Beneficial Trap-Nesting Wasps
Wasps on Flowers*
Kevin M. O'Neill and Ruth P. O'NeillIf you observe flowers while looking for bees, you will often see wasps that have similar body forms, but which are much less hairy than bees. In fact, from an evolutionary viewpoint, bees are actually just very hairy vegetarian wasps that we assign a different common name. Recent estimates based on DNA analysis, indicate that bees split from the wasp lineage about 125 million years ago. Anatomically, the close relationship of bees with wasps is partly evidenced by the possession of a stinger in both groups whose only function is to inject venom. For our interest here, however, true wasps differ from bees in ways that affect the kinds of flowers wasps visit, the reasons for the visits, and the value of wasps as pollinators. With relatively few exceptions, wasps are carnivores that visit flowers to obtain nectar for fueling their own activities rather than to collect pollen to feed their offspring. A few wasps, however, also visit flowers in search of prey, which are bees in the case of beewolves of the genus Philanthus (Crabronidae), cutworms for some Pterocheilus (Vespidae), and flies for some social wasps (Vespidae). Even when wasps do incidentally contact pollen on flowers, they may not carry much of it from flower to flower. This goes back to the difference in hairiness between bees and wasps: most wasps are very sparsely hairy, and their hairs are generally short, and always unbranched. The long, closely-spaced, branched hairs of most bees, on the other hand, readily attach to and carry pollen grains.
Wasps also visit a more limited set of flower types than do bees. Because most wasps have shorter mouthparts than bees, they can obtain nectar only from plants with readily accessible nectar not hidden within deep recesses of a flower. The flowers commonly visited by wasps include those from such plant families as Apiaceae (e.g., fennel, wild carrot, wild parsnip), Asteraceae (e.g., asters, golden rod, snakeweed), Caprifoliaceae (e.g., snowberry), Polygonaceae (e.g., buckwheat, sulfur flower), Ranunculaceae (e.g. buttercup, cinquefoil), and Rosaceae (e.g., blackberry, raspberry). Because they are not in search of pollen, wasps may also obtain sugars from other plant sources, including extrafloral nectaries of such plants as leafy spurge (Euphorbiaceae) and sunflowers (Asteraceae), sap oozing from trees, rotting fruits, or honeydew (the sugary secretions of insects such as aphids).
Because foraging female wasps visit many fewer flowers each day than do bees and because they carry fewer pollen grains between flowers they are usually (though not always) relatively ineffective as pollinators. The exceptions include 1) various species of wasps whose males are attracted to orchids whose flowers mimic the wasp’s own females; unfortunately, none of these fascinating insects occur in North America and 2) pollen wasps (see below). However, anyone who spends time observing insects on a variety of flowers is also likely to see a variety of wasps. The types of wasps (with the approximate number of North American species in each group) that one may observe on flowers include the following:
- Yellow jackets (genus Vespula), hornets (Dolichovespula), and paper wasps (e.g., Polistes) of the family Vespidae (40), the familiar wasps that harass picnickers and sometimes build conspicuous nests in trees or on the eaves of buildings. Among North American wasps discussed here, only these species are truly social (i.e., live in cooperative societies containing queens and non-reproductive workers, all of which are females). All of the others are solitary species whose females each build their own nests or parasitize the nests of other wasps or bees. Yellow jackets and hornets tend to have relatively stout bodies with distinct yellow or white coloration contrasting with areas of black, whereas paper wasps have more elongate bodies and slender waists, with somewhat less distinct color contrasts. The social vespids, as well as the solitary eumenine Vespidae mentioned next are also distinguished by their habit of folding their wings lengthwise while at rest, giving the appearance that the wings are narrower than they actually are. In our own yard in Montana, at least three species of social vespids of the genera Polistes, Vespula, and Dolichovespula are common visitors to the fennel flowers in the garden. And, by the way, any gardener not hypersensitive to the stings of these creatures should also welcome them to their yards since they are predators of pest insects such as caterpillars and flies. Recently, the European paper wasp (Polistes dominula) has invaded Montana and other states after it introduction to the U.S. via New Jersey in 1968. Although a voracious predator of garden pests, it also is likely displacing native species of Polistes, as well as killing butterfly larvae.
