Pests On Orchid

by Ray
(Nottingham, UK)

I have a lovely orchid which has flowered for the last two years and now that the last flower has dropped off I have noticed small black flies hovering at the base of the leaves. The plant looks healthy enough and what looks like a new flower spike is shooting up from the upper part of the stalk and is a darker green than the roots.
Is there a pesticide I can use that will not harm my orchid?.
I have left the old flower stem and cut it back to the first nodes in the hope that it will flower for a third year,but should I let this new growth carry on growing.
Yours in anticipation,

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Oct 26, 2011
Your Pests are Fungus Gnats
by: Melissa

Pests On Orchid

by Ray
(Nottingham, UK)

Hi Ray,

Thanks for your questions. The one about fungus gnats is a really great topic and one that has not been brought up before in this forum. Thanks for that.

The small black flies hovering at the base of the leaves are called fungus gnats.

They indicate a couple of things. Perhaps that your media is a little too wet and that the media may be breaking down somewhat providing a food source for the larvae of the fungus gnats. The adult flyers are not of any real consequence except as an annoyance. But they are an indication that some other things might need to be addressed. There are a couple of ways to deal with them, but prevention of what is encouraging them is the best. The larvae can weaken the plant some, the flyers just let you know the larvae are there.

You can buy some yellow sticky traps that you hand near your plants and the adult flyers with get stuck to the paper and that helps reduce the population. I have done this and it helps.

You can also make these traps yourself by getting yellow card stock and putting a thin layer of vaseline on it and using it in the same fashion.

You can also sprinkle the media of the plant with cinnamon (yes the kitchen spice). This acts as a fungicide for the plant and reduces what the fungus gnat larvae have to eat. I have not done this but I have heard that it worked well. You can also use BT to kill the larvae but I would try the other things first or they will just come back. You need to treat the sourse, not just the symptom.

You might consider repotting when your plant is not in spike or bloom. If you do, discard all the old media and wash your pot in dilute warm soapy water and scrub it some and rinse well. Or you could wash it in a very dilute bleach (less than 10%) solution. Again rinse well.

It is great that your plant is healthy it sounds like it is spiking. Although I am not sure what you mean by "shooting up from the upper part of the stalk". New flower spikes usually come out from the side of the crown of the plant and the end of the should look a little like a mitten before it starts differentiating into nodes and buds.

You say that you have left the old flower stem and cut it back to the first nodes in the hope that it will flower for a third year. So does this mean that you have a spike from the crown and have left the old stem expecting a spike from the node that you left?

If this is the case and you for sure have a new spike you could leave both. Or you could cut the old flower spike about a centemeter or so above the crown and let the new spike be the the flower spike. Leaving both is a little harder on the plant but might be interesting if the plant is healthy. But make sure you have a new spike before removing the old one.

Send a picture if you have more questions about this part.

Hope this is helpful.


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