While all succulents and cacti have different temperature tolerances, this guide will serve as a starting point from which to establish your own environment-specific temperature exposure practices.
As is the case with sunlight exposure, succulents and cacti prefer to live in the "sweet spot" when it come to temperature. Both ends of the spectrum, hot and cold, can cause irreversible damage to these resilient plants, so it's just a matter of paying attention to their condition and modifying the environment as needed.
According to Wikipedia there are over 8000 species of scale insects. The most likely species to be found on cacti are those in the Diaspididae family which is also known as "armoured scale". This name comes from the scale-like protective covering that these pests hide under. The attachment to the plant tissue is surprisingly strong and when a scale insect is removed, it leaves a little scar behind where it was attached and sucking the plant juices. One small scale insect could hardly affect a cactus that has passed the seedling stage. However, scale insects multiply quickly and can completely cover the surface of a cactus in just a few days. This gives the entire plant a fuzzy or frosted appearance. Scale seem to prefer shade may occupy every open space on the shady side of the plant and leave the sunny side untouched. Cacti with dense spination seems to provide sufficient shade for the scale and they will cover all parts of these plants.
Left untreated, scale will spread to other nearby succulent plants, but they seem to prefer certain species over others. Scale will often be found only on tender new growth and not elsewhere on a succulent or cactus.
Treatment of Scale:
The first thing to do with a plant that is infected with scale is to physically remove the scale from the plant. This can be done using your fingernail or a tweezers, but the most efficient method is with a spray nozzle on your garden hose. There is a good deal of satisfaction to blasting the little "blood-suckers" off the infected plant, but be sure that your water jet isn't strong enough to actually damage the plant. On cactus, also take care to get down in the cracks and under the spines.
Once you have removed them in this way, you can be sure that they make their way back. To prevent repeated outbreaks, you should treat your plant with a systemic insecticide. This is a poison that the plant will take in through the roots and into the plant tissue making the plant itself poisonous to the insect. For a systemic to work like this, the plant must be growing and taking in water an nutrients at the time. A good practice is to apply the systemic once at the onset of the growing season. This means you may have to keep an eye on the plant and physically remove the scale again while you wait for the systemic to take affect.
Mealybugs are the most tenacious of succulent pests. Not only are they a plague to nearly every grower, but they also affect different parts of the plant. There are mealybugs that affect the plant tissue and like to hid out in between ribs and tubercles, there are root mealybugs that live under the soil sucking on the roots, and there are even mealybugs that live on the spines of a cactus and suck the plant juices through the spines.
Treatment of Mealy Bugs:
Aside from being the most common of cacti-eating pests, mealybugs are also quite difficult to get rid of and is nearly impossible to do so without the use of a systemic pesticide. Contact insecticides will kill mealybugs, but do have to be added in fairly high concentration due to the protective covering that the insect makes for itself. Also, contact insecticides are risky due to their oily makeup. This can cause the plant epidermis to burn in the sunlight. This is called phototoxicity.
For best results, after physically removing the mealybugs with a toothbrush or high pressure water spray, it is best to treat the plant with a systemic insecticide. This should be applied only when the plant is growing or it will not be taken into the plant tissues. It is also not a bad idea to treat your plants at the onset of the growing season to prevent mealybugs from getting a start.
Since mealybugs affect different areas of the plant, unlike scale and other pests, it is always wise to check the entire plant when mealybugs are detected. In other words, if you have spine mealybugs, be sure to check down in the joints of the plant and unpot the plant and inspect the roots.
Red spider mites are not actually spiders, but they are mites. They get that name because they are red in color and they make a spider-web like silk on whatever plant they are attacking. The spider mites themselves are extremely small and depending on your eyesight, you might not be able to even see them. As real spiders often make webs on cacti, don't panic the first time you notice a silky web. However, it is worth taking a close look at your plant if you do see one.
If the web belongs to spider mites, you will see the plant turning dry-brown especially wherever there is new growth at the apex of the plant. Spider mites love the fresh new growth and will always attack the tender parts first. Eventually they will eat the entire outer layer off your cactus which will kill it if left untreated. After spider mites are killed off, the plant will continue to grow from the top and as it does the area damaged from the mites will "move down" the plant. The damaged areas will never look healthy again and you will have to wait for the plant to grow out of it. If you get a new plant that is dry-brown around the bottom, it is likely one that has survived a spider mite attack and started to grow out of it. Sometimes, this is just natural corking, however.
