Orange trees are relatively carefree to grow. Within one or two years after you plant one, you will begin harvesting the sweet fruit. However, people are not the only ones who like oranges--a number of insect pests also enjoy chewing on the tree's leaves and sometimes its fruit. A few insects won't hurt your tree, but if they reproduce quickly, a large population of bothersome insects can cause problems.
Snails and Slugs
Snails and slugs are common pests of orange and other citrus trees. They chew holes in the leaves and can leave the fruit looking scarred. You can sometimes identify this pest by the slimy trails they leave on your tree before they go into hiding after the sun rises. To check for snails and slugs, look for them with a flashlight at night. Hand pick as many as you can find and then squash them, but if the infestation is severe, sprinkle iron phosphate granules around the base of your tree. Also prune low branches that might allow these creatures access to your tree. Keeping the area under your tree clean of fallen leaves and other debris will limit the daytime hiding places for both snails and slugs.
Brown Soft Scale Insect
All of the types of scale insect are small sucking insects that resemble a small tank because they have a tough shell coating their bodies. The brown soft scale often attacks orange trees. This insect attaches itself to one spot on the tree and does not move. Leaves can turn yellow and fall when an orange tree has a scale infestation. Scale insects can be hard to control because of their shell: even though this one is called the "soft" scale, it does have a protective armor, making it resistant to treatments such as insecticidal soap. Hand pick and squash as many insects as you can find and then spray horticultural oil on your tree. In spring, spray your tree with insecticidal soap or a pyrethrin product to control newly hatched nymphs, which haven't yet developed the protective armor.
If you notice that the leaves on your orange tree are curled and have turned silver or the leaf buds are shriveled, the citrus thrips might be to blame. This pest is tiny--only 1/20 inch long--and eats the sap and juices of the tree. Many species of thrips exist: often they are light yellow or orange in color. Eggs hatch in spring and the insects then cause damage during the entire summer growing season. Damage might escalate during hot, dry weather, especially if your tree becomes too dry. Keep your orange tree well watered and use a fertilizer designed for citrus two or three times each year, following label instructions. You can also control thrips by keeping your tree healthy through proper water and fertilizer. Spraying your tree with nontoxic insecticide, such as a narrow range oil can help, according to the University of California, Davis. Some beneficial insects, such as the green lacewing, can also help to control a thrips population. UC Davis concludes that thrips do not normally harm trees or fruit quality.
The many species of aphids attack a large number of plants, from roses to oranges. This small insect sucks the sap from orange leaves and causes them to become puckered, twisted and yellow. In severe infestations, leaves will drop and the tree's branches can die back. Ants eat the sweet excretion of aphids and are often present when this insect is on your tree. Watch for ants and if you see them look for aphids. Spraying your tree with a strong stream of water can help. Sticky barrier products such as Tree Tanglefoot can be applied around the base of your tree to keep ants away. Insecticidal soap is effective in killing aphids: you must apply it every other day for a week or more and reapply if the insects return.