Introduction

This article is long overdue, and I hope it provides some value to anyone struggling with pests in their cactus collection.

Caring for cacti is a straightforward process if you keep the basics in check. Problems can arise when pests decide that your beautiful plant will become their next meal.

The most common pests and annoying insects are:

Root Mealies / Mealybugs
Spider Mites
Thrips
Fungus Gnats
Scale Insects
Slugs and Snails

I will deep dive into each pest and give potential treatments / preventative measures against them.

Note: I have included links to some products, if you click through and purchase, I will get a small commission for the sale.

Mealybugs

Root Mealies / Mealybugs

Mealy Bugs are probably the most common of pests for cactus growers. These little white buggers can wreak havoc if left untreated and can be very hard to eradicate, prevention is better than cure when it comes to mealybugs. Some people recommend throwing away the whole plant when mealybugs are spotted, obviously, this is not an option for cactus growers (unless you like wasting money and valuable plants).

Meely Bugs, a specific type of fluffy lice, are small and white in colour and mostly attack root systems (Root Mealies) but can also attack the cactus bodies. Plants with a mealybug infestation will generally suffer from poor overall health and can have secondary infections that are the result of open wounds left by the mealybugs.

Root Mealies are where the most amount of damage occurs, in the root system. They are extremely hard to spot and one of the most obvious giveaways of a root mealybug infestation is the presence of a white almost web like fluff in the roots. If you see one freely moving around mealybug, chances are extremely high that there are more below the surface of the substrate.

Once they are set in, they can jump between plants very quickly. All infected plants should be isolated straight away for treatment and to avoid cross contamination.

Treatment / Prevention

The eggs are the hardest to deal with as they have this cotton-like layer of protection which normally means that multiple applications of treatment will need to take place.

For mealybugs that are on the column of the cactus moving around freely, you can use a qtip saturated in either 70% isopropyl alcohol or 3% Hydrogen peroxide (H2o2) and kill each mealybug individually.

Now, for root mealies. You have some options in terms of treatment, Organic vs Chemical treatment.

Organic (Biological)

Neem Oil: Need oil is an effective organic treatment against a wide variety of pests, including mealybugs. Multiple treatments may be required and will not be effective for big infestations.

Mealybug Predators: One of the most effective organic treatments for Mealybugs is to introduce predators into the ecosystem, these predators are known for eating both mealybugs are their lavae. The most common would be:

Soapy Wash: Another effective root treatment is to do a H2o2 / dish soap root drench. Fill a tub with enough 3% h2o2 to full cover the plant root mass and add a teaspoon of dish soap. Fully drench the root mass for 10 to 15 minutes.

Chemical

In the UK, most harsh (extremely effective) pesticides have been banned like Imidacloprid. But there are some effective systemics like Resolva Bug Killer which will eradicate mealybugs quickly, just be mindful that these are systemics and can stay inside the plant for months. Due to the egg sacks being near indestructible, multiple treatments will be required.

Final Note

Prevention is better than cure when it comes to Mealybugs, there are a few precautions you can take to prevent outbreaks and they include:

Isolate all new plants to monitor for infestations.
Repot EVERYTHING, this allows you to inspect the roots for any root mealies, you will need a 10x to 45x magnifying glass as they are near impossible to spot with naked eye.

If any are noticed on the plant, isolate it immediately for treatment.

Red Spider Mites

Spider mites and red spiders on your cacti are typically from the Acari family Tetranychidae. These are usually less than 1mm in size and can cause extreme problems, especially when plants are being grown indoors. There are various Tetranychidae species and some of them can spin very fine webs like spider webs. Due to the small size of spider mites, these fine webs can sometimes be the only thing alerting growers to the presence of this pest.

Indoor plants can suffer badly from spider mites, simply because the plants are not exposed to the elements. Rain, humidity, strong winds and cold weather can greatly reduce spider mite populations. Over-fertilized plants are most vulnerable to spider mite damage and reducing the nutrients can already be effective.

In nature, the cold temperatures of winter reduce mite populations before they can turn into a severe problem. When growing plants in a heated greenhouse, this natural process of spider mite elimination does not take place. Due to the constantly warm climate and dry air of a heated greenhouse, pest populations can get out of control very fast. This is especially so with spider mites. If spider mites find themselves in ideal conditions, they can produce many generations of offspring in a very short time and cause substantial damage to the infested plants. Typical spider mite damage changes the cacti´s green skin colour to a greyish brown. Spider mites suck chlorophyll out of cactus skin, which causes lots of tiny wounds and secondary infections. Spider mite damage often goes hand in hand with orange rot or other fungal infections, and these conditions frequently confused with one another.

