Ants in an Oak Gall

Honeydew

• Gall wasp larvae are susceptible to attacks from insects that parasitize them. Several wasps in the Andricus, Disholcaspis and Dryocosmus genera have developed additional ways to protect themselves from predators, aside from the physical protection that the galls offer. The wasp larvae produce an enzyme that changes a tree’s starch into sugar. The galls on some oak trees produce so much sugar that it runs out of the galls and spreads over the surface of the tree where it attracts ants. The galls continue to produce honeydew as long as the larvae inside the galls are still active.

Gall Tenders

• Several species of ants, including Argentine ants, feed on honeydew. Argentine ants are a common species in California. These small, brown ants are only one-eighth inch long, but they will aggressively attack other insects that threaten their source of honeydew, including any parasitoids that try to attack the gall wasp larva. Another species, called honey ants, use sterile female ants to collect honeydew. The sterile females have elastic abdomens that can hold eight times their weight in honeydew. Worker ants feed them until they can’t move, then they hang themselves from the ceiling of the ant nest where they wait until they’re needed to supply food to the rest of the colony.

Decreased Parasitism

• Honeydew tending ants have a significant impact on gall wasp survival. In a 1984 study published by the Ecological Society of America, J.O. Washburn showed that parasitism doubled from 25 to 48 percent when ants were kept from reaching nectar-producing galls. In a study done at the University of California and published by the Royal Entomological Society in 2004, parasitoids increased by 36 percent and the number of gall wasps that emerged from their galls decreased by 54 percent when ants were prevented from reaching honeydew.

Control

• Although leaf galls don’t usually harm oak trees, galls that appear on twigs and branches can cause serious injury and may even kill a tree that’s heavily infested. Since the larvae inside galls are protected from insecticides and adult wasps aren’t usually seen, it’s not effective to control the gall-making wasps. Controlling ants may be more effective, and will allow natural predators, like parasitical wasps, to control the gall wasps. Bait stations designed for outdoor use are effective in controlling honeydew-tending ants.