Parasitoid wasps
Also known as Parasitic wasps, Aphid parasitoids, Aphidius, Encarsia formosa, Braconids, Ichneumon wasps, Chalcid wasps
In short
Parasitoid wasps lay their eggs in or on other insects, and the wasp larva eats its host from the inside, killing it. Most are tiny and harmless to people, and they are among the most important natural controls of aphids, whiteflies, scale, mealybugs and caterpillars. You usually notice them by their handiwork: swollen, papery "mummies" left behind in pest colonies.
How it looks
Adults range from specks under 0.5 mm to about 25 mm, and many are slender, dark or yellowish, with long antennae and clear wings. Aphidius aphid parasitoids are slender black-and-yellowish wasps about 3 mm or less. Encarsia formosa, used against greenhouse whitefly, is about 0.4 mm with a black head and thorax and yellow abdomen. The larvae live hidden inside or on the host and are rarely seen. What you will see are the parasitised hosts: aphid mummies are puffy, crusty, smooth and tan, gold or brown (black for some groups), with a round exit hole once the wasp has left; whitefly scales parasitised by Encarsia turn black.
Signs
Bloated, tan or bronze aphids that no longer move, often with a neat round hole in the back; black instead of pale whitefly nymphs on leaf undersides; tiny wasps walking over pest colonies tapping with their antennae.
Life cycle
Egg laid in or on the host, larva feeding on the host, pupa (often inside the dead host) and adult. Aphidius completes a generation in about two weeks at summer temperatures, with several generations a year.
What favours it
They are most active where pests are present and where flowering plants supply nectar. Ants tending aphids or soft scale attack them, and dust and broad-spectrum insecticides reduce them.
Prevention
- Leave aphid mummies and blackened whitefly scales on the plant: new wasps are about to emerge from them.
- Grow a few flowering plants that offer nectar and pollen near your collection, keep ants off infested plants, and avoid broad-spectrum, persistent insecticides.
- Reduce dust on outdoor plants.
Treatment
Start with the first step and move down only if the problem persists.
- No control needed. In greenhouses and conservatories, commercially reared parasitoids such as Encarsia formosa (whitefly) or Aphidius (aphids) can be released while pest numbers are still low.
- Where a spray is needed, choose short-residual products such as insecticidal soap or horticultural oil, spot-treat and follow the product label.
Pros and cons
Pros
- Very specific: each species attacks a narrow range of hosts, so they do not harm plants or people.
- Individual Aphidius species can attack at least 40 kinds of aphid.
- Several species are commercially reared for greenhouse use.
Cons
- They work slowly and are best at keeping low pest numbers low rather than rescuing a heavy infestation.
- Releases generally need an enclosed space such as a greenhouse to be worthwhile.
- Easily killed by insecticide residues and kept away by ants.
Look-alikes
- Aphids (live)
- Healthy aphids are soft, move and produce young; mummies are rigid, puffy and papery, often with an exit hole.
- Fungus gnats
- Fungus gnats are grey-black flies with long legs that hover over wet compost; parasitoid wasps have a pinched waist and walk over pest colonies on leaves.
- Stinging social wasps
- Social wasps are much larger, often yellow-and-black and build nests; parasitoids are usually tiny, solitary and do not sting people.
Where you find it
Any plant with aphids, whiteflies, scale, mealybugs, leafminers or caterpillars; Encarsia is used on greenhouse ornamentals, tomatoes and strawberries.
Related
Sources
- Aphid Aphidius parasitoids — UC IPM Natural Enemies Gallery
- Whitefly Encarsia formosa parasitoid — UC IPM Natural Enemies Gallery
- Parasitoid wasp — Wikipedia
Written in our own words from these sources. Check a product label before using any treatment.