By Athayde Tonhasca
If you watched the film Alien and jumped out of your seat when the creature burst from the unfortunate astronauts’ chest, an entomologist in the audience may have nodded knowingly: ‘Ah, a human parasitoid!’ Indeed, the screenwriters acknowledged entomological inspirations for the alien’s life cycle.
Here on Earth, a parasitoid is an insect that is free-living as an adult, but completes its larval development inside the body of a host (usually another insect), eventually killing it. A female parasitoid lays her eggs on or inside the host; the eggs hatch and the larvae consume the host. This type of life history lies between a predator’s and a parasite’s: a predator such as a dragonfly takes several prey and kills them outright, while parasites such as lice, fleas and ticks live off hosts without killing them.
Most parasitoid species are wasps, followed by flies. Among the latter, there are about 800 species (24 in Britain) of thick-headed flies, also known as bee-grabbers or conopids (family Conopidae). Many conopid species look more like wasps, bees or perhaps hoverflies. The size of their heads is another noticeable feature, which explains their common name.
Thick-headed flies hang around flowers, mostly between June and August. Sometimes they are looking for a sip of nectar, but if a female is lurking, she may be waiting for an opportunity to lay her eggs. Which is bad news for a visiting bee or wasp, and in some cases for crickets or grasshoppers.
It goes like this: an unsuspecting bumble bee approaches a flower. A female conopid closes in and grabs the bee in mid-air. Still afloat, she pries open the bumble bee’s abdominal segments with her theca, which is a pad-like, hardened structure at the end of her abdomen. Sometimes attacker and victim fall to the ground, but the outcome is the same; the female fly lays a single egg inside the bumble bee and lets it go.
The drama is over within seconds, and both insects fly away. The fly will wait for another opportunity to attack. But the bumble bee is done for.
The egg hatches and the larva develops inside the bumble bee, consuming its innards. But the larva does not penetrate the host’s thorax, thus keeping her flight muscles intact. The bee carries on with her life, feeding and taking nectar back to her nest, although less and less efficiently as the parasitoid grows. Within 10 to 12 days, her abdomen is completely taken up by the larva, which has nothing more to eat. The bee dies and falls to the ground (if you find a dead bumble bee with a swollen abdomen, conopid parasitism could be the causa mortis). The larva pupates and overwinters inside the bee’s body, and the adult emerges in the following year.
Conopids are among many species of parasites and parasitoids capable of changing hosts’ behaviour for their own benefit. There are examples of wasps that turn ladybirds into inert bodyguards over parasitoid eggs, and fungi that make ants climb up plants so they can release spores. Perhaps the most notorious case is the effect of toxoplasmosis cells on rats and mice. Infected rodents become attracted to cat’s urine and are less likely to hide. This altered behaviour is a death wish: they became easy prey for cats, in which toxoplasmosis cells complete their development. See other examples here.
Some conopids increase the chances of their pupae making it through the winter with a trick that may seem macabre to human eyes: they induce their bumble bee hosts to dig their own graves. In America, bumble bees parasitized by the conopid Physocephala tibialis bury themselves in the soil just before death. This grave-digging behaviour does not make a difference for the doomed bee, but it shelters the parasitoid pupa from cold and dehydration during winter months, and reduces its exposure to pathogens and its own parasites. Hibernation in the soil also leads to larger and healthier adult flies.
How common are conopid attacks against bumble bees? Accounts differ: in Britain, parasitism rates vary from 13 to 30%; in America, local figures can reach 80%. But bees don’t take it lying down. When parasitism pressure becomes high, some bumble bees shift their reproduction cycle to later in the year to avoid peaks of conopid populations. And bees have an immune system against invasive agents. Some bumble bees – like many other insects – secrete melanin, which encapsulates and suffocates the parasitoid larva. Some studies have shown that melanisation kills up to 30% of invading conopid larvae.
We don’t know the consequences of conopids for bumble bee populations, but parasitism is a fact of life for every insect. About 10% of all known insect species are parasitoids, although specialists believe this figure is a huge underestimation.
Parasitism seems gruesome and cruel. Even Darwin was dismayed by it, as he expressed in one of his letters: ‘I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidæ [ a group of parasitic wasps] with the express intention of their feeding within the living bodies of caterpillars.’ But such anthropomorphism (attribution of human values to natural phenomena) is misguided and biased. Parasitoids, like predators and parasites, are important regulators of the natural world: they prevent excessive population growth, including of agricultural pests and disease vectors. Parasitism helps shape biodiversity and ecosystems, so it is not intrinsically bad or good. It is a characteristic of life on our planet.
Scottish pollinators 