The solitary parasitoid wasp lays her egg in the body of a caterpillar or similar host. After it hatches, the developing larva dines on the caterpillar’s insides until eventually tearing its way through the skin of the host, killing it. Since the parasitoid depends entirely on the caterpillar for its development, the quality of the host is quite important, and having an entire caterpillar to oneself is critical for offspring survival. According to new work published in PLOS ONE this week, female wasps can tell if competing parasitoids have laid their eggs inside a host. They can even count how many eggs are already there.

Previous studies on parasitoid behavior revealed that the tiny 2-millimeter (0.08-inch) long Leptopilina heterotoma uses its ovipositor – the needle-like egg injecting organ (pictured above) – to discriminate between unparasitized and parasitized fruit fly larvae. The species is also known to discriminate between hosts with different numbers of parasitoid eggs already inside them. This skill requires some experience, and how females choose the best possible host has remained a mystery.

A Wageningen University team led by Sara Ruschioni wanted to see if there was a sensory basis for this so-called host discrimination. Using hemolymph (a blood-like vital fluid) from fruit fly larvae, the team stimulated tiny structures similar to taste buds – called sensilla – located at the very tip of the ovipositor of the wasps. There are seven sensilla at end of an ovipositor, and each are innervated by six neurons.

The researchers made electrophysiological recordings of neural signals as they stimulated just one sensillum using hemolymph from unparasitized fruit fly larvae, from one-time-parasitized larvae, and from those that had been parasitized twice already.

The neural responses of sensillia to these hemolymph samples varied dramatically depending on both the presence and number of parasitoid eggs inside the host – suggesting that host discrimination is encoded by taste receptor neurons at the ends of ovipositors. And activity from just three of the six neurons will suffice.