The Relationship Between Aphids, Ants, and Natural Enemies

Have you walked by a garden or flowerbed recently and wondered why some plants have a sheen on them when it hasn’t rained recently? That sheen is actually a sugary substance called “honeydew” that is secreted from aphids. Aphids (Family: Aphididae) are soft-bodied insects that use their piercing sucking mouthparts to feed on plant sap. They typically occur in colonies on the undersides of young plant growth and they can be found in large numbers infesting plants near the home. Heavily-infested leaves can wilt or turn yellow due to excessive sap removal. While the plant may look damaged, aphid feeding typically does not harm healthy, established trees and shrubs. Aphid species vary in color; some are green, others pink, brown, yellow, or black. They often possess a pair of horn like structures known as cornicles at the posterior end of the abdomen and these cornicles are where the honeydew is secreted. A fungus called sooty mold can grow on honeydew deposits that accumulate on leaves and branches, turning them black. The presence of sooty mold on plants may be the first indication that an aphid infestation is occurring. Furthermore, the drops of honeydew can attract other insects such as ants, that will feed on the sticky deposits.

The Ant/Aphid Collaboration

In some situations, ants tend aphids and feed on the honeydew aphids expel. The most common species we encounter that will tend aphids are carpenter ants (Camponotus pennsylvanicus DeGeer), argentine ants (Linepithema humile), fire ants (Solenopsis spp.), and acrobat ants (Crematogaster ashmeadi Emery). This tending of aphids is accomplished by chemicals on ants’ feet that sedate and subdue colonies of aphids, keeping them nearby as a steady food source. Put simply, aphids are farmed by ants and the harvest is sweet, sugary honeydew. Also, ants have been documented attacking and fighting off beneficial insects that have tried to eat their aphids. An insect is considered “beneficial” if it parasitizes or preys on pest insects or if it aids in pollinating plants. In the case of dealing with aphids, beneficial insects are natural enemies that parasitize or prey on the aphid pests. Some of the most important natural enemies are various species of parasitic wasps (Family: Braconidae) that lay their eggs inside aphids. The skin of the parasitized aphid turns crusty and golden brown, a form called a mummy. Once mummy aphids are noticed, it won’t take long for the wasps to eliminate the entire colony. Several other beneficial insects are predators that will feed on aphids. The most well-known are lady beetle adults and larvae (Family: Coccinellidae), lacewing larvae (Families: Chrysopidae, Hemerobiidae), and soldier beetles (Family: Cantharidae). An adult lady beetle can consume several hundred aphids in a day and each larva may eat 200-300 aphids as it grows. While homeowners may want to treat their entire garden for aphids, it is natural enemies that will do the best job. However, pest management professionals can certainly help these beneficial insects by controlling the ant species that will hinder them or prevent them from doing their job. So, the next time you’re around your own garden or a client’s flowerbed, look to see what type of biological control could be unfolding before you.