Because of a few aggressive species, wasps often get a bad rap, but they are vital to the ecosystem and helpful to gardeners for many reasons. Wasps — of which there are 18,000 species in North America — also have fascinating biology and behaviors, and you’ll love learning about them. To tell us all about the diversity and importance of wasps, my guest this week is biologist, pollinator conservationist and award-winning author Heather Holm.
Heather is an expert on the mutually beneficial relationship between native plants and native pollinators. Her most recent book is “Wasps: Their Biology, Diversity, and Role as Beneficial Insects and Pollinators of Native Plants,” a National Indie Excellence Award Winner and a natural follow-up to her earlier work “Bees: An Identification and Native Plant Forage Guide.”
Heather lives in Minnesota and grew up in Canada. She has long had an interest in plants and studied horticultural science at the University of Guelph in Ontario. After she began to practice ecological horticulture and worked on ecological restorations, she was amazed by the diversity of insects that her native plant landscapes attracted. That led her down the road to study pollinators and learn everything there is to know about bees and wasps.
Though bees are Heather’s first love, she couldn’t help but fall for wasps too. When studying flower-visiting insects, wasps are part of that equation, she says. She found that wasps have many of the same behaviors as bees and they nest in the same places. “I thought, ‘Wow, they’re equally as cool and interesting,’” she recalls. She really began to study wasps in depth four years ago when she began research for her new book.
“Wasps” is a page-turner, and I can’t say enough good things about it. It explains wasp behavior and lays out how the insects navigate and how they find their prey. Though some species can only be positively identified under a microscope, the book provides the means to identify many wasps, such as range maps and preferred nectar plants. Most importantly, “Wasps” empowers us gardeners by providing us with a better understanding of these creatures and the important role they play in the ecosystem.
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Wasps’ Undeserved Bad Rap
Wasps are unfortunately painted with a broad brush. Most people don’t like wasps because they have been stung by one, but it’s only a small minority of wasp species that sting or actively want to defend their nest, Heather points out. There are many beneficial wasp species that live in our gardens and have no negative interactions with us.
Humans tend to have a knee-jerk reaction to wasps — to be scared. But in all likelihood, that big wasp is a solitary wasp that nests in the ground or in a cavity, and it will not have any negative interaction with a person. That wasp has a positive impact on your garden as a pollinator and as a predator.
What Separates Bees and Wasps
Wasps are the ancestors of bees. “Bees are really just hairy wasps,” Heather says. But there are a number of other traits that also set bees and wasps apart.
Diet is a major difference between bees and wasps. Bees are essentially vegetarians. They collect pollen and nectar to feed their offspring. Wasps hunt insects and spiders to feed their offspring.
Though wasp larvae eat the insects and spiders provided by their mothers, adult wasps subsist off sugary substances, such as flower nectar and honeydew.
Wasps as Pollinators
It can be hard to quantify if an insect visiting a flower is truly pollinating, Heather says. The question is, are they successfully moving pollen from flower to flower?
Because wasps, in general, are not as hairy as their bee cousins, they don’t have the structure to carry pollen efficiently. However, when wasps visit flowers frequently, the likelihood of them moving some pollen around is pretty good, according to Heather.
Social vs. Solitary Wasp Nesting Behaviors
Of the nest-building wasps, there are species that are social and species that nest singly. Stinging wasps are generally social wasps because they have more reasons to defend their nests than solitary wasps.
Social wasp nests have an egg-laying queen, workers and multiple broods a year. Many wasps work cooperatively to operate and protect the nest. Every wasp has a different job: Some collect prey to feed offspring, others stand guard, etc.
If you have ever been stung by a wasp, it was likely a yellowjacket, Heather says. Yellowjackets are usually ground nesters. Mowing or digging around a yellowjacket nest will cause the insects to be aggressive, and they will attack.
Other wasp species build nests above our heads in trees, sheds and eaves. In the Eastern United States, the above-ground nesters that gardeners are most likely to encounter are paper wasps and aerial yellowjackets. When their nests are high up in trees or on a house, they are less likely to defend their nests from humans because we don’t get close enough to alarm them.
Wasps that are tasked with guarding the nest hang out near the entrance and look for threats. Vibration, rather than visual cues, are typically what they react to first, Heather says. An 8- or 10-foot buffer should be enough whether a nest is above ground or underground.
Wasp nests are annual in most cases, so you can wait out a nest by maintaining a buffer zone until winter comes. But in a warm climate like Florida’s, for example, wasp nests can be perennial, Heather says.
How Social Wasps Build Nests
Social wasp nests are made of paper, which the wasps produce by chewing up wood fiber and plant fiber that they have collected. The wasps scrape their mandibles on exposed wood and then mix the wood fiber with water, Heather explains. They use their saliva to help turn the wood fiber into pulpy material that they then carry back to the nest, where they continue to chew it. Once the material is the right consistency, they carefully layer it onto a cell of the nest.
