🐝 Two Wasp Approaches to Securing the Next Generation: Builders vs. Infiltrators in Nature

📖 Introduction

Wasps are among the most versatile predators in the insect world, and their methods for raising young are as varied as their shapes and colors. Two solitary groups, the mud‑nest builders and the internal parasitoids (the “infiltrators” in our comparison), both provide their larvae with fresh food, but they achieve this through entirely different strategies. The accompanying figure shows these two strategies side by side, highlighting differences in prey handling, larval feeding location, and developmental sequence. Understanding these differences reveals how inventive evolution can be.

🧱 Mud‑Nest Builders

As shown in Figure (left), mud daubers and potter wasps are expert architects. The female collects mud and shapes it into smooth‑walled cells or pot‑like chambers in sheltered spots such as under eaves or inside sheds. Once the structure is ready, she hunts for prey, typically spiders for mud daubers or caterpillars for potter wasps, and uses her ovipositor to deliver a precise sting that causes complete paralysis without killing the victim. Several immobilized prey are placed inside each cell. A single egg is laid on one of them, and the cell is sealed. In these solitary species, each cell contains only one egg and one developing larva. When the egg hatches, the larva feeds externally on the prey body, starting with non‑vital tissues to keep the prey fresh, a common parasitoid strategy. After feeding is complete, the larva pupates inside the same cell and later emerges as an adult.

Examples include the Common Blue Mud‑Dauber (Chalybion californicum), which often reuses old nests and stocks them with paralyzed spiders; the Yellow‑Legged Mud‑Dauber (Sceliphron caementarium), known for its boxy mud cells; the Organ Pipe Mud‑Dauber (Trypoxylon politum), which creates long tubular nests in rows; and the Fraternal Potter Wasp (Eumenes fraternus), famous for its small, vase‑shaped mud pots provisioned with caterpillars.

🧬 Internal Parasitoids – the Infiltrators

As shown in Figure (right), braconid and ichneumon wasps take a very different approach. They typically do not build constructed nests for their offspring, although some species protect cocoons or choose unusual oviposition sites. Instead, the female locates a suitable host, often a caterpillar, and uses her ovipositor to lay eggs directly into the host’s body. Many internal parasitoids are solitary and lay a single egg per host, but some, such as Cotesia congregata, are gregarious and deposit multiple eggs in a single large host, with all larvae developing together. Along with the eggs, she may inject venom and immune‑suppressing viruses that prevent the host from attacking the developing larvae.

Internal parasitism begins when the eggs hatch and the larvae feed within the host’s body cavity. The host remains alive and active, continuing to feed and grow while the larvae consume hemolymph, fat reserves, and other non‑vital tissues, a common parasitoid strategy. Only when they are ready to emerge do the larvae consume vital organs, killing the host. They then pupate in one of two ways: either inside the host’s remains, or externally in silk cocoons on or near the host’s body, as seen in Cotesia congregata. In both cases, the adult wasps later emerge from the pupal stage.

🕸️ Cotesia congregata, a braconid, exits tomato hornworms and spins clusters of white, rice‑grain‑like cocoons on the host’s skin. This is a textbook example of external pupation.

🌾 Aphidius colemani, another braconid, develops inside aphids and leaves behind hardened “mummies” in which pupation occurs.

🌲 Megarhyssa macrurus, an ichneumon, drills into wood with its extraordinarily long ovipositor to reach horntail larvae, pupating within the host’s tunnel.

🦋 Trogus pennator, also an ichneumon, parasitizes swallowtail butterfly caterpillars and pupates inside the host chrysalis, emerging before the butterfly can develop.

⚖️ Key Differences

Both strategies end with the host’s death, but the timing and method differ. Mud‑nest builders immobilize their prey immediately and store it in a constructed chamber, while internal parasitoids keep their host alive as a living incubator until the final stage of larval development. These contrasting approaches show how two closely related insects can solve the same problem, feeding their young, in completely different ways.

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