As the old folk song proclaims, “Lemon tree very pretty, and the lemon flower is sweet...,” but a substance even sweeter than its blooms lures ants to a lemon tree (Citrus limon), which grows in U.S Department of Agriculture plant hardiness zones 9 through 11. Called honeydew, it's the sugary, sticky waste of sap-stealing insects. Several varieties of these pests infest lemon trees, and ants go to great lengths to protect these insects. Eliminating honeydew from lemon trees requires first eliminating the ants.
Insects capable of drenching a lemon tree with honeydew include aphids, mealybugs, soft scales, whiteflies and -- less commonly -- Asian citrus psyllids. All of them puncture the stem or leaf tissues with hypodermic-sharp mouthparts and drain sap.
Nitrogen in the sap fuels the growth of all these pests, but it's present in such small amounts that they consume far more sap than they actually metabolize. They excrete the undigested portion as gooey, carbohydrate-dense honeydew.
Farmers, Herders and Protectors
Ants with swollen abdomens streaming down the trunk of a lemon tree are transporting freshly collected honeydew back to their nests. Examination of the tree's tender new growth reveals other ants farming the sap-sucking insects by stroking their midsections to stimulate honeydew excretion.** The ants drink the honeydew until they have enough for transport.
Herders as well as farmers, ants move the honeydew producers to unpopulated parts of a lemon tree so they can form new colonies. They also serve as protectors by killing lacewings, lady beetles, predatory wasps and other insects that feed on the sap thieves.
Evicting the Ants
To prevent ants from reaching the honeydew, remove the weeds from around the base of the lemon trees and cut back branches to at least 2 feet from nearby plants, structures or the ground.
Make barriers around the trunks by wrapping them with 6-inch bands of duct tape coated in petroleum jelly. Check the barriers frequently during the year and move them at least once to reduce the chance of bark injury. Replace them when the petroleum jelly becomes clogged with ants or debris.
Line the ant trails leading to and from the trees to the ant nests with enclosed, slow-acting bait formulated for sugar-loving ants. Boric acid bait is a good choice. The foraging ants transport it back to the nests, where it kills the colonies within a few weeks.
To keep honeydew off lemon trees after the ants are gone, treat the invading insects with organic, ready-to-use insecticidal soap. It suffocates them without leaving residue harmful to bees and other beneficial insects after it dries.
On a calm day with no rain in the 24-hour forecast, spray the trees until the soap runs from all their surfaces, including leaf buds and leaf backs. Time the treatment for early morning or after dusk, when no bees are foraging and the trees are out of direct sun. Repeat every one to two weeks, or at the label's suggested frequency, until the infestation subsides.
Always follow the insecticide label's application instructions and precautions. Dress in protective clothing, including a long-sleeved shirt, long pants, waterproof gloves and safety goggles, when spraying.