Soft scale parasitoid wasps
Metaphycus wasps are 1/32–1/10 inch (0.8–2.5 mm) long, brown, orange or yellow, with bulging greenish eyes, and they are among the most important enemies of soft scales. M. helvolus parasitises at least 22 scale species and gives substantial control of black scale in Southern California citrus. On brown soft scale, UC says a complex of Metaphycus species attacks it, most often M. angustifrons in Southern California. UC's citrus guidelines say to release M. helvolus only "if available", and none of the retail suppliers checked list it now. In practice, protect the ones already there.
At a glance
- Kind
- Wild — rarely sold now
- Adult
- 0.8–2.5 mm; brown, orange or yellow
- Hosts
- Soft scales, black scale above all
- Attacks
- Small, young scales; females host-feed
- Sign
- Round exit hole in a puffy scale
- Works best
- Coastal, where scale generations overlap
How it works
The female lays one to several eggs inside a scale, and the larvae feed within it through four instars before pupating. UC says females live one to several weeks and can each lay several dozen eggs. She also punctures young scales and drinks their body fluids, which kills them without parasitising them and helps her live longer and lay more. M. helvolus parasitises only the smaller stages of black scale.
Because it needs small scales, it works best where scale generations overlap. UC's citrus guidelines say it is more effective near the coast, where susceptible stages are present for longer, than inland. For releases, UC advises late summer or early autumn, 1,000–2,000 per acre per year (a minimum of 1,000), and ant control, because ants guard soft scales for their honeydew. Growers also move parasites between groves by cutting scale-infested branches before exit holes appear.
What it is confused with
Parasitised soft scales look different from live ones, but several wasps leave the same signs.
- Also goodCoccophagus wasps — Another common soft scale parasite. UC credits it and Metaphycus together with keeping citricola scale almost unseen. It also leaves a round exit hole, so the hole alone does not tell you which one it was.
- Also goodAphytis wasps — Yellow, with three tiny red eye spots in a triangle. They parasitise armoured scales, working under the cover, not soft scales.
- PestLive soft scale — A soft scale's cover and body come away together. Squash one: a live scale exudes fluid, while a parasitised one does not, and is often puffy or darkened, sometimes with the parasite visible through it, and later has a round hole.
How to keep it around
UC's advice for these wasps is conservation, and it warns that it can take several months, or until the next season, for scale numbers to fall.
- Control ants: they protect soft scales for their honeydew and attack natural enemies
- Avoid broad-spectrum, persistent insecticides
- Check scales for exit holes before spraying. UC calls spraying a highly parasitised population a common error
- Grow flowering plants so the adults have nectar and pollen
- Rinse dust off small plants, because dust interferes with natural enemies
- In citrus, cut scale-infested branches from a grove with many parasites before exit holes appear, and move them to where parasites are scarce (UC)
Pesticide sensitivity
UC IPM warns that broad-spectrum, persistent insecticides are toxic to natural enemies, and that scales normally kept down by parasites escape control when these sprays disrupt them. Check scales for parasite exit holes before any spray. Open the compatibility matrix →
When to choose something else
Metaphycus are soft scale parasites to protect, not ones you can count on ordering. Instead:
- For brown soft scale, UC names the lady beetle Rhyzobius lophanthae among its predators, and it is sold
- For armoured scales, release Aphytis melinus, because Metaphycus specialise in soft scales
- Where ants are running up the trunk to honeydew, control the ants first, or no parasite will get to work
- For a few scaly twigs on a small plant, UC says to prune them off
Soft scale parasitoid wasps by target
What it eats and when it turns up differ by pest. Each page below is the picture for one target.