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Ants

Also known as Argentine ant, Yellow crazy ant, Weaver ant

PestDepends
Black garden ants tending aphids Photo: ImagePerson · CC BY 4.0

In short

Ants are rarely plant-eaters themselves, but many "farm" aphids, scale insects, mealybugs and whiteflies for their sugary honeydew and protect them from predators, which lets those pests multiply. At the same time ants eat many insects, and some, such as weaver ants in Southeast Asia, are used to protect fruit trees.

How it looks

Social insects with a narrow waist between the thorax and abdomen and elbowed antennae, living in colonies with queens and workers; winged males and females appear at swarming time. On plants, trails of ants running up stems to clusters of aphids, scale or mealybugs are the typical sign. Weaver ants (Oecophylla smaragdina) of tropical Asia and Australia are mostly orange and build nests by stitching live leaves together with silk.

Signs

Lines of ants on stems and leaves; ants gathered around colonies of aphids, scale or mealybugs; sticky honeydew and black sooty mould; soil heaped up in or under pots where ants nest.

Life cycle

Egg, larva, pupa and adult; colonies grow from a queen and may produce winged reproductives that fly off to start new colonies.

What favours it

Plants with honeydew-producing insects; dry, undisturbed pots and soil for nesting.

Prevention

  • Control aphids, scale and mealybugs so ants have no honeydew source.
  • Check potted plants for ants nesting in the mix, particularly after they have stood outside.

Treatment

Start with the first step and move down only if the problem persists.

  1. Band stems or trunks with a sticky barrier (on a protective wrap for young bark) to keep ants off, so natural enemies can deal with the sap-feeders.
  2. Treat the honeydew-producing pests themselves.
  3. Where ants must be controlled, use ant baits placed along trails rather than sprays; baits are carried back to kill the colony — follow the product label.

Pros and cons

Pros

  • Eat termites, fleas and other insects.
  • Weaver ants have been used for pest control in citrus and other tree crops since at least 304 CE.

Cons

  • Protect aphids, scale, mealybugs and whiteflies from their natural enemies, making outbreaks worse.
  • Some species bite or sting; weaver ant bites are painful.

Look-alikes

Parasitoid wasps
Tiny winged wasps that fly and rest singly on leaves rather than walking in trails.

Affected plants

Any plant carrying honeydew-producing insects; fruit trees, citrus and shrubs outdoors, and potted plants.

Sources

  1. Ants — UC IPM Pest Notes
  2. Oecophylla smaragdina — Wikipedia

Written in our own words from these sources. Check a product label before using any treatment.