Wasps make consistent, hexagonal cells that come together in an amazing way. The cells are similar to the cells in a honeybee wax comb. The wasps use their antennae as a measurement guide to keep a consistent diameter.
Yellowjacket nests get quite large as the wasps build multiple combs, or tiers. They will build up to four combs as they continuously enlarge the nest. Meanwhile, paper wasps build just a single comb, which is exposed with no envelope, and attached to a horizontal surface.
Thermoregulation in Wasp Nests
Hot, humid summers don’t bode well for developing wasp larvae. To regulate the temperature in their nests, wasps collect droplets of water. They deposit the water in the nests and then fan the water with their wings, creating an air conditioner effect. In cold weather, they can also use their wings to regulate the temperature: Vibrating their wings creates heat.
How Solitary Wasps Build Nests
For solitary wasp species, the female builds a nest by herself and puts prey in it for her larvae to consume. The majority of nesting solitary wasps build their nests in the ground, excavating cavities themselves and creating cells inside the nest. The minority nest above ground in preexisting cavities, such as hollow plant stems and holes in wood.
Cavity-nesting wasps come in various sizes and they look for just the right size cavity. For the smallest species, the cavity diameter can be as small as a 16th inch. Mason wasps look for cavities that are about a half-inch in diameter.
Stem-nesting wasps create nests in the dry stems from the previous growing season. (This is one of the reasons why gardeners should leave stems standing over winter rather than cleaning up in fall.) Because stems of flowers and perennials tend to stand at an angle rather than perfectly vertical, rain stays out of the stem for the most part. Wasps will also use resin, mud, leaf pieces or the stem pith to cap off the stem.
In nature, wasps use the stems of plants that were browsed by deer or rabbits. Even heavy snow load can break stems and create opportunities for wasp nests. In our gardens, it’s our pruners that create topless stems that wasps can lay eggs in.
Wasps that nest in stems use mud for partitions in order to create multiple cells. The deeper the cavity, the more cells they create. They also build a vestibular cell, which is like a vestibule or foyer, at the entry of the cavity. The empty cell is designed to keep out predators.
Wasps can be picky about what type of mud they use to create partitions. One species may prefer sand while another seeks loamy soil. They then combine the material with water and sometimes their saliva to make mud balls.
The cicada killer is an example of a solitary digger wasp. Cicada killers excavate ground nests that are quite large in diameter in order to get a cicada down the hole.
A solitary wasp nest underground may have a 12-month life cycle from egg to adult. If the soil is tilled or otherwise disturbed over the course of the year, the larvae won’t mature. But generally, Heather says, ground nests are better protected from human disturbance than aboveground nests.
Wasps That Don’t Build Nests
Parasitic wasps make up most of the wasp species in North America. The females are recognizable and noticeable because they have a long ovipositor that looks like a stinger. Parasitic wasps inject their venom and an egg through that ovipositor into prey. When the eggs hatch, they eat the still-living prey, which is typically insect larva.
Other non-nest-building wasps behave like cuckoo bees: They invade the nest of other insect species and lay their eggs. When the eggs hatch, they feed on either the insect larvae in the nest or the prey that the nesting wasp had gathered.
Hunting and Orienting Behavior in Wasps
Solitary wasps are not inclined to waste their energy or venom on protecting their nests. Heather explains it in terms of energy expenditure and conserving resources: Wasps want to use their venom to sting and paralyze their prey, not us.
Wasps have a number of crafty ways that they capture prey. Once they have a hold of an insect or spider, they sting with venom that causes paralysis. The immobile prey can then be more easily transported back to the nest. Still, the prey can be two times as heavy as the wasp, so the wasp may take a couple of rest breaks on the ground on the way back to the nest. When the prey is just too big, the wasp may drag the prey across the ground the whole way, taking breaks occasionally to climb up vegetation to reorient herself and make sure she is headed in the right direction.
Small and medium wasps may need to take multiple trips to capture many small insects and fill their nests with prey. For big wasps, capturing just one sphinx moth caterpillar or one cicada can be enough. The prey is alive when delivered to the nest and will be eaten alive when the wasp eggs hatch.
Solitary wasps can sometimes be found nesting in one big aggregation, such as a big sandy area that makes for an attractive nesting spot. With hundreds of nests in one area, how can a wasp tell which nest is hers? A wasp will often do an orientation flight in which she hovers over her nest entrance and makes either arcing or figure-eight flight patterns. “What they’re doing is they’re orienting themselves to the sun, and then that gives them sort of a picture in time of how to find their nest when they return with prey,” Heather says.
