Commonly known as West Indian lime or Mexican lime, the key lime is a shrub-like tree that grows 5 to 6 feet tall. It features slender branches with evergreen, oval and aromatic leaves, and fruit smaller than that of a standard lime tree. Although considered a hardy plant, the key lime tree is susceptible to a number of insects that can cause severe damage if untreated.
Scale insects, aphids and spider mites are the main insects that damage the key lime tree.
More than 400 species of scale insects exist, with each falling into one of the three main categories: soft scales, armored scales and mealy bugs. According to the University of Florida Extension, soft and armored scales cause the most damage to ornamental plants and attack woody plants, including key lime trees. Both armored and soft scales secrete a wax-like covering that resembles a flat shield or armor over their bodies, hence the name. Armored scales are 1/16-inch wide with bodies that are oval, circular, oblong, or pear-shaped, while soft scales are 1/8 to 1/12-inch wide with an almost flat or spherical shape.
Aphids have oblong bodies in colors depending on the plant they were feeding on. They have antennae-like structures that arise from the lower end of their bodies. Spider mites exist on the host plant all year round but are difficult to see with the naked eye because an adult measures only 0.44 mm in length.
Scale insects damage plants by sucking vital juices from them. Heavily infested key lime trees produce little new growth and appear unhealthy and stunted. The surface under the leaves develop yellow spots that become larger with consistent feeding and eventually drop. Soft scales that feed on stems or the trunk of the key lime secrete honeydew that serves as the ideal growing medium for a fungal disease called sooty mold. Aphids feed on young growth, causing leaves to curl or twist, permanently distorting the shape. Spider mites feed on the leaves of key lime trees, forming yellow spots along the midrib and causing them to appear etched and stippled.
Control insects that damage key lime trees with horticultural oils as an alternative to chemical insecticide. Because these oils are refined petroleum products, they are odorless and safe for use indoors and around children and pets, according to Iowa State University Extension. Thoroughly douse the plant with the spray to cover it completely. Spaces as tiny as gaps in between the bark or under leaves that missed the application provide a safe haven for the insects.
Raise the humidity levels for the key lime tree to prevent infestation by spider mites, and improve overall health of the tree. Mist the foliage frequently or place a humidifier in close proximity.