Posted by: Lisa Mason, Arapahoe County Extension
In case you missed it, click
here to read Part 1. We covered the purpose of wasps in the ecosystem and
answered questions on the Asian giant hornet that made headlines in 2020. Part
II will cover social wasps, their role in the ecosystem, and possible control
methods if they become a nuisance.
Wasps sometimes get a bad reputation because they can sting
and are sometimes a nuisance. We have one species of wasp that can be particularly
aggressive: the western yellowjacket. Don’t let one or two species of nuisance
wasps ruin your opinion of all wasps. Wasps are a diverse group of insects that
provide important ecosystem services such as pest control.
Social wasps are probably the most familiar wasps to people
because they are easily seen in the yard and landscape. Social wasps live in a
colony together. They have a similar lifecycle to a bumble
bee (Bombus spp.). A new colony is started each year by a fertilized
queen that survived the winter. She will lay several generations of female
workers throughout the season. Towards the mid-to-end of the summer, she will
lay eggs that are male wasps and potential queens. The males and potential
queens will leave the colony to find a mate. Once cold temperatures arrive, the
current colony will die except for the newly mated queens. The cycle will
repeat and the following spring, when the new queens begin a new colony. Social
wasps always build a new colony each year. They never reuse old nests, which is
important to note if you’re looking to control nuisance wasps. Social wasps
make their nests out of chewed up wood, creating a paper nest. Social wasps
also feed on insects like caterpillars, providing important pest control in our
backyards. The western yellowjacket is a scavenger feeding on carrion and human
sources of food such as trash.
Let’s discuss five species of social wasps that are found in
Colorado. Understanding the life history of social wasps can help you control them
if they become a nuisance in your landscape, and build appreciation for their complex social biology, along with the pest control services they provide.
|A western yellowjacket. Photo: Lisa Mason|
yellowjackets (Vespula pensylvanica)
are a native, social wasp that you will find at your family BBQ, picnics, trash
cans, etc. They are very common in urban landscapes and can become a nuisance. Like
social wasps, they create a new colony each year. The paper comb nest is
usually underground or in a cavity that is well-protected. While yellowjackets
are commonly seen, their nesting site can be difficult to find.
Yellowjackets can be aggressive, especially when defending
their nest and are responsible for 90% of the insect stings in Colorado. They
are scavenging insects feeding on carrion, dead earthworms, garbage, human
foods including meats, and sweet, sugary foods. They also will feed on
honeydew, a sweet substance excreted by aphids and soft scale insects. Scavengers are the clean-up crew for
ecosystems and play an important role in the food web. Yellowjackets tend to
get more aggressive in the fall as food can be harder to find.
If yellowjackets tend to be a nuisance in your landscape,
you can purchase a yellowjacket trap available at hardware and garden stores.
The traps contain heptyl butyrate which yellowjackets are attracted to. Traps
will be most effective if they are placed outdoors in the early spring to
capture the overwintering queens before they start their new colonies. Nest
removal can be a dangerous task and difficult because their nests are so
well-protected. Insecticide treatments often aren’t effective because it is
difficult to get the insecticides inside the colony. Hiring a professional is
often necessary. Remember, the colony will only last for one season.
|An underground entrance to a western yellowjacket nest. Photo: Nancy Bonita|
European Paper Wasps
paper wasps (Polistes dominula) are
much less aggressive, but they often build their nests close to human activity.
They are a non-native insect that has become well-established in Colorado. They
first appeared in Colorado in the late 1990s/early 2000s. They prey on
caterpillars and other insects and feed their young live insects. Common prey
includes hornworms and cabbageworms. They also will feed on honeydew secreted
from aphids. The papery comb nests are often found under house eaves,
overhangs, sheds, pipes, and other hollow spaces in human infrastructure.
If the paper wasp nest is located in an area that won’t be
disturbed by people, the nest can be left alone, and the wasps likely won’t be
a nuisance. The current colony won’t survive when temperatures cool in the
fall. If the nest is close to human activity, there are insecticide treatments
to destroy the nest. Following instructions on the insecticide label is
critical. Insecticides should be applied at night when most wasps are present
at the nest. The nest should be destroyed afterwards to also kill the capped
larvae in the nest. The location of the nest site should be thoroughly washed
to prevent any remaining wasps from building a new nest.
Traps that attract yellowjackets will not attract paper
wasps. There are no effective trap methods for paper wasps.
|A European paper wasp. Photo: Lisa Mason|
Baldfaced Hornets and Aerial
hornets (Dolichovespula maculata)
arenaria) are common in Colorado but are less likely to be a nuisance
around human activity. They are only aggressive when their nest is threatened.
They develop a large papery comb nest usually high in large trees and shrubs. They
feed on caterpillars, other insects, and honey dew. If you find a nest in your
tree or shrub, it may look intimidating, but the nest can likely be left alone
if the nest can be left undisturbed. These two wasps can be common visitors in
our landscapes but often go unnoticed by people.
Western Paper Wasp
The western paper wasp (Mischocyttarus flavitarsis) is a native paper wasp in Colorado and the western US. They have a similar biology to the European paper wasp. They are capable of building paper nests close to human infrastructure and activity, but they are not nearly as common as the non-native European paper wasp. They can sting if their nest is threatened, they often prefer to “ram” into the person or animal that is threatening the nest (Snelling, 1953). Like other paper wasps, they prey on caterpillars, flies, and other pests, bring the prey back to the nest to feed the young wasps the live insects. Adult wasps may also forage for nectar on flowers.
|A western paper wasp visiting my cup of tea earlier in April. Photo: Lisa Mason|
A Note About Insect Stings
Western yellowjackets are responsible for 90%+ of all stings
in Colorado. When someone says, “I was stung by a bee,” they were likely stung
a yellow jacket.
Both bees and wasps can sting. A stinger is a modified
ovipositor (the egg-laying mechanism in insects), so only females have the
ability to sting. The purpose of a stinger is defense, and in some species,
predation. Generally, insects will only sting if they are provoked or their
colony is disturbed. Both social and
solitary wasps and bees can have the ability to sting, but social insects are
more likely to sting because they need to protect their colony.
Honey bees prefer to forage on flowers and go about their
business in their hive, but they can sting if they need to protect their hive. Honey
bees can sting only once. They have a barb at the end at the end of their
stinger that stays in your skin. The barb is attached to the internal guts of
the bee, so when the bee tries to fly away, the guts are ripped out of the bee’s
body, which kills the bee.
Bumble bees have the ability to sting but will only sting if
their colony is disturbed. They can also sting more than once unlike honey
bees. Bumble bees are not aggressive and prefer to forage on flowers and go
about their business. Many native bees are not able to sting or will only sting
Wasps can sting more than once. Solitary wasps will only
sting if they are pressed up against your skin, or you try hard to provoke
them. They prefer to fly away and stay away from human activity. Social wasps
can be very defensive if their nest is disturbed. They also can sting if they are
away from their nest and provoked. The western yellowjacket is much more likely
to sting because they are scavengers and attracted to human foods sources.
They tend to get more aggressive in the fall when temperatures cool down and
food is harder to find. Other social wasps including the European paper wasp
are generally not aggressive unless their nest is disturbed. The European paper
wasp tends to build nests close to human activity on buildings, sheds, and
other structures, which can increase the chance of nest disturbance.
Western yellowjackets and European paper wasps can be a
nuisance to people and often attract attention, but these wasps and other
social wasps represent a small part of wasp diversity. Look for a future post
on the CO-Horts Blog about solitary hunting wasps. These wasps often go
unnoticed in the landscape, but provide valuable pest control services!