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I have a young Acacia bonsai which has been invaded by mealy bugs (I believe?), primarily on the branches circled in red. They are brown, stick themselves to the plant, and can be seen walking up and down branches (they have legs), especially after I poke one. And they have excreted white hairs, crystals and blobs.

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I first used rubbing alcohol and an earbud to directly kill most of them, but have since been spraying daily with this insecticide. From what I can tell it has been killing off the stragglers. I have some questions:

  • Can I keep the infested branches or would the plant recover better if I pruned them off completely?
  • How much longer should I keep with the spraying?
  • Can I leave the dead mealy bugs on the plant? There are hundreds of them and it would take hours to remove each one and I'll likely further damage the plant in the process. Will they eventually be worked off as the plant grows and recovers?

See close ups (these mealy bugs are all DEAD):

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  • I would also love to know HOW they arrived on my bonsai, which is completely isolated from other plants. It has been sitting by this window sill for a good month or more without a single trace of pest or damage. – Dave New Feb 12 at 8:56
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Let's start with what I can see:

  • I know you say those mealybugs are dead but they look quite healthy to me.
  • The brown bumps on the stems could be scale or might be ladybugs who have arrived to eat the mealybug
  • That is a major infestation of a pest that is hard to control.

What you have been using is a contact pesticide.The active agent is the oil which is supposed to block their spiracles that they breathe with. Mealybugs protect themselves with a white waxy fluff so you could spray every day and not kill them.

I recommend that you find out if this root mealy bug which I have never managed to control. They live in the roots and when it gets crowded they move upstairs to the branches and leaves. Take the plant out of the pot and look for white fluffy masses on the roots.

If it is root mealy then throw the plant out. If the roots are clean then cut the plant back so it is just branches and stems with no leaves. Then apply your insecticide at five day intervals. You get better coverage by wetting a paper towel or rag with the solution and wiping the stems so you crush the bugs and remove them.

I also recommend trying 25 ml dish soap mixed in 1 liter of water as this also an effective contact insecticide.

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  • Thanks for the response! It's possible that I've got it wrong. It's the brown bumps which I am referring too. The white stuff (hairs or crystals) seems to be something which they have excreted as it only covers the areas where they have invested. I can describe the brown bumps (hundreds of them) as little flat bugs which stick themselves to the plant, but I have seen them walking around (especially if I disturb them) and they have legs. I am adding another photo where you can see the legs on one of them. – Dave New Feb 12 at 13:34
  • If you have live bugs on the plant after using that oil spray then it's not working. Cut it back to the main branches, bag the clippings so the bugs don't move to something else. Wipe every surface with a rag or cloth, repeat at five day intervals. Isolate this plant so what's on it does not spread – kevinskio Feb 12 at 14:26
  • Found out what they are - cottony cushion scale (pestnet.org/fact_sheets/cottony_cushion_scale_343.htm). Quite different from other scale as they actually keep their legs. I have now cut the plant right back and will follow your suggestions – Dave New Feb 15 at 10:01
  • @DaveNew Good luck, be patient... – kevinskio Feb 15 at 12:27

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