Lemon trees attract a variety of pests, including leaf miners, whiteflies, gall wasps, aphids, orangedog caterpillars and various scale insects. To control them and keep them from damaging your trees, you must identify the pests and deal with them accordingly. Some pests can be well controlled with insecticides, while others are resistant and require another course of action. Pest control methods include pesticides, white oil, physical removal and biological control.
Insect infestations often resemble diseases, leaving telltale signs on leaves, stems, trunks and fruit. Distorted leaves, for instance, can be a sign of several pests. If the leaves are curled with silver-grey discolouration, this may be a sign of citrus thrips, small, winged, yellow-orange insects that also cause scabbing and silver discolouration on the fruit.
Curled leaves combined with a sticky, mouldy substance are a sign of citrus whiteflies. These flies lay their eggs on the underside of lemon tree leaves and feed on the tree's sap. Severe infestations can negatively impact fruit production.
If leaves are twisted with puckered marks, it may be a sign of aphids. These small, plant-eating insects, also known as plant lice, can be extremely destructive to trees if left uncontrolled.
Leaf miners are another common lemon tree pest. These moth larvae live inside young leaves, distorting the shape and leaving silvery trails on the surface. These pests can be particularly hard to get rid of if the infestation is allowed to grow too serious, so early detection is key.
If you find partially eaten leaves, it may be a sign of orangedog caterpillars. Unless you have a large infestation, however, they generally do little harm to mature lemon trees.
Lumps on the stems and branches of a lemon tree are a sign of a gall wasp infestation. Gall wasps lay their eggs inside tree branches and can cause lemon trees to become unproductive.
Scale insects, another common lemon tree pest, feed on the sap of the tree. Attaching themselves to the tree branches and trunk, they look like waxy, crusty bumps on the tree's surface.
Citrus thrips can be controlled with insecticides, but this is recommended only in severe cases to avoid insecticidal resistance. Pesticide use for any pest also can serve to eliminate beneficial insects such as natural predators. Biological control methods for citrus thrips recommended by the University of California Pest Control Program include the introduction of natural predators such as spiders, lacewings and Euseius tularensis mites.
Pesticides also can help control aphid infestation, but again, they should be used only as a last resort. Instead, try spraying aphids off with a garden hose whenever possible.
Though scale insects tend to be resistant to pesticides, petroleum oil or white oil is often effective.
Citrus whiteflies, leaf miners and gall wasps cannot be controlled with insecticides. To control whiteflies, biological control through the introduction of lacewings, minute pirate bugs and big-eyed bugs often does the trick.
Combat leaf miners and gall wasps by trimming away the affected areas and destroying them. To trap adult gall wasps, try hanging sticky wasp paper in trees.