Treatment of Red Spider Mite:
If you even suspect spider mite damage on your succulent or cacti, quarantine all infested plants immediately. Treatment with some form is pesticide is the only cure. It is best to look for a pesticide that specifically states control of spider mites on the label. Multiple treatments will be required as the eggs will not be killed by the pesticide. Typically, reapplication will need to be done every week or so depending on temperatures. At higher temperatures, the mites reproduce faster and more frequent treatment will be necessary. Since most pesticides will create a phototoxic reaction on the plants epidermis, it is best to keep your plants out of direct sunlight after treatment for several weeks.
Because spider mites are difficult to detect until they have done a lot of damage and because of their small size, a systemic pesticide is typically not a useful treatment. Too much damage will be done before the systemic could take effect. Using a systemic as a preventative measure is a good idea, but mites are not insects and are not controlled as easily with insecticides. Finally, it is also a good idea to spray the areas surrounding the infected plant and to pay close attention to uninfected plants. The chances are good that the spider mites have been on the move before you noticed them.
Slugs and snails are soft-bodied mollusks called gastropods. If conditions are optimal, they can exist in your garden in very large numbers. It is rare to have slugs or snails indoors. Because they are prone to drying out, they wait until night to come out of hiding. As they move along the ground or on your cacti, they leave a tell-tale slime trail behind them. In the daytime, this is usually dries out and glitters in light. Even though these animals are very soft, they can crawl unharmed right across the top of the spines on your cactus to get to the part they want to eat.
Snails and slugs prefer the softer new growth of cacti and seldom bother with the older growth. On the cactus stems, they scrape off the top layer of the tissue as they go. This leaves behind a rounded patch of exposed tissue that quickly scabs over. Flower buds are particularly enjoyed by snails and slugs and they will almost always eat the flower buds before eating on the stem at all.
Treatment of Slugs & Snails:
If only a plant or two is being munched on by nails or slugs, you can most likely find them and squish them. On potted plants, lift the pots and check underneath them. Any cool/shady spot is a good hiding spot and they don't seem to go too far from where they had their last meal. Going out at night with a flash light, is another way to physically eradicate these pests.
On a larger scale, particularly with plants in the ground, snail/slug bait seems to work best. Using a saucer of beer may kill some, but is not terribly effective. Snails and slugs are repelled by copper and could be kept off with strips of it around the base of the plant, but obviously isn't practical or economical. On the other hand, there are liquid treatments that can be squirted on the ground in a circle surrounding each plant or pot. Snails will start to cross this and die soon afterwards. As soon as the sun comes up the slugs and snails will quickly dry up and nothing will be left but their shells.
Snails seem to be seasonal and are only a real problem when conditions are right - warm and moist. Treatment will need to be continual during these times as neighboring snails will move in to replace the last batch you killed.
There are a few moth species whose caterpillars will eat some succulent and cacti species. The most notorious of these is Cactoblastis cactorum which feeds on Opuntia species. Typically a moth will lay its egg in a protected spot somewhere on the cactus. Then, when the egg hatches, the caterpillar starts eating its way through the plant. In some cases, the caterpillar will be eating on the inside of the stem unseen by the cactus grower. When these are discovered, it is often too late.
Treatment of Worms & Caterpillars:
Since there are usually only a few caterpillars on a plant, it is easiest to simply squish them when detected. Continual inspection of your plants is important even if you've never had any of the pests listed on this page. The only good thing about cactus moths is that the caterpillars are easy to spot and kill. Of course, since they are larger, they can also eat more of your plants in less time.
Most everyone is familiar with the little green monsters known as aphids. While there are many different kinds of aphids, their overall appearance and behavior is fairly consistent. They congregate on soft plant tissue in masses and suck the plant's juices. On succulents and cacti, these are rarely found on the plant body and usually only affect the flower buds and flowers themselves. They can be found on very soft, new growth. If you have ants on your plantsi, be on the lookout for aphids as they are often brought in by the ants.
