Aphids on Kaffir Lime Leaves

Aphid Identification

• Kaffir lime trees occasionally attract melon or cotton aphids (Aphis gossypii), black citrus aphids (Toxoptera aurantii) and spirea aphids (Aphis spiraecola). Black citrus aphids always have black bodies, while spirea aphids are always green. Melon aphids can be green, dull black or yellow in color. All of those aphid species have soft, pear-shaped bodies and rarely get any larger than 5 mm in length.
  Like other aphid species, those that feed on lime leaves have needlelike mouthparts that allow them to easily pierce plant tissue. The pests form colonies on the undersides of lime leaves. In warm climates, the aphid colonies consist entirely of females that bear live young. Male aphids sometimes appear in colder climates to help produce overwintering eggs.

Aphid Damage

• Aphids use their long, slender mouthparts to tap into leaf cells and suck out the contents. This robs the leaves of nutrients and often causes light yellow spots to form on the foliage. Infested leaves might also curl toward the stem while looking puckered, deformed or stunted in size. Although not a common occurrence, prolonged, severe infestations can eventually kill entire trees. Aphids also excrete an abundant amount of a sticky substance called honeydew, which provides the ideal growing environment for sooty mold fungi. Aphids can also damage lime trees by transmitting various plant diseases, including the citrus tristeza virus. Fortunately, the three aphid species most commonly found on Kaffir limes have low transmission rates.

Nonchemical Treatments

• Spraying your lime tree foliage with a hard blast of water from the garden hose helps reduce aphid populations by washing the pests off the plant. Make sure you spray the undersides of leaves where the aphids gather to feed. Spray your tree once a day until populations are under control. The water also washes away the honeydew and helps prevent sooty mold infections.

  Nitrogen promotes lush new growth that attracts aphid pests, so avoid feeding your tree with a nitrogen-rich fertilizer. Instead, apply a slow-release, 21-0-0 ammonium sulfate fertilizer. Following the instructions on the manufacturer's label, use about 1 cup of fertilizer for every year your tree has been alive. Divide that annual amount into thirds and feed your tree in February, May and then December. Scatter the fertilizer onto the soil beneath the canopy and water the treated area thoroughly.

Chemical Control

• Small aphid populations typically don't require any type of chemical control because the lime trees can handle light infestations and still bear fruit. Spraying your Kaffir lime with a pesticide might help treat severe aphid infestations, but the chemicals also kill off the pests' natural predators, which include ladybugs, lacewings, parasitic wasps and praying mantis insects. In addition, some aphid species have become resistant to commonly used insecticides containing carbaryl or pyrethrins.

  If chemical control proves necessary, insecticidal soaps can effectively control the soft-bodied pests and prevent severe infestations. The Colorado State University Extension suggests using a 2 percent insecticidal soap solution. Following the instructions on the product's label, mix about 5 tablespoons of soap product into 1 gallon of water. Soap sprays only work on contact, so thoroughly cover the foliage, including the tops and undersides of leaves, for optimal pest control results. Repeat applications every four to seven days until the aphids are gone.