Citrus trees, such as lemon, lime, orange and grapefruit, are prone to different diseases, but definitely attract some insects that damage their growth and fruit production. You can prevent these insects by using some spray deterrents and getting rid of them at the first sign of infestation. The method of getting rid of them varies with each pest, so make sure you understand exactly what pest it is before using any treatment.
Aphids do a lot of damage to a citrus tree only if they are present in large numbers, but the problem is they can grow rapidly, so a couple of aphids can turn into a lot within one growing season. They damage the tree by sucking the sap out of the leaves, leaving small needle prick marks. This eventually causes the leaves to yellow, drop off and die. One way to tell if aphids are on your citrus tree is to watch the underneath of the leaves. Aphids leave a sappy dew substance there, which attracts ants. You can control aphids by using insecticides such as Bug Buster. Usually a couple treatments do the trick.
Citrus whiteflies are small winged insect that feed on the underside of citrus tree leaves and lay their eggs there, which is where the larvae feed once hatched. This causes the leaves to curl and become covered in a sticky mold-like substance that the whiteflies secreted, so much that it seems to be dripping. These pests can multiply rapidly, so use something to control them, such as Trounce, an insecticide.
A large brown caterpillar called the orangedog caterpillar is about 1.5 to 2 inches in length. It eats citrus tree leaves, and one sign of them is that the leaves on the tree are chewed through and around the edges. The only way to get rid of the caterpillars is to physically remove them with your hands and destroy them. Keep in mind that when the caterpillar feels threatened, two red antennas come up from its head and produce a pungent odor. Numbers of them can be controlled by spraying the tree with Garden Insect Spray with Spinosad or BTK Biological Insecticide Bacillus thuringiensis (Bt).