Cabbage aphid parasitoid
A black-and-brown wasp about 1/12 in (2 mm) long that lays single eggs inside aphids. Cabbage aphid is its most common host, and it attacks about 100 aphid species in all. A parasitised aphid turns about a week later into a tan, gold or brown mummy, and a new wasp chews its way out through a round hole. It is treated here as a wild ally that turns up on its own, because it homes in on the mustard-oil scent of brassicas. Leave mummies alone, keep broad-spectrum sprays off the crop, and grow flowers nearby.
At a glance
- Kind
- Wild — arrives on its own
- Adult
- About 1/12 in (2 mm); long, beaded antennae
- Sign
- Tan, gold or brown mummy with a round hole
- Hosts
- About 100 aphid species; cabbage aphid chief
- Egg to adult
- About 3 weeks at 68 °F (20 °C)
- Output
- About 25 eggs per female over a week
How it works
A female stings an aphid and leaves one egg inside it. She can use adults and any nymphal stage, although wasps laid in the youngest nymphs usually die with their host. The larva feeds inside the living aphid through three instars. About 7–10 days after the egg was laid it kills the aphid, which hardens into a mummy. The wasp pupates there and chews a round hole to get out. At an average 68 °F (20 °C), egg to breeding adult takes about three weeks, and an adult lives about a week and lays about 25 eggs. Because nothing shows for the first week, more aphids are parasitised than the mummies suggest.
It finds brassica crops by smell. When cabbage-family plants are damaged they release isothiocyanates, the mustard oils, and John Innes Centre field trials showed that raising one of these compounds (but-3-enyl isothiocyanate) drew more wasps to Brassica oleracea, the cabbage species, and to oilseed rape. It is active all year where winters are mild and has several generations a year. UC IPM believes it native to the western US, while a 2025 study treats it as probably Mediterranean in origin and now found on every continent but Antarctica. The same study moves it into the genus Aphidius, as Aphidius rapae.
What it is confused with
Mummies are the sign it is working. Tell them apart from other mummies, and from signs of trouble.
- Also goodAphidius mummies — UC IPM says any aphidiid wasp leaves a brown, gold or tan mummy, so a mummy alone does not prove it was Diaeretiella. Either way, it is an ally.
- Also goodBlack aphid mummies — Black mummies come from Aphelinus and other aphelinid wasps, not from Diaeretiella. They are allies too.
- PestLive aphids — An unparasitised aphid is soft, keeps its species' colour and has no hole. A parasitised one looks normal for about a week before it mummifies.
- PestHyperparasitoid wasps — The genus Alloxysta includes known hyperparasitoids, tiny wasps that attack the parasitoid inside the mummy. UMass caught them on the same flowers as Diaeretiella.
How to keep it around
Adults feed on nectar, and the wasp needs aphids to breed, so it follows cabbage aphid into the crop. Help it by feeding the adults and not killing them.
- Ammi majus (bishop's flower): UMass collected the most Diaeretiella on it in its 2018 flower trial beside Brussels sprouts
- Flowering insectary plants near the brassicas to give adults nectar, as UC IPM recommends
- Hedgerows around cole crops, which UC IPM says shelter natural enemies
- Leave mummified aphids on the plant; each one is a wasp about to emerge
- Control ants, which attack aphid natural enemies, and reduce dust, which disrupts them, as UC IPM advises
- Avoid broad-spectrum, persistent insecticides. If you must spray a brassica pest, delay until just before heading, as UC IPM advises
Pesticide sensitivity
Broad-spectrum, persistent insecticides kill it. In a French oilseed rape study, deltamethrin residues had little effect on wasps that had somewhere untreated to shelter, and the wasp still held green peach aphid down after the spray. That is a reason to leave untreated refuges, not to spray. Open the compatibility matrix →
When to choose something else
The wasp is free, but it lags behind the aphid and may not stop a big autumn outbreak. Look elsewhere when:
- The brassicas are under glass or in a tunnel. Buy Aphidius colemani for green peach aphid, or use lady beetles under row cover
- Colonies are already large. UMass says aphid predators control high populations better than parasitoids do; it names hoverfly larvae and lady beetles, and lacewing larvae are another predator option
- It is a cool autumn and Brussels sprouts are filling with aphids. UMass warns natural enemies may not prevent those densities; see cabbage aphid for sprays that spare it
Cabbage aphid parasitoid by target
What it eats and when it turns up differ by pest. Each page below is the picture for one target.