until its inevitable death.
Finally, after several weeks, the wasp larvae complete their development and emerge from the caterpillar’s body, often by chewing their way through its skin. The caterpillar, now hollowed out and severely damaged, cannot survive this process. Its death provides the ultimate resource for the wasp larvae, which then pupate, often forming cocoons on or near the remains of their former host.