At some point in your life, even if you only ever have just one houseplant at a time, you will have to deal with houseplant pests. They live all around you, seeking out the ideal conditions to reproduce. They can arrive in potting mix, in new plants, on your dog, through the open window or doors, or even on your clothes when you come in from the garden. Everywhere has houseplant pests just busting to feast on your tender, juicy, precious plants.
To avoid becoming a planto-chondriac, it helps to know what to look for when it comes to houseplant pests. Improve your houseplant pest understanding and you can remain calm when you see something odd going on with your plant.
Don’t panic about every single leaf – look to the health and vigour of the whole plant. Plants do shed leaves on a regular basis so get used to that. It doesn’t always mean something is horribly wrong with your plant. A plant shedding a single lower leaf is standard practice – that’s an old leaf and is due to retire at some time. If more than ¼ of the leaves are looking crook, then it’s time to investigate what’s going on.
Don’t Panic.
Houseplant pests are best defeated early and often – pay frequent attention to your plants so you can observe and notice a pest early on – before it damages your plant and the infestation becomes hard to control.
When you water the plant, check 3 places:
- Underneath the leaf
- The centre of the plant
- The potting mix
They are the 3 main locations where pests get their start in life.
Spider mite
What is it?
Red spider mite AKA two spotted spider mite is a common houseplant pest. It is in the same family as spiders, so has 8 legs. They lay small, spherical, initially transparent eggs. For much of its life, the spider mite is yellow or pale green. It turns red as the weather cools. They are tiny, not at all easy to spot against a green leaf. Look for two dark dots which are at the front end of the bug.
What does it do?
It hides on the underside of leaves and feeds by sucking the sap of the plant. They prefer the edge of soft leaved plants like Alocasias and Syngoniums. This causes the surface of the leaf to go pale and mottled. Leaves attacked by spider mites will yellow, discolour and drop off.
You will also see faint spider webbing which the mite creates for its own protection. Mild infestations won’t cause long-term damage but heavy infestations can weaken the plant, causing the leaves to drop.
If left unmanaged, the plant will die.
What are the ideal conditions?
Spider mites thrive in warm, dust-dry conditions where plants are already stressed. They breed in temperatures over 10°C and thrive at 27°C. It takes 55 days to go from egg to adult and as conditions become more ideal for them, that will go down to 12 days. One female can lay 20 eggs a day and will live up to 4 weeks, laying every day. That is 560 eggs.
Spider mites aren’t too fussy about their preferred plant, and will have a go at anything that is dry, dusty and under stress. Once the temperatures drop, the females go hide somewhere dry and sheltered to wait out the winter. This could be in the crevice of a leaf, thick foliage, under the rim of a pot.
How can I avoid it?
Keep actively growing plants properly watered so they’re never struggling. Spider mites thrive on weakened, dry plants. Make sure there is plenty of airflow around your plants. Regularly wipe down both surfaces of the leaves of the plants with a damp, soft cloth. This way you will spot the webbing first, then maybe the bugs.
Spider mites do not like high humidity. Pay frequent attention to your plants so you can observe and notice a pest early on – before it damages your plant and the infestation becomes hard to control.
Don’t Panic. Pull out your magnifying glass and have a close look at that webbing the insects it hides. When you water the plant, check 3 places:
- Underneath the leaf
- The centre of the plant
- The potting mix
How can I get rid of it?
Wipe away the webbing. Wipe away and squish any bugs you see. Water can blast off mites and their eggs and discourage further breeding. Water the plant and wash it down in the shower or with a soft watering sprayer – make sure you get into all the surfaces and stems of all the plant. Repeat in a week’s time to break the breeding cycle. If there is a massive infestation on just one leaf, cut it off, bag and bin the leaf.
If the whole plant is covered in mites, wash and water as above and place the plant into a plastic bag to keep the moisture in.
If your whole green house is infested, investigate the use of Phytoseiulus persimilis which is a predator of the spider mite.
I prefer to avoid the use of broad-spectrum insecticides on my houseplants. If you do want to go there, isolate your infested plant, and apply a pyrethrum spray, or pest oil as per the instructions on the bottle. Be aware that these sprays will also kill beneficial insects living in your plant.
