Scale Insects

How to know it's scale

Small immobile bumps along stems and leaf veins — brown, tan or grey, sometimes flat and shell-like, sometimes raised and waxy. They don't move, so people mistake them for part of the plant, or for a fungal spot. Soft scale leaves sticky honeydew and sooty mould; armored scale doesn't, which makes it even easier to miss.

The mobile stage is the crawler, and it's brief. Everything after that is a female sitting under a protective cover, feeding, which is precisely why sprays underperform: the cover is doing its job.

What eats them

Rhyzobius lophanthae is a small dark lady beetle that specialises in armored scale. Both adults and larvae feed underneath the scale cover — they get to the insect the spray can't wet.

Montrouzieri (Cryptolaemus montrouzieri) is best known for mealybugs but also takes soft scale, and it's worth having if you have both, which is common.

What to expect

Scale is slow, and so is its control. Dead scale stays stuck to the plant — an empty shell looks identical to a live insect from across the room. Press one with a fingernail: if it's dry and hollow, that battle's over. Judge progress by whether new crawlers are appearing, not by whether the old bumps have fallen off.

Both beetles are warmth-loving and slow down in a cold room. Give them a stable, warm spot and time to find the colonies.