- Potter wasps and their relatives of the family Vespidae, subfamily Eumeninae (250). These solitary wasps, most of whom are caterpillar hunters, build free-standing nests of mud or else nest in existing cavities in wood or those that they excavate in soil. Eumenine wasps vary considerably in size (typically 10–25 mm) and shape (some have slender waists), and sport white, yellow, or even orange markings. Common North American genera include Ancistrocerus, Eumenes, Euodynerus, Odynerus, Pterocheilus,Stenodynerus, and Symmorphus. As with their social relatives, some eumenines also benefit humans as predators of pest insects, such as spruce budworms and alfalfa weevils.
- Pollen wasps of the family Vespidae, subfamily Masarinae (12). These are the only wasps in North America that visit flowers to collect nectar and pollen which, like bees, they provide as food for their young. Their long mouthparts allow them to take nectar from deeper flowers than most wasps. The North American species are all in the genus Pseudomasaris and all are restricted to the western U.S. About the size of a small yellowjacket (10-20 mm in length), Pseudomasaris also possess yellow and black stripes, along with a distinctive knob at the end of each antenna. Although Pseudomasaris may visit a wide variety of flowers for nectar, they take pollen from a more restricted set of species, mainly in the families Hydrophyllaceae (Phacelia) and Scrophulariaceae (Penstemon). Among Penstemon, those species with violet-colored flowers are pollinated by Pseudomasaris, whereas those with red flowers are hummingbird-pollinated and those with blue flowers bee-pollinated.
- Digger wasps and sand wasps of the families Sphecidae and Crabronidae (1200), which are the closest relatives of bees. Sphecids are an extremely diverse group of solitary wasps that exhibit great variation in body size and form, from tiny and slender aphid-hunters of the genus Passaloecus to the relatively large (40 mm long) and robust cicada-killers of the genus Sphecius. Sphecids also vary tremendously in color, from homogeneous black to various combinations of white, yellow, orange, red, and metallic blue or green. Though digger wasps are typically much less abundant on flowers than are bees, it is not unusual to encounter particular species in high numbers on certain flowers. At the NRCS Bridger Plant Materials Center in Montana, the great golden digger wasp (Sphex ichneumoneus) is a ubiquitous visitor to slender-white prairie clover (Dalea candida). But it is unknown how much this species contributes to pollination and seed production of the clover, relative to the honey bees or the various native bee species that vastly outnumber it.
- Spider wasps of the family Pompilidae (220), that are distinguished by their predominantly jet black color, long spindly legs, and habit of constantly flicking their wings while walking. Though less variable as a group than the digger wasps, spider wasps occasionally sport white, yellow, or red body markings, along with some combination of orange, black, or smoky gray wings; they typically range in size from 10-25 mm, but the spectacular tarantula hawk wasps (genera Pepsis and Hemipepsis) of the southwestern U.S. can be up to 40 mm long and deliver correspondingly powerful stings. Within their ranges, tarantula hawks can be found on such flowers as milkweed (Asclepiadaceae) or creosote bush (Zygophyllaceae).
The above list includes many of the North American solitary and social wasps likely to be seen on flowers, but is by no means exhaustive. Thus, positive identification even to the family level may require more detailed knowledge of this group. Other wasp families seen on flowers may include the Mutillidae (velvet ants, often orange and black, whose winged males visit flowers) (435), Scoliidae (of which members of the genus Campsoscolia are sometimes effective pollinators) (20), Tiphiidae (220), and Chrysididae (the brightly metallic green or blue “cuckoo wasps”) (230). To this list we can also add sawflies of the family Tenthredinidae (800), which are distantly related to the other wasps we have mentioned and whose plant-feeding larvae often resemble caterpillars. We have commonly encountered sawflies of the genus Tenthredo on species of geranium in the mountains of Montana.
Field guides to insects can provide general information on identifying social and solitary wasps, whereas details on the biology of the solitary species can be found in Solitary Wasps: Behavior and Natural History by Kevin M. O’Neill (Cornell University Press, 2001).
* this article is based partly on material written by K. O'Neill for the book Attracting Native Pollinators, published by the Xerces Society (2011).