Photo Credit to Lauren Kaplan
No need to run for cover—unless you’re a cabbage worm. These are not your garden variety wasps!
These wasps are mostly stingless, at least to humans. What looks like a stinger is really an
ovipositor, used to deposit their eggs into or on top of crop pests, which they use as hosts.
Because of their small size, these beneficial insects often fly under the radar, and outside the
notice of many farmers… but they are worth looking out for, as they are capable of performing
significant ecosystem services, especially in organic farming systems.
Depending on the type of wasp, these parasitoids can help with managing a
lot of herbivorous pests
, including “aphids, beetle larvae, bagworms, cabbage worms, Colorado
potato beetle, corn ear worms, cucumber beetles, cutworms, gypsy moth caterpillars, Japanese
beetles, leaf-miners, mealybugs, Mexican bean beetles, moth caterpillars, sawfly larvae, scale,
squash vine borers, tent caterpillars, tobacco budworm, tomato hornworm and whiteflies.” The family
of Braconid wasps includes many aphid- and caterpillar-parasitizing species. The Chalcid wasps,
including the familiar Trichogramma wasp (and the incomparably cool fig
), largely parasitize eggs of various foliage-feeding caterpillars. Ichneumon wasps are
perhaps the most widespread, present in nearly all terrestrial habitats, and are aggressive
parasitoids of herbivorous pests.
Ichneumon wasps, with their tiny waists and prominent ovipositors, look more wasp-like than
Braconids and Chalcids, which—while generally varied
in overall appearance— are generally so much smaller that it’s hard to tell what they look like
without a magnifying lens. Chalcids can be as small as 1/64 of an inch! Due in part to their tiny
size, it can be difficult to identify the presence of parasitoid wasps by spotting adults. Farmers
who are tuned into crop plants and pests may more easily recognize parasitoid wasps earlier in their
life cycle, at their larval or pupal stages.
Chalcids, which generally parasitize eggs rather than larvae, are perhaps best identified by
looking at host eggs, which turn black once parasitized, as in this corn
and these codling
. Chalcid wasps commonly emerge either from infected eggs or chrysalis
both of which will become darkly discolored prior to emergence. Inconspicuous as they are, these
tiny wasps are a highly beneficial insect because they effectively kill the host before it can do
any damage—unlike, for example, the hornworm, which can turn a lot of foliage and fruit into a lot
of frass before succumbing to parasitism.
Adult parasitoid wasps feed on pollen, sap, and nectar from plants with collections of tiny
flowers. Many of these—such as coriander, dill, and other members of the carrot family; sweet
alyssum; yarrow; buckwheat; and faba bean—can
as insectary rows or in hedgerows, and also provide habitat for other beneficials
such as syrphid flies and minute pirate bugs.
Native species also provide important habitat. In the west, coyote brush was found to be a key
species for parasitic Hymenoptera habitat, including all three families of wasps. Another source of
native habitat is the widespread boneset, which prefers low-lying, wet areas of the east and south.
In addition to feeding on nectar and pollen, adults will also use these plants as shelter from wind,
areas to rest, mate, and pupate.
CHALLENGES AND COMMERCIAL USE
The plants that provide adults with sustenance and shelter are generally not the same plants that
pest hosts prefer. This means that adult wasps will need to travel between their own habitat and
host habitat. Given that they cannot travel very far, it can be a challenge to establish habitat
within a few thousand feet of crop plants and hosts. Locating habitat in
close proximity to the presence of appropriate hosts
is ideal, as well-fed wasps devote more
time to parasitizing pests.
Another challenge to providing good year-round habitat is that these wasps overwinter with the
pupae or cocoons of host pests. The Big Bug Hunt suggests leaving
a small amount of host habitat through the winter
to provide shelter for the wasps that
parasitize those hosts. This, however, involves identifying your particular wasps (or at least their
preferred host species), which is no easy feat. It also requires leaving shelter for your
pests—though if this patch of habitat is small, it’s possible to be able to monitor it for the
quantity of overwintering hosts and presence of wasps.
In lieu of cultivating habitat for your own native species of parasitoid wasps, Trichogramma and a
few other wasps are available commercially to provide annual pest control. Tips for successful
release, establishment, and quantity per acre are generally available with purchase.
There is much we have yet to learn about these tiny wasps, including the degree of service they can
provide, and in which kinds of agricultural systems. In the meantime, we can plant tiny flowers for
them, and allow them to provide ecosystem services as part of sustainable farming practices. And we
can continue to observe how they work on our own farms and for our own unique agro-ecosystems.
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After graduating from UCSC's
Center for Agroecology and Sustainable Food Systems, Lauren Alexandra Kaplan spent a season at
an organic CSA farm in California before returning east to farm in the Hudson Valley. Prior to
pursuing farming full time, she worked in book publishing and helped to launch an urban farm in
NYC. Alexandra is an avid salsa dancer and maker of jams, pickles, and kraut.