Final Note: If you can see red spider looking bugs with normal sight, these are NOT red spider mites and are actually predator mites that consume red spider mites. If you have them, keep them as they will look after your collection. Once they have eradicated all the existing red spider mites, they will turn on each other.

Prevention

Amblyseius californicus – These are predator mites and are very effective at controlling spider mite infestations as well as a preventative measure to avoid any infestations.

Treatment

Garlic extract is very effective at keeping unwanted pests away and is a great general pest repellent.
3% H2O2 (Hydrogen Peroxide) – You can put the H2O2 in a spray bottle and spray the affected areas every few days, this will greatly reduce numbers and keep the infestation under control.
Spidex Boost – These are Phytoseiulus persimilis predatory mites for the control of Two Spotted Mites, commonly known as the Red Spider Mite. The predators will feed on the Two Spotted Mites, especially at the egg stage of which they pierce and consume the contents.
Phytoseiulus persimilis – Phytoseiulus persimilis is a predatory mite used to kill the Red Spider Mite, and is particularly effective in tackling severe infestations and treating Spider Mite hotspots.

Phytoseiulus predatory mites are the most commonly used form of biological pest control against the Red Spider Mite in greenhouses and protected growing environments. These predatory mites feed on all stages of the Spider Mite’s life cycle, from egg to adult. The life cycle of the Phytoseiulus is faster than that of the mites they are feeding on, and in suitable conditions they are able to double their population every two days. During this time, they are capable of consuming five adult Spider Mites and 20 Spider Mite larvae each day. This leads to a rapid reduction in the Spider Mite population and helps prevent plant damage.

Note: I will not be recommending any chemical treatments as the biological treatments are effective.

Final Note

Spider mites are nasty and if left untreated, can really destroy all your plants. Keep on top of inspections and act quickly if you spot anything funky. If you introduce predator mites effectively, they will form a colony and will continue to do so till there are no spider mites left. At that point, the predator mites will then turn on each other and kill themselves off. It’s a win-win.

Thrips

Thrips are the devil and will wreck any garden if quick action is not taken to treat and get numbers under control. They can get to around 2mm in size, they are small but deadly for cacti and often target new tip growth which is a cactus growers’ nightmare.

Being so small, they are easy to miss, and it is normally only once the scarring has happened, is it obvious you have a problem on your hands. Thrips also look different during their different stages of growth; it is a good idea to familiarise yourself with the look at each stage so that you can spot them when needed.

Prevention

Prevention is tricky, the only tangible way to prevent thrip infestations is to isolate all purchased plants to monitor them for any signs of thrips. You will need a magnifying glass.

Treatment

Thrips are notoriously hard to control and even harder to eradicate. If you see one of your plants with thrips, isolate it immediately to get started on treatment. Systemics like Imidacloprid are very effective against thrips, however, it is banned in the UK / EU due to its harmful effects on bees and wildlife.

Spinosad can also be used to treat a thrip infestation, multiple applications will be required.
Ethanol can also be used to treat the infected areas, again, multiple applications will be required with close monitoring. Neem oil can be used, it will kill thrips but may also burn your plants and should never be applied during sunny periods.

Final Note

Thrips are the hardest infestation to get rid of and treat, be vigilant with new plants entering your growing space. I cannot state this enough, isolate ALL NEW plants for a while so they can be monitored for any infestations. Most pests get introduced by purchased plants being brought in.

Fungus Gnats

Fungus gnats (Sciarid Fly) are one of the most common pests for cactus growers and pose most of the risk to seedlings, adults are largely unaffected by fungus gnats. They are annoying as all hell, too.

The larvae can eat young seedling root systems and can stunt or kill younglings. They are so aggressive, they could destroy a full tub of seedlings overnight.

Prevention

Heavy mineral (inorganic) soil mixes are one of the best preventative measures you can take against gnats as they dry out a lot quicker and gnats don’t do very well in dry conditions. That is why gnats are far less of a problem during Summer than early Spring.

Fungus gnat larvae are tiny transparent worms that crawl around on top of the soil. Adult fungus gnats are little black flies that can be confused with fruit flies. Many cactus breeders use glue traps to reduce fungus gnat populations.

Treatment

Sticky Fly Traps – these traps work well, albeit a little cruel. These are effective at keeping populations at bay and only affect adult fungus gnats.
Tanlin – Organic pest control that kills fungus gnat larvae when consumed.
Lava-Lite No-Gnats – This has been very effective for me with Tanlin drops. Being an inorganic lava rock, it creates a barrier between the soil an air and effectively inhibits the ability for gnats to lay eggs, breaking the reproductive cycle.
There is a domestic fly spray called “Raid” that kills all of the adults in minutes. It’s available at Tesco’s and Wilkinson’s.

