Friday, June 25, 2021

A Wasp-Benefit Analysis – Part II: Social Wasps

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

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

Western Yellowjackets

Western 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

European 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 Yellowjackets

Baldfaced hornets (Dolichovespula maculata) and aerial yellowjackets (Dolichovespula 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.

A baldfaced hornet nest. Photo: Joe Boggs, Ohio State University Extension

A baldfaced hornet. Photo: Joe Boggs, Ohio State University Extension

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 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 if handled.


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 and garbage. 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.

Learn More

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 about solitary hunting wasps. These wasps often go unnoticed in the landscape, but provide valuable pest control services! 

Friday, June 11, 2021


By Yvette Henson, San Miguel Basin, CSU Extension

Colorado has a semi-arid to arid climate with recurring periods of drought.  Yet, plants in the landscape provide many benefits like cooling the air temperature around and inside our homes.  There are also physical and mental health benefits from growing gardens.  Growing at least some of our own food contributes greatly to food security.  So, how do we use water responsibly to grow a garden, without waste?  

This blog posts gives some tips to conserve water in the vegetable garden: 

·         Grow only what you need.

·         Group plants with similar water needs.

·         Plant in blocks, not rows, to shade the soil.

·         Incorporate organic matter into soil before planting to hold water.

·         Apply mulch, after planting, to reduce evaporation.

·         Cover plantings with row cover fabric to reduce evapotranspiration.

A freshly watered vegetable garden. This garden will benefit from an application of mulch to the fall peas to reduce evapotranspiration.

Some other water conservation ideas to investigate are Hugelkultur, planting in natural depressions and paths of runoff (“rain gardens”).  Or you can create your own depressions, swales, and underground trenches to direct water.  Check water law before creating ponds or other larger water holding/directing structures.

“waffle gardens,” depressions in the ground hold water for crops, and were used by first nations peoples in the southwest. (Photo credit: Jodi Torpey)

When watering your garden:

·         Water only when needed, rather than on a schedule.

·         Use a trowel or shovel to check soil moisture, rather than guessing.

·         Water more deeply, to depth of the root zone, less frequently.

·         Water slow-draining soils, like clay, in several short intervals to reduce runoff. 

·         Water during the coolest times of day or when your garden is shaded.

·         Don’t water when it is extremely windy.

·         Don’t water automatically when you’ve received adequate rainfall (> ½”)

·         Use soaker hoses or drip irrigation.

·         Use harvested rainwater (see this Fact Sheet )

In general, water is most critical during the first few weeks of development, immediately after transplanting, and during flowering and fruit production.  Young plants need less but more frequent watering.  Older plants use more water but need it less frequently.  They have deeper roots and larger canopy. 

You can target the timing and amount of water to add during the most critical periods of each specific crop.   

Root crops need water most when roots are sizing up.  Water stress can cause bolting and/or strong flavor.  Fruit- and seed-bearing crops need extra water when fruit is setting and filling.  Too little water can reduce yield significantly but can also make fruit more flavorful.  Too much water before fruit set can delay it. Bulb crops, like onions and garlic, need more water when young and less water when maturing and none when curing (in ground).

Certain plants require more water than others.  Are they worth it?   For example, beans and strawberries (especially when establishing) need 0.25 to over 0.50 inches of water per day when blooming and setting fruit. Of course, this is variety dependent. Certain beans, like terpary beans, do not require much water.


Follow your local water restrictions, garden responsibly and reap the benefits!