Good Wasps in Small Packages
The word “wasp” usually brings to mind some sort of pesky insect: the yellow jackets that join us for a picnic, or the paper wasps that dangle a threateningly large nest over the front door. Yet most wasps are innocuous garden residents, usually non-aggressive, diminutive, and stingless. Aphid parasitoid wasps are so small that they go about their business virtually unnoticed, but their presence is easily ascertained by examining an aphid colony. Look for papery black or beige “mummies,” many with a neat exit hole gnawed by an emerging adult wasp. Several wasp families include species that attack aphids. The small, non-stinging wasps in the family Braconidae include aphid parasitoids, as well as wasps that attack other garden pests such as caterpillars, flies, true bugs, and beetles.
Braconid wasps can be distinguished by the typical, narrowed “wasp-waist.” They may be shades of black, red, brown, or yellow, often with a dark spot at the edge of the forewing. The antennae have sixteen or more segments, and are shorter than the forewing. Although they may be slender or stout-bodied, the abdomen is always longer than the head and thorax combined. For most garden purposes, braconids can be recognized as the slender dark wasps commonly found hovering around aphid colonies or caterpillars. Virtually all tiny wasps found lingering in the vicinity of pests, regardless of family, are garden allies.
Parasitoids, Parasites, and Predators
A parasite lives on (or in) a host; although it may move from host to host, it is not free-living and rarely kills its comfortable abode. A parasitoid lives on (or in) a host for only part of its life cycle, and invariably kills its unfortunate victim. In contrast to these parasitoid organisms, each of which typically kill just one host, a predator kills and eats many individuals over the course of its life, and is usually bigger, not smaller, than its prey.
Different species of wasps attack different stages in their host’s life cycle, but all are helpful in controlling pest populations. Most effective are the egg parasitoids, whose larvae develop in the host’s egg, which never has the opportunity to hatch; many commercially available species of wasps are egg parasitoids. Larval parasitoids deposit eggs on (or in) the larval stage of the host. In some host species, the egg hatches, but the victim has little opportunity to damage plants before its untimely end. Although some caterpillars, once parasitized, quickly lose their appetite for prized garden plants, some continue to eat, apparently oblivious to their internal guest. When parasitized, the familiar imported cabbageworm has even been found to eat up to one and a half times the usual amount of food over its larval lifespan. Pupal parasitoids also parasitize the pest after the damage has already been done to plants. The benefit, in these last two cases, is a reduction in the next generation of insects, as the parasitized larvae will not reach maturity or reproduce. This may seem like meager comfort, as you watch your kale being munched. The objective, for the patient gardener, is to provide “wasp-friendly” habitat, allowing populations of these helpful insects to build, resulting in better control of pests over the long term.
Parasitoids often have a restricted range of hosts, attacking only one or several closely related species of pests. Specialist parasitoids are often more effective at controlling pests than are generalists, which are less discriminating in their tastes and may attack beneficial insects as well as pests. The large and diverse family of braconids includes many specialists. Aphid parasitoids pupate inside their hapless host and emerge as adults, but many braconids, especially those parasitizing caterpillars, emerge to pupate outside the host. Caterpillars may be host to more than one braconid at a time, and can sometimes be found in the garden festooned with numerous small silky cocoons. Tomato hornworms, attacked by Apanteles congregata (formerly Cotesia congregata), or imported cabbageworm, attacked by A. glomerata (formerly C. glomerata), are commonly parasitized; if you come across them in the garden, it is best to leave them to complete the life cycle of the wasps.
Feeding Adult Wasps
Parasitoid wasps, while depending on arthropod hosts in their larval stage, are nectar-feeders as adults. Sweet alyssum (Lobularia maritima) and small-flowered members of the daisy family, such as German chamomile (Matricaria recutita) and fleabane (Erigeron), are ideal wasp food sources. The best plants for attracting parasitoids are those in the umbellifer family (Apiaceae). In my garden, over the last few years, I have grown every umbellifer I could find. The beautiful flowers contribute far more than useful habitat for wasps. In addition to punctuating the landscape in a highly satisfactory manner, the umbellifers also provide a banquet for lady beetles, lacewings, syrphid flies, soldier beetles, and other beneficial insects—in addition to wasps. Anecdotal evidence suggests that most of the small-flowered species, such as the annual Orlaya grandiflora or perennial sea holly (Eryngium ×tripartitum), are effective insectary plants. If allowed to flower, umbelliferous herbs, such as parsley, dill and cilantro, offer copious nectar; angelica flowers virtually drip with nectar. A garden full of ornamental umbellifers is sure to please both beneficial insects and gardener alike.
In a Nutshell
Order: Hymenoptera (includes many families of bees and wasps, as well as the ant family).
Common Garden Species:
Aphidius colemani and A. ervi are cosmopolitan species that parasitize aphids. Some now widely established species are introductions: A. matricariae, a parasite of green peach aphids, and Diaeretiella rapae, which attacks the common cabbageworm.
Over 1,700 known species in North America, and more than 100,000 worldwide. Several hundred species are known in California and the Pacific Northwest, and it is likely that many species remain undescribed.
Holometabolous (complete metamorphosis from egg to larva, pupa, and adult).
Eggs: microscopic. Larvae: maggot-like. Pupae: External cocoons are usually white or yellowish and fluffy. Adults: typically under a half-inch; may be slender or stout, with long antennae; frequently, a dark spot on the forewing; color varies.
Some of the smallest braconids have brief life spans, but may have several generations per year. Other species have only one generation annually.
A wide range of pests, such as aphids, beetle larvae, leaf miners, flies, bugs, and sawflies. Braconids often parasitize the larvae of lepidopterans, such as sphinx moths, cabbage butterflies, and gypsy moth (helpful for our friends on the East coast).
Nectar plants such as yarrow (Achillea) and other small-flowered members of the daisy family, alyssum (Lobularia maritima), and most plants in the umbel family (Apiaceae).
Braconids attack many common garden pests, and can be effective at parasitizing great numbers of individuals. Aphidius colmani, for instance, may lay more than 350 eggs over a four to five day lifespan.
In a butterfly garden, braconids may exercise a bit too much control, since they will parasitize caterpillars.
Parasitoids are sometimes themselves parasitized. These secondary parasites are known as hyperparasitoids; they can reduce the beneficial effect of a primary parasite, but, in our gardens, it is all simply part of the web of life. For a closer look at a parasitoid, insert an intact aphid ‘mummy’ into an empty gelatin capsule and wait a few days. If you are lucky, you may even see the wasp chew its way out of the aphid carcass.
Some parasitic wasps are commercially available, but expensive for the back-yard gardener; a better strategy is to provide suitable habitat. In some cases of large infestations on important landscape plants, the expense may be justified.
Great Garden Companions, by Sally Cunningham, has a wonderful chapter on backyard garden beneficials that includes several braconid wasps, and a long list of plants that will attract garden beneficials.