Appearance: I think we all know what ants look like. 6 legs, 3 body segments, often in numbers, ya?
Damage caused: Ants are not so much damaging the plants directly, but they are doing things that indirectly damage plants. The only thing they are attracted to are the sweet juices that are excreted from the plants, and nesting sites in the pot. They will however eat the fruit bodies of many cacti and ruin your chances of harvesting seeds!
Ants are carriers of other, more harmful pests, such as mealy bugs and spider mites. These are not welcomed at all. When ants make a nest, sometimes in the pot of our plants, they provide great breeding grounds for fungus, bacteria and other animal pests. These should be seen as "disease factories" and this is one of the main issues with ants.
Solutions: There are all sorts of ways of getting rid of ants. There are probably 10,000 different insecticides that will kill them. We often just soak the pots in a tub of water when its watering time (in growing season). Ants drown easy, some escape, but they are not in your pot anymore. Once there is a nest in your cactus pot, at some point after they are gone you will need to repot with new substrate. The old substrate should be thrown out and not reused (or you can sterilize it if you want).
Prevention: I think poison is the only prevention. If you can find a fool proof way of preventing ants, I think you are set for life!
Appearance: Mealy bugs are a white fluffy looking bug. They have a cotton appearance. Up close you can see they look like a frosted sow bug. Males have wings and can fly, females are without wings.
Damage caused: These are sucking bugs, meaning they suck juices from plants. They can do 2 things to hurt our plants (not just cacti): suck juices and leave little blemishes, they also spread plant disease by sucking a diseased plant then sucking a new plant, thus infecting it.
Solutions: Luckily they are soft bodied and easily killed by hand. Unless your collection is large, killing by hand is relatively easy to do.. There are numerous chemical insecticides that do a number on these bugs, if your into that route. Removing lush growth around the cacti area will alo help keep numbers down.
Prevention: They spread via other animals and also by themselves (not as much). Males are able to fly, females are not. They often attack lush growth so getting rid of large bushy plants around you cacti will help. Also keeping ants and other things that can carry these pests is a good idea. But in reality, if they are going to come, there is little to prevent them. Luckily they are easily controlled. They prefer dry, warm times...but will often attack despite that.
Another great way is to use a liquid dishsoap. Mix the soap (use a water soap) with water and spray the affected cacti. Let sit for a day and rinse the soap off the cactus, this may take a few times to get them all.
Appearance: These are very tiny, appearing only as tiny red spots with the (very good) naked eye. Mites are actually arachnids, and look a tiny bit lick tciks. Without magnification, they will simply look like red spots.
Causes: Spider mites area wide spread pest in most countries and affect all different plant groups. The prefer hot dry climate (just like cacti). Unfortunately they infest perfectly healthy plants as well as sick ones. You will see damage done on the skin oi fcactus asthey become lighter and almosta little scabby. The skin will appear more rough. Spider mites will also leave behindsmall webbing, like mini spiders. Both these 2 things are the most common indicator.
Solutions: Along with having a huge range in distribution and food types, they are also incredibly difficult to kill and reproduce fast. Chemical warfare is unfortunately the best option. generally soaking plants all the times helps dispalce them, but for cacti this is not real feasible. Others have used things like neem oil, alcohol, home made poisons with tobacco, peppers, garlic etc,all with varying success. The key with sider mites is to spray it in EVERY crack and crevice and keep at it. Most chemicals don't affect the eggs, soafter you spray and teh eggs hatch the baby mitesare free to roam allover the plants. The idea is to spray these baby mites before tehy themselves creat eggs. Try to spray every few days for a couple weeks to be sure.
Prevention: Sometimes they will just come from outside, little can stop that. Other times its from infested plants being brought in. Quarantine is good. Check the plants carefully and even rinse tehm under water well before introducing them to the collection. Once spider mites get into a collection, they generally don't leave without leaving their mark!