Monday, January 16, 2012

Common House Plant Pests Part 1 - Thrips

If you own house plants, you've probably had many of these critters invade your home without even knowing it.

Today, I watered my house plants, and paying attention to some signs, I knew I had an unwanted invader.

I found this leaf damage on my Prayer Plant (Maranta leuconeura).

Now for some of you, you've probably seen various forms of this sort of browning (the loss of green colour, leaving a blotchy mark on your plants) without ever doing anything about it. However, my gut told me to examine things more thoroughly.

During the winter, lack of humidity causes similar effects. Although, loss of humidity usually shows up on the tip or the base of the leaves.

Here, the damage is toward the side and in the middle of the leaf. One thing plant lovers begin to learn, is to see the damage on both sides of the leaf. It helps determine the source of problem.


As I suspected, the damage was coming from beneath the leaf, which then allows me to find other evidence of damage.

If you take a moment to enlarge the above and below photo (both viewing the underside of the leaf), you'll also see small pin prick sized areas of discolouration. And if one looks even closer, you'll see the bug itself. Can you see it?


Forgive the over exposure of this shot, but there, towards the bottom of the leaf, along the stem, to the right of my thumbnail, can you see a dark, sliver of something?  This is an adult insect, known as a Thrip.

Thrips are aggressive sap suckers. Having small mouth parts that prick the under-surface of a leaf, they individually suck sap from leaf cells, causing this decolouration and visible damage.  You can see similar damage at the base of the leaf, near the stem where this Thrip fed as well.  A lot of damage for one small insect. As the leaf ages and gets tougher at the base, the thrip moved along the leaf's edge to more tender tissue. Unfortunately, when you see one, you will find others on the plant. Others of varying ages too. Eggs, hatched nymphs and adults. One adult can do a lot of damage.

Not to fret if you have this insect on your house plants. This isn't the first time for me, nor will it be the last. I work in areas where I can easily bring this insect home on my clothes or other house plants. Sometimes, you can also bring this insect home with the plant themselves. Thrips are slender and move into crevices easily. Some species can fly. I'm most likely the culprit, I probably brought it home, not even knowing it. :(

I find the best way to combat this bug, is simply to use your fingers and water. I'm so used to using non-chemical ways to kill off invaders, but you certainly can use over the counter products available to you at the local garden centre. Some of you might be saying...eww.  So be it. I rather do it this way.

First, I examine and thoroughly look under each leaf, its stem and on any new forming leaves and squish any signs of insects (eggs, adults). With your fingers you can feel grit and their remnants. I then take the plant to a large sink and rinse the leaves and stems as much as I can with water, making sure each leaf is clean by the eye and by feel. It's an arduous task, but worth the results.

Unfortunately, this control is not a one time deal. I have developed patience and perseverance in examining plants for several days, until I see no evidence of a pests presence. It's rather time consuming, but I find I don't have to worry about misapplications of pesticides and since using chemicals indoors don't appeal to my health consciousness, I avoid it as much as I can. When certain leaves are more damaged than others, I also prune them off the plant entirely. Having damaged leaves is unattractive anyway, and their removal helps reduce insect populations - it's rather effective to prune away anything that is disfigured. One hint though: you must not leave bits of leaf remnants. These are fabulous hiding grounds for crawler stages of insects. I prune the leaf as far back to where it's attached, and remove any leafy bits that would be areas to look out for down the road.

If you have infections on numerous house plants, the above treatment would be too difficult to do. There are effective, more natural products out there, like EndAll and Safers Insecticidal Soaps. However, one application is not enough and Thrips hide and move about quickly when leaves are disturbed. For the most part, the application can miss spots and well, rendering the application useless. Spraying indoors is messy and the vapor off of these sprays is awful.  If you're desperate to save the plants, use my squishing and rinsing method, after spray applications (1 week or so later).  Two methods are better employed to be thorough. Although, if the infestation is severe, sometimes it is better to toss the plant out rather than to waste efforts in battling something too hard to rid of.

Two website links I would like to share, and will share often are: 



Each define, describe and give great samples of preventative measures, how to recognize plant damage, pointing to pest problems, and varied methods to control household invaders. I especially like the Colorado State University website for their photographs and warnings regarding chemical controls.

As the weeks progress, and as I examine my plants regularly, I will post more problems for sure...

As they say, "An ounce of prevention is worth a pound of cure".  I'll keep you posted!

5 comments:

  1. I also have a mini garden of my own here at the balcony of my apartment. It is amazing how you can have something like it in your place of zen. But you can never avoid these irritating pests.

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    1. Hi Eliza - agreed! I figure pests and disease go hand in hand with anything natural. We just need to be wise and look for the problems before they become night-mares. Have a great upcoming growing season!

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  2. Hi Heidi - Wow, this is the best information I've found about thrips on houseplants. Thank you! I never had them until I bought some vase-shaped sansavieria from a big indoor nursery. I thought tSansavieria would be immune to almost anything. Instead I found odd raised white strips like scars. I did the shake-off on white paper method - and they were black thrips. When I told the garden store where I'd bought the plants - it took a year to see this damage - they said, yes, we discovered we had a thrip problem. After doing everything you say - including using a granular systemic insecticide - (and throwing away plants too), I thought I was through with them. I bought blue indicator sticky-traps also. After about a year, I suddenly discover my walking iris plants (Neomarica gracilis) are COVERED with them - in every stage of development. I worked like a dog to get rid of them again. Threw the worse cases aweay. A year later - I have them again on the walking Irises! I can't fingure out why this keeps happening! I never had any problem with either hosueplant species before. Any idea why I can't seem to get rid of them no matter what? Thanks - Jud

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    1. Hi Jud, I feel your pain. They are the worst! In my research, thrips can reproduce asexually, so if you even leave one...they can multiply! It's also known to prevent flowers from blooming, as the thrips feed on pollen. Others feed on fungus too. Try ridding pollen and fungus as best you can, whilst doing what you have with safers soap. Another natural control is Thripex, a preditory mite that eats thrip larvae. I've not tried it myself, but it wouldn't hurt to try. Frustrating isn't it! Keep fighting the good fight! Good luck!

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  3. By the way - I only used the granular systemic insecticide after washing and safer insectical soap repeatedly applied didn't work.

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