Your lemon tree can produce a healthy harvest for many years, but it is important to regularly watch for the insects that can sabotage the vitality of your tree. Bugs, such as the aphid and citrus thrip, cause tree leaves to go yellow and begin curling. In extreme cases, the lemon tree experiences severe leaf drop and stunted growth. By knowing what to look for, identify any bugs on your own lemon tree so you know how to get rid of them.
Inspect your lemon tree's stems and leaves for aphids. These small, flying insects come in various colors, such as green or brown, and leave a sticky honeydew residue on the leaves that they suck the sap from. In many cases, a sharp blast of water dislodges them, but if they continue to reappear, apply an organic insecticidal soap to the foliage.
Shake the branches of your tree to see if small, white insects appear. The citrus whitefly measures 1/12 inches long and makes itself known by leaving a sooty excretion on the foliage and laying eggs underneath the leaves. Apply an insecticidal soap to the foliage and make sure to cover the undersides of the leaves.
Check your lemons and the underside of the tree leaves to see if you find small, colored insects. The citrus bud mite in particular has an elongated body and prefers to absorb nutrients from budding lemons, while the red spider mite enjoys sucking nutrients from the leaves. Use a forceful spray of water to get rid of them, or spray an insecticidal soap.
Look for scabbed-over or shiny attachments on the wood of the tree, which signify the presence of scale. These non-moving bugs also make an appearance on the foliage and sometimes the lemons of your tree. Use a horticultural oil and follow the directions on the package to get rid of these sap-sucking insects.
Monitor your tree for orangedog caterpillars, which have a brown body and are 1.5 to 2 inches long. These insects are found chewing on the leaves and if their eggs are allowed to hatch, multiple caterpillars can eat through all of the foliage in a matter of days. Remove these bugs by hand, and if that is not possible, use a biological insecticide or garden insect spray with spinosad to get rid of them in an organic way.
Watch for small pale yellow or orange insects that fly. The citrus thrips feed on tree sap and leave a silvery gray color on the foliage and lemons of your tree. Use a garden insect spray with spinosad every 14 to 21 days until you see a decline in the population.