With its long, wavy, ridged stems and fiery red fruit, it’s no wonder the pitahaya (Hylocereus undatus) plant is commonly called dragon fruit. Its strong and menacing name contradicts the plant’s ability to protect itself from predators.
 
Scales, Mealybugs and Aphids
Sap-sucking pests can get out of control on a dragon fruit. Scale insects and mealybugs are tiny, flat, disk-shaped insects and the females have no legs or wings. They hide under a protective waxy coating and feed by piercing plant flesh and sucking out the sap. Aphids are pear-shaped, approximately 1/8-inch-long insects, and they're often wingless. These insects excrete a sweet liquid, called honeydew, which attracts ants.

 
Ants and Beetles
If your dragon fruit has aphids, ants are not far behind. The sugary honeydew aphids produce is a favorite ant food. While they might not eat your plant directly, the ants protect aphids from predators. More aphids survive with the help of their bodyguard ants and further damage the dragon fruit. Some ants are also attracted to the sweet fruit and may damage it. Beetle larvae may also feed on the plant or the fruit.
 
Mites
While not insects, spider mites may affect your dragon fruit. These arachnids are tiny relatives of spiders and, unlike their eight-legged cousins, feed on plants. You may need a magnifying glass to identify mites on your dragon fruit. Silk webbing on the plant, along with speckled damage to the stems, usually indicate a mite problem.

 
Thrips
Four fringed wings and short antennae distinguish thrips from other insects but it is difficult to see them on dragon fruit as they are less than 1/20 inch long. Their feeding behavior leaves a stippled pattern on the plant that is mostly an aesthetic problem. Thrips rarely kill a plant.

(Source: homeguides.sfgate.com)