Braconid wasps
Braconids are small parasitic wasps whose larvae feed inside another insect and kill it; the Cotesia species are only 2–3 mm long. The one gardeners notice is Cotesia congregata: the white, rice-grain objects standing up on a hornworm's back are its cocoons, not eggs. A hornworm carrying them eats about a fifth as much as a healthy one and then dies, so leave it on the plant. Other braconids attack cabbageworms, fruit moths and aphids, which they turn into papery brown mummies. None of the wild species is sold; you keep them with flowers and fewer sprays.
At a glance
- Kind
- Wild — attracted, not bought
- Adult
- 2–3 mm; dark, slender, long antennae
- Eggs per hornworm
- 50 to several hundred
- Inside the host
- About 12 days (C. congregata)
- Egg to adult
- About 2 weeks in summer (a related Cotesia)
- Adults feed on
- Nectar and honeydew; Cotesia adults live 2–3 weeks
How it works
A female Cotesia congregata lays eggs inside a young hornworm (first to third instar, under about an inch long). A single wasp may inject anywhere from 50 to several hundred, along with a virus that suppresses the caterpillar's immune response. The larvae feed on its body fluids and fat body for about 12 days, then chew out through the skin and each spins a white silk cocoon standing up on the caterpillar's back. The adults emerge a few days later by cutting a cap off the cocoon. The hornworm, which has been eating about a fifth as much as a healthy one, stops feeding and dies while they pupate. The wasp is native to the United States and common throughout the eastern states; it attacks around a dozen species of sphinx moth caterpillars and a few other caterpillars.
Braconidae is a large family, with over 1,000 species in the US, and most attack the larvae of caterpillars, beetles, flies or sawflies. Cotesia glomerata and C. rubecula take cabbageworms, Macrocentrus fruit moth larvae, and Microctonus adult flea beetles. The aphid parasitoids (Aphidius, Lysiphlebus) are sometimes classed as braconids and sometimes as a family of their own. Their larva eats the aphid from inside and leaves a crusty brown or gold mummy with a round exit hole. One of them, Aphidius colemani, is the aphid parasitoid sold for greenhouses. Braconids have enemies of their own: more than a dozen species of hyperparasites attack C. congregata.
What it is confused with
The mistake that matters is killing a parasitised caterpillar. The cocoons are the wasps' next generation, and they are often mistaken for eggs.
- Also goodHornworm 'eggs' — The white objects standing up on a hornworm's back are wasp cocoons, which UMN likens to grains of white rice. Hornworm eggs are oval, light green and laid on leaves, not on the caterpillar.
- Also goodTachinid fly eggs — Small, oval white eggs glued to a caterpillar, often on the head or thorax. They are also a parasite, so leave that caterpillar too.
- Also goodAphid mummies — Puffy, crusty brown, gold or tan aphids that do not move, some with a round exit hole. A wasp developed inside, so leave them.
- Also goodCabbageworm cocoon clusters — A mass of small yellow silk cocoons beside or on a green cabbageworm is Cotesia glomerata, not the caterpillar's eggs.
How to keep it around
UC IPM says adult Cotesia wasps feed on honeydew and nectar and live two to three weeks. The University of Kentucky notes that nectar is often an important food for adult parasitoids, and that planting a variety of flowers is known to raise their numbers. Parasitoids are often more sensitive to insecticides than predators are, so what you do not spray matters as much as what you plant.
- Leave any hornworm or cabbageworm carrying cocoons on the plant so the wasps can emerge
- Grow a variety of flowering plants near the vegetables; sweet alyssum and buckwheat raised Cotesia parasitism on brassica trap crops beside cabbage in a Missouri trial
- Small sunflower plantings near peach trees, which UC IPM suggests for the oriental fruit moth parasitoid
- Keep ants off aphid colonies, because they drive off the parasitoids
- Keep dust down along paths and field edges, which UC IPM says disrupts natural enemies
- Avoid broad-spectrum, persistent insecticides; if a caterpillar spray is needed, use Bt kurstaki on small larvae
Pesticide sensitivity
Parasitoids are often more susceptible to chemical insecticides than predators (Cornell), and Cotesia glomerata is susceptible to several pesticides used on cole crops. UC IPM rates Bt kurstaki low in toxicity to parasites. Open the compatibility matrix →
When to choose something else
The wild braconids cannot be ordered, and they work in the background rather than on demand. Look elsewhere when:
- Hornworms without cocoons are stripping a plant now — pick them off by hand, or use Bt kurstaki on small ones
- You want to stop caterpillars at the egg stage — Trichogramma egg parasitoids are sold
- Aphids are building on greenhouse or indoor plants — release Aphidius colemani
- The caterpillar carries oval white eggs glued to its skin rather than upright cocoons — that is a tachinid fly, also a friend
Braconid wasps by target
What it eats and when it turns up differ by pest. Each page below is the picture for one target.