Braconid Wasps: Good or Bad for the Garden?

article image
PHOTO: TOM QUIRK
An adult braconid wasp.

Learn all about braconid wasps.

Many years ago, at a time when my wife was 8-months
pregnant, we took in a movie but were not very particular
in our choice. Two hours later, after seeing the original
Alien , she said, “That was a big mistake. Where
was Mary Poppins when we needed her?”

This visceral reaction to the idea of foreign “things”
living inside other animals, including ourselves, must come
from a long human history of living with the likes of
tapeworms, pinworms and scabies. Or, taking an analytical
view of the natural world, it could be described as one big
transfer of nutrients from one organism to another. Within
that simple model, unlimited ways exist in which the
materials of one organism can be borrowed or hijacked by
another. The braconid mini-wasps fit the profile; they are
insects that fulfill their life cycle by inhabiting the
bodies of other creatures.

The braconid family is in the Order Hymenoptera, which
includes other bees and wasps, but all 1,700 North American
species in this family are stingless, and so small you have
to pay close attention to see them. They can be almost
invisible, at 2 to 3 millimeters long, or veritable giants
at 15 millimeters (about 5/8 inch). Antennae will usually
be noticeable and a visible ovipositor (not a stinger) is
common to many species. Most will be black or brown, but a
few have some color. Braconids are short and
stocky — the abdomen is about the same length as the
head and thorax combined. Unlike other wasps, braconids do
not have skinny “waists.” They can be confused with small
flies.

Different braconids are parasitic on army worms, eastern
tent caterpillars, corn borers, cotton bollworms, alfalfa
weevils, wheat-stem sawflies and Douglas-fir bark beetles,
just to name a few. In the garden and orchard, this
beneficial parasitism occurs on aphids, coddling moths,
tomato hornworms, garden webworms and on many different
caterpillars, beetles and flies.

The parasitism occurs in a variety of forms. Some adult
braconid wasps will lay eggs in or on the host. The resulting
larva consume the host and then emerge to become adults
elsewhere. (The white bumps on the hornworm in the photo
above look like insect eggs, but they are actually cocoons
from which adult mini-wasps will hatch.) Other braconids,
like the aphid parasites, lay only one egg per aphid nymph,
and the invader completes its cycle and emerges as an
adult. Diversity is a strength of ecosystems, so we
shouldn’t be surprised that some beautiful and harmless (to
us!) species, like the larva of swallowtail butterflies and
sphinx moths, also are hosts for braconids.

The genus Aphidius is of particular interest to
gardeners, especially greenhouse gardeners. These wasp
adults are only about two to three times the size of an
aphid. They lay a single egg in each aphid nymph, which
kills the aphid; then they emerge as an adult. Aphid
colonies often show different colors. Live aphids are
off-white or light green in color, while dark to light
brown bodies are likely the shells of dead aphids from
which parasites have emerged. A hand lens comes in handy
here. The presence of dead aphids means that many of the
live nymphs are likely parasitized as well, but can still
move. The braconid “alien occupation” of the host for these
species can take as little as 7 to 10 days. Females of
different species of wasps can lay from 50 to 400 eggs
each, so their effect can be substantial. Many braconid
species are sold commercially to protect different
greenhouse crops. These beneficial mini-wasps feed on
nectar from flowers. To attract them to your garden, grow
plants with tiny flowers including dill, fennel and
mints.

— John Stuart