Another thing with pest sprays – as the spider mite reproduces so rapidly it is able to adapt and resist pesticides. Prolonged use of sprays become ineffective. These pests are wide spread and very common, in houseplants, veggie gardens and orchards.
Ensuring your plants are not allowed to become dust-dry and stressed is the most effective way to manage the red spider mite.
Mealy bugs
What is it?
A tiny white or grey oval shaped ball of fluff that squishes easily. In heavy infestations they pile up to form a thick mat. The egg mass looks like a fluffy white cloud.
It’s a scale insect, without external protective armour. They excrete honeydew – a sticky, sugar rich substance which comes out the bum of the bug as food goes in the mouth. Ants love to eat honeydew. Ants are known to farm the mealy bugs, moving them around the plant so the mealy bug has something new and fresh to eat. Ants harvest the mealy bug poo. Nice.
The lifecycle of the mealy bug: The longtailed mealy bugs produces live young. The others lay eggs. The young go through several nymph stages before maturity. Depending on temperature and humidity the immature crawlers mature in about 6 weeks to 2 months. Mature females die after laying eggs. Reproduction is more rapid with higher nitrogen levels of the host plant. Males have wings and do not cause plant damage because they don’t have mouth parts. That’s right – they can’t eat, just mate and die in 2 days. Nice.
What does it do?
Mealy bugs are parasitic insects that feed by sucking the sap of the plant. They move around slowly on their own, but when ants get involved, they will be moved around the plant to new, fresh tasty parts of your plant. Honeydew encourages the growth of sooty mould. Mealy bugs cause stunting, leaf yellowing and distortion of the leaf.
What are the ideal conditions?
Mealy bugs proliferate in moist, warm conditions with little air flow where the plant is not disturbed. Pay frequent attention to your plants so you can observe and notice the mealy bugs early on – before it damages your plant and the infestation becomes hard to control.
How can I avoid it?
Yes, you want to grow tropical plants that need moist, warm conditions – so ensure there is plenty of air flow around your plants. Pay frequent attention to your plants so you can observe and notice the mealy bugs early on – before it damages your plant and the infestation becomes hard to control.
Don’t Panic.
When you water the plant, check 3 places:
- Underneath the leaf
- The centre of the plant
- The join where the leaf stem (petiole) meets the main stem
They are the 3 main locations where these pests live and eat.
How can I get rid of it?
I have had success with a cotton bud dipped in methylated spirits and dabbing it on top of the fluffy white bugs and their egg clusters. This will dry their outer protective coating and they will die. Horribly. This may help you to feel better. For a heavy infestation you can use a 50/50 metho and water solution in a pray bottle and apply it to the plant. Wait a few minutes until you see the bugs die, then shower the metho off the plant.
Ladybirds are brilliant at controlling mealy bugs and work best in outdoor situations. Cryptolaemus (Cryptolaemus montrouzieri) are Australian native ladybird beetles. They are very efficient predators of many species of mealybugs. Cryptolaemus are recognised worldwide as effective biocontrol agents of these pests and they have been exported to many other countries. The adult Ladybird beetle and the larvae feed on juvenile stages of mealy bugs. The large Ladybird beetle larvae also eat adult mealy bugs.
Scale
What is it?
A sucking insect related to mealy bugs. – it pierces the plant and sucks out the sap. Females have a soft body and no limbs. They live under a domed scale and exude a waxy coating for protection. Males have legs, some species have wings, too and look like small flies.
Like the mealy bug, the scale insect exudes honeydew – the sticky substance pumped out the rear as the plant sap gets sucked up the mouth. Ants also farm and protect scale insects living off the honeydew. Sooty mould grows on the honeydew which also has a detrimental effect on the plant.
Newly hatched juveniles crawl about the plant. After mating, the mature female scale insect, having no limbs, stays in one place and exudes the waxy scale cover for protection. The adult males die after mating.
What does it do?
It pierces the plant and sucks out the sap. It exudes honeydew which allows for sooty mould to grow, which also reduces the vigour of the plant. Scale insects cause stunting, leaf yellowing and distortion of the leaf.
