Tachinid Flies

A white maggot emerges from a dead or dying caterpillar or chrysalis. The maggot moves to a dark corner or under whatever is around and changes into a small brown capsule shaped pupa. In a week or two, an adult fly emerges. Yes, that is a Tachinid fly!

Tachinid Fly adult - butterfly caterpillar parasitoid

Tachinid Fly adult – butterfly caterpillar parasitoid

Tachinid fly eggs  How many do you see?  There are two on this caterpillar.

Tachinid fly eggs
How many do you see?
There are two on this caterpillar.

Tachinid fly egg on a Tawny Emperor caterpillar

Tachinid fly egg on a
Tawny Emperor caterpillar

Tachinid fly maggot emerged from Tawny Emperor butterfly chrysalis

Tachinid fly maggot emerged from
Tawny Emperor butterfly chrysalis

One Tachinid fly egg The fly larva has begun its damage as evident by the darkening around the egg

One Tachinid fly egg
The fly larva has begun its
damage as evident by the
darkening around the eggshell

Tachinid fly larva and pupae

Tachinid fly
Larva and pupae

Mourning Cloak caterpillars died from tachinid fly larvae

Mourning Cloak caterpillars
died from tachinid fly larvae

Tachinid fly mucus strings from a J'ing Monarch caterpillar

Tachinid fly mucus strings
from a J’ing Monarch caterpillar

Tachinid fly mucus strings from a Monarch chrysalis

Tachinid fly mucus strings
from a Monarch chrysalis

Tachinid fly mucus strings from a J'ing Monarch caterpillar

Tachinid fly mucus strings
from a J’ing Monarch caterpillar

Tachinid fly larva dropping from its mucus string from a J'ing Monarch caterpillar

Tachinid fly larva dropping
from its mucus string from
a J’ing Monarch caterpillar

Tachinid fly larva dropping from its mucus string from a J'ing Monarch caterpillar

Tachinid fly larva dropping
from its mucus string from
a J’ing Monarch caterpillar

Tachinid Fly adult

Tachinid Fly adult

Empty moth cocoon with Tachinid fly pupae and dead adult Tachinid flies

Empty moth cocoon
with Tachinid fly pupae
and dead adult Tachinid flies

Tachinid Fly maggot emerging from a Mourning Cloak chrysalis

Tachinid Fly maggot
emerging from a
Mourning Cloak chrysalis

A Tachinid fly sits near a Gulf Fritillary caterpillar

A Tachinid fly
sits near a
Gulf Fritillary caterpillar

An Oakworm Moth pupa A Tachinid fly empty pupa shell An adult Tachinid fly

An Oakworm Moth pupa
A Tachinid fly empty pupa shell
An adult Tachinid fly

Healthy Tawny Emperor chrysalis with wing color showing just before emerging

Healthy Tawny Emperor chrysalis
with wing color showing
just before emerging

Infected Tawny Emperor chrysalis Tachinid fly infected Brown/red death color showing just before emerging

Infected Tawny Emperor chrysalis
Tachinid fly infected
Brown/red death color
showing just before emerging

Tachinid fly larva Emerged from a Tawny Emperor butterfly chrysalis

Tachinid fly larva
Emerged from a
Tawny Emperor chrysalis

5 or 6 Tachinid fly larvae are seen inside this dead oak caterpillar

5 or 6 Tachinid fly larvae
are seen inside this
dead oak caterpillar

Yellow Shouldered Oak Slug Caterpillar with 3 fly eggs

Yellow Shouldered
Oak Slug Caterpillar
with 3 fly eggs

Tachinid Fly

Tachinid Fly

Flies around caterpillars can be deadly. Tachinid flies lay eggs on young caterpillars. The eggs hatch and the fly larvae (maggots) begin to drink the hemolymph (blood) of the caterpillar. Just before it pupates, the maggot eats it way out of the caterpillar or pupa. It finds leaf litter, and moves underneath the leaves and twigs. Hidden, it pupates into a small brown pupa.

There are different species of Tachinid flies. Some kill the caterpillars and they fall to the ground. When the butterfly or moth caterpillar is lying on the ground, the fly larvae emerge from the caterpillar and move away to become pupae.

Other species will leave the caterpillar while it is J’ing or after the caterpillar changes into a chrysalis. The fly larvae lower themselves about three or four inches on a mucus string before dropping to the ground.

A caterpillar may have one to six fly larvae growing inside it.

If you are raising caterpillars indoors, keep them safe from flies. It just may NOT be a housefly!

We wild collected butterfly and moth caterpillars to raise and photograph their life cycle. We collected some species to raise indoors and re-release outdoors. Every tachinid fly we have seen on our property has come from wild-collected caterpillars because our caterpillars are raised in a closed room in closed rearing containers. Flies cannot fit through the mesh into the rearing containers.

From the wild, we find about a 25 – 50 percent infection rate in some species.

When you find a caterpillar, you can’t always tell that it has a Tachinid fly larva inside its body. You can see eggs. When the fly larva hatches, it bends the shell of the egg outward, into the caterpillar.

In some cases, you can see that the caterpillar is infected with Tachinid fly larvae.

Butterfly chrysalises and moth pupae are discolored before the fly larva emerges. You can compare the Tawny Emperor butterfly chrysalis photos to the left and below. One is healthy with its wings showing through the chrysalis. The other is dark in the abdominal area. A Tachinid fly larva (at least one) is inside the chrysalis and will soon emerge.

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