Parasitic Fig Wasps Sniff Out Hosts With Their Butts

Their babies eat other wasps' babies.

Pratibha Yadav

You might not know this, but most figs have dead wasps in them at some point. That’s right. Wasps. And if that surprises you, then hold onto your hat: Scientists have found out how another species of wasp preys on the larvae of the first wasp. Here’s how it works.

Most varieties of fig trees have symbiotic relationships with wasps that lay eggs inside their figs. In return for providing a cozy place where the wasp’s eggs can develop and hatch, the fig receives the vital pollination services that can only be provided by the wasp. You see, the female that climbed in to lay her eggs in the fig is still covered in pollen from the fig she was born in, making her life cycle pretty dull but super important to everyone involved.

If you know how flowers and fruits usually work, this may sound like a strange arrangement, because pollination typically happens before a fruit develops. But a fig is different. It’s actually one big inflorescence (flower structure), with all the flowers growing on the inside. These specialized fig wasps depend on the fig to complete their life cycle. In a big twist, though, some species of wasps parasitize the fig wasp’s larvae to feed their own young.

In a study published Wednesday in the Journal of Experimental Biology, Pratibha Yadav and Renee M. Borges show exactly how Apocrypta westwoodi Grandi, a parasitoid wasp, can sniff out other wasps’ larvae in figs with its ovipositor. According to the researchers, the wasp’s ovipositor — an adapted stinger, thin as a hair, that the wasp uses to insert its eggs into the fig — actually detects CO2 from the larvae, as well as volatile compounds produced when the host larvae munch on the insides of the fig. This helps the parasitoid wasp lay its eggs right near the larvae they’ll eat when they hatch.

Scientists say the Apocrypta westwoodi Grandi wasp, which parasitizes fig wasps' larvae, can actually sniff out the larvae with its long, thin ovipositor.

Pratibha Yadav

This unique trait differs greatly from the way fig wasps — the victims in this scenario — use the figs. Most female fig wasps crawl into a tiny hole at the bottom of the fig, often losing their wings and antennae on the way in. Once they lay their eggs in the fig’s tiny flowers, pollinating them in the process, they die since they’re basically trapped in there without wings or antennae. Little do they know, their babies could become fodder for A. westwoodi. (As a side note, the dead wasp usually gets “digested” by an enzyme in the fig, so you’re probably not going to find any actual remains of dead wasps in your fig, just their chemical signatures. Once the larvae mature, the newborn males chew a hole in the fig for the newborn females to escape through.)

The way A. westwoodi detects fig wasp larvae, the researchers found, is through some sensors in its ovipositor’s tip that are actually quite similar to those found in other insects’ antennae. They figured this out by harvesting figs that had A. westwoodi larvae in them, then when they hatched, the scientists removed the females’ ovipositors and hooked the tiny appendages up to electro-sensing equipment. They found that when they blew CO2 over the ovipositor tip, it reacted with a small nerve impulse.

The researchers also tested live wasps. In one trial, they blew air towards the ovipositor. When they blew air with a puff of CO2 in it, the wasp responded by pointing its ovipositor into the stream of air.

A. westwoodi, a parasitoid wasp, uses its specialized ovipositor to find and lay its eggs near those of another fig wasp. It does this by detecting CO2 and fig volatiles released by the larvae.

Pratibha Yadav and Renee M. Borges 

This, they concluded, must mean that A. woodsi detected CO2 produced by host larvae in the figs. By detecting where these larvae are concentrated, the parasitoid wasp can ensure it lays its eggs very close to the tender morsels — other wasps’ babies — that its own babies would eat upon hatching. It’s a cruel world out there.

Additionally, the researchers found that A. woodsi uses its ovipositor to detect volatile compounds produced when the host larvae eat the insides of the fig. This would only increase the chances that this parasitoid was on-target to let its offspring prey on their tender hosts.

Taken together, this research suggests that the Apocrypta westwoodi Grandi wasp is an even more formidable parasitoid than previously thought, with a highly sophisticated method of searching out host organisms in a fig.

Abstract: We show that the insect ovipositor is an olfactory organ that responds to volatiles and CO2 in gaseous form. We demonstrate this phenomenon in parasitic wasps associated with Ficus racemosa where ovipositors, as slender as a human hair, drill through the syconium (enclosed inflorescences) and act as a guiding probe to locate highly specific egg-laying sites hidden inside. We hypothesize that olfaction will occur in the ovipositors of insects such as parasitic fig wasps where the hosts are concealed and volatile concentrations can build up locally. Relevant stimuli such as herbivore-induced fig volatiles and CO2 elicited electrophysiological responses from the ovipositors. Silver nitrate staining also revealed pores in ovipositor sensilla, indicating their olfactory nature. Insects could use volatile sensors on their ovipositors to evaluate ecologically relevant stimuli for oviposition. Further investigations on the sensory nature of ovipositors can provide designs for development of ovipositor- inspired micro-chemosensors.
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