What are the ideal conditions?
Scale insects proliferate in moist, warm conditions with little air flow where the plant is not disturbed. Pay frequent attention to your plants so you can observe and notice the mealy bugs early on – before it damages your plant and the infestation becomes hard to control.
How can I avoid it?
Yes, you want to grow tropical plants that need moist, warm conditions which is ideal for scale insects. Ensure there is plenty of air flow around your plants. Pay frequent attention to your plants so you can observe and notice the scale insects early on – before it damages your plant and the infestation becomes hard to control.
Don’t Panic.
When you water the plant, check 3 places:
- Underneath the leaf
- The stems of the plant
- The petioles (leaf stem) of the plant
They are the 3 main locations where these pests live and eat.
How can I get rid of it?
Insecticide sprays are only effect at the first, crawler stage of the life of the juvenile scale insect. The scale stage can be controlled with horticultural oil that suffocates the insect. It is only effective if it actually covers the scale. It has no residual affect. Mix the oil with water as per the directions on the bottle, and apply with a spray bottle all over the plant, making sure it gets down into all the nooks and crannies. Start by testing the solution on a single leaf, as some plants react badly to oil.
Ladybirds are brilliant and work best in outdoor situations. Cryptolaemus (Cryptolaemus montrouzieri) are Australian native ladybird beetles. They are very efficient predators of many species of scale insects. Cryptolaemus are recognised worldwide as effective biocontrol agents of these pests and they have been exported to many other countries. The adult Ladybird beetle and the larvae feed on juvenile stages of scale insects.
Fungus gnats
What is it?
Fungus gnats are the most annoying of houseplant pests. They can easily zoom straight up your nose as they are attracted to carbon dioxide. They are small, flying insects that hover around the surface of potting mix.
What does it do?
Apart from the annoying adults that zoom into your face, the larvae live in potting mix with high organic matter and feed off fungus which grows when potting mix is kept too wet. The larvae eat the new, tender roots of your houseplant, causing stunted growth. Kill them.
What are the ideal conditions?
Moist, warm soil with high organic matter and low aeration – conditions that encourage the formation of fungus.
How can I avoid it?
Use potting with plenty of space between the particles – a good, chunky Aroid mix is ideal – lots of airflow in the mix reduces the chance for fungus to grow. Use potting mix with low organic matter that doesn’t rot quickly – reduces the chance for fungus to grow. Commercially made potting mixes will almost certainly have fungus gnats. Once I stopped buying that and made my own I radically reduced the number of gnats. I also repot into my own mix every new plant that comes home with me, regardless of the seller.
I prefer to make my own mix with orchid bark, coconut husk chunks, perlite, tree fern fibre. There is no organic matter in there that will grow fungus to feed the larvae, and the chunky nature of the mix will allow for plenty of air in the mix. As there isn’t any matter than can easily rot there isn’t much in the way of nutrients to feed the plant. Be sure to use a quality liquid feed for your plants.
How can I get rid of it?
I’m not a fan of this method: dry the potting mix out to break the breeding cycle. The juveniles can’t survive in the potting mix without water, but a dry mix won’t benefit most of your plants.
This is slightly better: for a heavy infestation, or if you are really annoyed, make up a drench of one part hydrogen peroxide to four parts water to kill both the gnats and their eggs. Pour it through the potting mix until the water pours out the bottom of the pot.
Exclusion: I have tried a physical barrier product called gnat barrier. It’s a fine grit that is spread thickly over the top of potting mix. It stops the female burrowing down to lay eggs, and stops the juvenile reaching the surface. It worked ok, but once you want to repot the plant, all that gnat barrier just gets mixed up into the potting mix and the surface protection is gone. Expensive technique.
Better: changing the potting mix to a less fungus-inducing type worked the best for me, along with sticky traps.
Image: Houseplant Jungle 2023
Gnat traps are yellow and sticky on both sides. You can hang it from the plant or sit in the pot. Any flying insect will be attracted to the yellow and when they get close will get stuck to it and die. We sell them in our online shop – click here.