Final Word

Although not one of the most dangerous pests out there, they can wreck a seedling tub quickly if left unchecked. Prevention is best in this case and keeping a nice inorganic substrate mix is the first step to being fungus gnat free. I highly recommend all 3 treatments mentioned above, the sticky fly traps can be in the grow area year-round and the Lava-Lite No-Gnats is a very good top layer solution for seedlings.

Scale Insects

Scale insects or scale is a catchall term for a group of insects that are very common in cactus culture. Scale insects are round and look like little brown spots on your plants. Some species of scale have a protective layer that can shield them from pesticides and scale often scars plants permanently.

Many plants on the commercial market suffer from scale, and cactus growers may fail to recognize them as an infestation because there are so many varied species. Even experienced cultivators have trouble recognizing scale. They can look very similar to scars or blemishes, especially because they cause constant scarring.

Distinguishing varied species of scale is rather difficult. In the grand scheme of cactus culture, scale species does not really matter because all scale causes similar damage and hence require extermination. To be effective, scale treatments require numerous applications, and a systemic pesticide is usually necessary. Scale causes extreme scars that can kill a plant via secondary infections.

Treatment

Personally, I like to use regular ethanol treatments for scale population control, followed by use of a pesticide. Ethanol treatments are not as effective as a systemic pesticide, but if done repeatedly, often kill the current generation of scale insects. Methanol and ethanol treatments have a slightly lower risk of causing scars and chemical burns than oil-based pesticides like neem oil. Systemic pesticides like imidacloprid (banned) or other neonicotinoids (most likely banned) are effective. Spinosad can be used to treat scale as well.

Final Word

You might be noticing a trend here, go and buy some Ethanol and Spinosad as they treat a wide range of pests.

Slugs

Slugs & Snails

These slimy buggers are a right nuisance and for whatever reason, they really like munching on your brand new, beautiful tip growth. Slugs are hard to find during the day as they tend to come out for snack time late at night when its wetter. If you walk into your greenhouse at the right time, you can hear them munching on your babies, brutal stuff.

If they are left to do what they want, they can cause enough tip damage to abort the tip which would cause that tip to stop growing and could even damage the vascular bundle to the point where it won even pup.

Prevention

Most people would recommend salt for slugs, although it is effective, its not good for plants etc.

Copper, copper and more copper. Slugs do not like copper, best to have a fair amount of it about. You can buy copper tape which you can tape around the top of your pots which creates a barrier and should stop them from crossing it.

I use copper pipping that I have pressed down so that it is a flat pipe and I lined the inside of my greenhouse with it, fingers crosse it works, I will update here with effectiveness.

Treatment

A free and effective way to prevent snails and slugs is to have all your plants off the ground, even if it’s a few inches. This won’t stop them entirely but will greatly reduce incidents.
Slug & Snail Killer Pellets – This uses ferric phosphate which is touted as a safer alternative to Metaldehyde which has been banned (good) in the UK due to its toxicity to wildlife and pets. Its takes longer to kill the slugs but is far less toxic to animals and humans. Be careful, in enough quantity, it is still toxic to pets. There is also a question of slug treatment as they take quite a while to die.
Beer Traps – These traps are quite effective but hit and miss. You just add beer, the slugs / snails are attracted to it and fall into the trap trying to drink the beer.
Copper Tape – Apparently, copper is toxic to them and gives them a shock. I am not quite convinced as I have seen slugs inside copper rings I’ve created, the slugs may have been inside the ring already. I have lined all my pots and trays with copper tape.
Go out late at night with a torch and manually remove and destroy them. You can be virtually slug and snail free in just a few nightly visits. Be aware, you can literally hear them eating your plants, not the greatest sound in the world.

Final Word

Slugs & snails can really cause a lot of damage to cacti (plants in general, they got to eat) and preventative measures need to be taken to keep these slimy douchebags out of your grow area. I wouldn’t recommend salt for anything other than killing slugs on the spot, salt is not good for cacti so avoid getting it into the substrate.

Conclusion

Pests, both annoying and harmful are just a part of having a plant collection. Just to remind you, the best thing you can do is isolate any new plants that you receive so that you can monitor them for anything funky that can potentially infect the rest of your collection. As a rule, repot everything you receive into new substrate, this allows you to have a look at the roots too.

Just a few small steps can make the world of difference.

Some items I always have a steady supply of:

Hydrogen peroxide (H2O2) 3%
Resolva Bug Killer
70% ISO Propyl Alcohol

That’s it, I hope you got value out of this. As always, feel free to reach out to me if you have any questions.

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