Much of the natural history research done on wasps was conducted between the earlier 1900s and the ’60s, when the work was funded, Heather says. Today, there is more research being done on solitary bees and wasps, but wasps are not getting the same level of attention as bees. Heather scoured the old research to write her book and includes excerpts of the Victorian era materials.
Wasps, like bees, have two sets of eyes. There are compound eyes that we can easily see on the side of their heads and then three simple eyes on the top of their heads, which look like little buttons. The simple eyes are used for orientation with the sun and the large compound eyes are for visual purposes.
Some wasps, such as sand wasps, don’t have functional simple eyes anymore, so they have lost that orientation function. They must use their compound eyes to remember the shapes of plants or trees on the horizon, and that’s how they orient back to their nests.
Wasps have a number of sensory organs. For example, along the length of their antennae are olfactory sensors. A parasitic wasp can sit on a tree snag and tap her antennae to detect if there are beetle larvae eating wood fibers inside the snag. That gives her the direction she needs to drill her ovipositor through the wood and into a beetle larvae. Scoliid wasps similarly detect scarab beetle larvae belowground, dig down and inject their eggs.
One way to recognize a wasp is by its constricted waist between its abdomen and its thorax. This separates wasps from bees and flies. Heather says that though this long, narrow constriction has been studied, there is no clear answer as to why wasps have this anatomical difference. One factor may be the mainly liquid diet of adult wasps — their digestive systems don’t have to be wide because they aren’t eating solids. Plus, that abdomen–thorax connection is pliable so thread-waisted wasps are agile enough to inject venom into their prey.
Wasps and Pest Control
Encouraging wasps in your garden is a form of biological pest control.
You may have seen the white, rice-like cocoons of parasitic wasps on the backs of tomato hornworms or tobacco hornworms. Once the eggs hatch, the larvae race to eat the nonessential parts of the hornworms and then the organs, killing their hosts.
Not only do parasitic wasps control pest populations, so do prey-hunting wasps. They both consume pest larvae before the pests can reach maturity and lay more eggs.
By increasing the diversity of native flowering plants in our gardens, we can attract more diverse wasps to provide prey control services.
Predators of Wasps
A number of insects prey on wasps. For example, robber flies that look like giant bumblebees are voracious predators of wasps of other insects. A number of parasitic flies and beetles have their larvae hitchhike on wasps’ backs back to wasp nests, where they prey on wasp larvae. And thick-headed flies hang out around flowering plants, where the females inject their eggs into the abdomens of bees and wasps.
Birds also prey on wasps, particularly male wasps because the males don’t have a stinger, Heather says.
Social wasps, particularly ground-nesting yellowjackets, can be scavengers and cavernous, consuming roadkill and other mammals and birds that they find dead. They also consume other insects.
Besides flower nectar, a sugary substance that wasps enjoy is aphid honeydew. You may see wasps perched on a flowering plant on the leaves below the flower, where they collect the honeydew that aphids excrete.
Social wasps are attracted to fruit including apples and crabapples. For that reason, you should be careful when walking around during fruit downfall.
Sap is a good sugary food source before plants are blooming in spring, and then during the summer, sap can become fermented when airborne yeast gets into a tree, Heather says, and wasps can get a little drunk on the alcohol.
Some solitary wasps have very specific diets, like the cicada killer. For wasps that hunt a specific type of wood-boring beetle, their habitat is limited to where that type of wood grows.
Wasp Life Cycle
The life cycle of social wasps is quite similar to the life cycle of bumblebees, Heather says. The nest is established in the spring by a female that has overwintered as an adult. She finds the perfect place to construct her nest and then goes out and collects wood fiber to make the first paper nest cells. At the end of the growing season, she starts producing new queens. All the other wasps will perish at the end of the growing season, but the queens survive.
With paper wasps, the sisters overwinter together in an insulated place such as a crevice in a rock. They will then jockey for hierarchy when spring approaches. They may each establish their own nest or they may partner up to build nests. Yellowjacket queens overwinter by themselves under heavy leaf litter or a log, or in a hollow stem.
Solitary wasps may have one generation per year, or multiple generations. The great golden digger wasp, for example, emerges in July, builds a nest, lives for four to six weeks, then perishes with her offspring develops in the nest. Some cavity-nesting wasps such as mason wasps have up to three generations per year. The first generation builds a nest in a cavity, the offspring emerges, and then the offspring builds their own nests elsewhere.
I hope you have newfound affection for wasps after listening to my conversation with Heather Holm. If you haven’t listened yet, you can do so now by clicking the Play button on the green bar near the top of this post.
Have you observed wasps and their interesting behavior in your garden? Let us know in the comments below.
Links & Resources
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“Bees: An Identification and Native Plant Forage Guide” by Heather Holm
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