Treatment of Aphids:
Aphids are among the more easily disposed of pests and can be blasted off with a high pressure water hose. It may be necessary to do this several times. If the aphid problem persists, spraying the infected parts with soapy water will be a stronger deterrent. If this still doesn't work, then a systemic insecticide is recommended, but it will have to be taken up into the plant tissues before it is effective. Again, be sure the water pressure isn't so intense that it damages your plants.
You might end up with a mouse in your house, but rodent and bird damage is something you would usually look for on plants grown outdoors. The damage from rodents and other small animals looks just like something has been eating on your cactus. This makes a lot of sense, however, bird damage is a little less obvious. Since birds peck at things with their pointed beaks, bird damage usually looks like someone chopped up your cactus and spread it around. In some cases, especially with smaller plants, the entire cactus may disappear overnight. It may have been eaten all in one sitting or carried off. It also seems that one of these critters is content with a small nibble here and there as opposed to munching down the whole plant. Also, spines don't seem to be as much of a deterrent as would be expected.
Treatment of Rodents & Birds:
Rodents will work at night while birds will probably be doing their damage in early morning. Covering your plants with a wire mesh of some sort is an easy non-lethal method of preventing this sort of damage, but is not very attractive. For rodents, poison can be set out, but should be used with caution. Mouse traps can also get the job done. Once a rodent has moved into an area, it will systematically keep munching your plants. Birds seem to move from one area to another and usually are more isolated attacks.
Clearly, you can't do anything about damage that has already occurred. Once an animal intruder is discovered munching your plants, you can only fight to prevent future damage.
Fungal attacks are typified by a rust-colored or black spot surrounded by dried brown sections on the stem of a cactus or succulent. Rot is sometimes a fungus "eating up" the dead plant tissue, but in that case something else triggered the start of tissue death. In a true fungal attack, there is a fungus growing inside the plant tissue that kills it as it goes. This is what makes it turn brown and dry.
Treatment of Fungus:
Fungal attacks are extremely difficult to stop. The best option is to find any uninfected stems and re-start a new plant from them and throw the rest of the plant away. Fungicides are available that could slow down the fungus attack, but the fungus rarely is eliminated in this way.
Rot is very likely the number one growing problem encountered by new growers. Succulents and cacti are susceptible to rot because they are mostly water-filled fleshy tissue. This is just the type of environment that bacteria and fungi thrive in. In proper conditions, rot does not occur because the moisture in the environment is low. In the wild, most succulents grow in rocky, fast-draining soil, and high temperatures. While the amount of moisture a succulent can tolerate varies from one species to another, as a general rule rich soil with a high organic content that is continually damp will rot pretty much any succulent, especially if the temperatures are low. The low temperatures exasperate the problem because the most species are dormant at low temps and not using any of the available moisture and it takes longer for the soil to dry out.
Rot usually starts from the bottom up. Very often the roots will rot off first, but the plant body will not be rotting. This might happen where top-dressing keeps moisture away from the plant body. Sometimes, portions of the plant higher up might rot first. This usually occurs with over-watered plants. In this case, the cells will fill with water until they burst open like a water balloon. After the cell wall is ruptured, the cell dies. If a group of cells die from this, it is the perfect place for rot to set in.
Rot can vary in appearance from red to black. Rotted parts of cacti are mushy, slimy, and typically have a bad odor. Rot may not be visible on especially spiny plants even after the plant is long dead. Additionally, since rot usually works from the bottom up, by the time the problem is noticed, the plant is too far gone.
Rot is usually a bacteria, but can be fungal as well. Plants in dry environments can sometimes get rot if they have been damaged in some way to let infection in.
Treatment of Rot:
As mentioned above, once rot is detected, it is usually too late to save the plant. This isn't always the case. Sometimes rot is caught just when it starts or only a portion of the plant has started to rot. To save a cactus from rot, the rotted portions must be removed entirely. This might mean cutting the plant in half or more. The remaining healthy portion of the plant should be light green, firm like an apple, and not have any spots of discoloration in it. If you can still see some spots, you must keep cutting. If you cannot get to blemish-free, healthy tissue, then it is too late and the plant is doomed.
It is said often that prevention is by far the best cure for any problem. To avoid rot in your succulents, be sure that you have a good soil that does not sit damp for long periods when you water it.
Now that you've become acquainted with some of the main pests and diseases associated with growing succulents, it's time to learn about something a little more enjoyable—grooming!