Eelworms

• Eelworms, or nematodes, are parasitic worms that attack cacti roots. They're very difficult to control. If you have eelworms, disinfect all gardening tools, plant containers and greenhouse benches with boiling water. Use chemical decontamination on objects, like plastic pots, that can't be treated with boiling water. Decontaminate soil by heating. Remove any infected roots when transplanting. Kill eelworms by bathing cacti in water at a temperature of 113 to 122 degrees Fahrenheit for 30 minutes.
  
  

Spider Mites

• Spider mites are tiny brownish-red mites that suck the sap from cactus tissue, causing brown, gray, rusty and scabby areas to spread over the plant. Destroyed tissue never grows back. Spider mites dislike anything wet, so water cacti from overhead to discourage them. Miticides can also be used, but effective miticides are usually only available to professional growers. You can use predatory mites called Phytoseiulus persimilis for biological control, but only in temperatures of at least 70 degrees Fahrenheit.
  
  

Mealybugs

• Mealybugs are small gray or light brown bugs that resemble woodlice. Since they're only about 1/10 inch long, they can be hard to see. Small balls of white fluff on cactus spines or around the base of your plant are an early symptom of mealybugs. They also infect the roots of cacti. Brush root mealybugs off with a stiff brush, then dip the roots in an insecticide, before repotting or transplanting. Control small numbers of mealybugs on the plant itself by dabbing rubbing alcohol on them to kill them. Use an insecticidal spray for large infestation. Using a high-pressure water jet to wash the bugs off the cacti is also effective.
  
  

Longhorn Beetles

• The cactus longhorn beetle is a long, shiny black beetle with antennae longer than its body. The beetle is easily identified by the white markings on its antennae. Cactus longhorn beetles feed on cactus buds. Control longhorn beetles by picking them off infested plants by hand. Chemical control isn't recommended since handpicking easily controls them.
  
  

Other

• Ants damage cacti by eating young stems, the stamens of flowers and grains of seeds. In addition, the presence of ants often indicates an aphid problem because ants eat the honeydew that aphids excrete on the plants. Aphids are tiny green bugs that live in the soft parts of a cactus, such as its flowers. Both pests can be washed off with a strong stream of water. You can also use an insecticide to control them.