Gall wasp, an insect that parasites plants and trees

A gall wasp laying eggs on an oak twig.

Gall wasp is an insect that lays eggs in soft tissues in trees, flowers and other plants. As the larvae develop, a gall forms and usually hinders the proper development of the plant.

Gall wasp general facts

Name – species of various families
Common name – gall wasp
Appears in – spring to summer

Type – insect (wasp)
Spread – depends on species
Target – shoots, leaves, buds

Size – from 1/16th of an inch (2 mm) up to 1/3rd of an inch (8 mm), depending on wasp species
Generations per year – variable

Usually, a single given insect will only contaminate a single plant species, or at most only members of the same plant family. In severe cases, the harvest is compromised and the plant may die.

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Symptoms of gall wasp

There are different kinds of gall wasp, and their impact on host species also differs, but there is one symptom that all have in common: formation of a gall or several of them.

  • A gall is a swelling of tissue such as leaf tissue, bark tissue, or bud tissue resulting from infection by a gall wasp.
  • The gall wasp infects the host by laying eggs into the soft tissue.
  • Depending on the case, the gall may take on many shapes, from perfectly round marbles to formless gobs on leaves, stems, twigs, bark, flowers or buds.

What is gall wasp

A gall wasp is a flying insect of the same family as the common wasp. It has the particularity of requiring a specific host plant as part of its reproductive cycle.

  • A particular gall wasp species usually only infects a single specific plant species or genus.

For example, the citrus gall wasp will only infect citrus trees. Other plants and trees will not be infected by it. If no citrus tree is available, the citrus gall wasp will not be able to reproduce. Originally, it only infected finger lime, but when other citrus trees were introduced, it infected those, too.

  • Note: not all galls result from gall wasp. Some galls are due to other insects like mites and midges, and still others are due to fungal or bacterial infections.

Gall wasp on a chestnut leaf

How does a gall wasp infect plants

  • Gall wasp infects trees and plants by laying eggs in soft tissue, usually in spring or during growth periods
  • Chemicals and enzymes and other toxins are released by the eggs or hatching larvae
  • These force the host plant to produce extra tissue around the parasites, creating a swelling or gall
  • The gall protects the hatchlings and usually provides gall wasp larvae with food
  • When mature, larvae transform to adult insects and drill through the gall to mate and lay new eggs

Usually, a gall becomes visible to the eye around 1 to 6 weeks after the eggs were laid.

Plants vulnerable to gall wasp

Every day, botanists and entomologists discover new species. Only a fraction of all possible gall wasp and host plant relationships are know and described so far.

In gardens throughout the world, all types and families of plants have been known to support at least one or more gall wasps.

Gall wasp on trees

  • Oak gall wasp – there are hundreds of different gall wasp species within the Cynipidae family that infect one or all oak tree species.
  • Beech gall wasp – since this tree is related to oak, some gall wasp infect both species.
  • Chestnut gall wasp – infects young branch shoots, flower buds and leaf buds. Interestingly, the female wasp doesn’t need to mate and produces eggs that are only female.
  • Coral tree gall wasp – gall wasp lays eggs on leaves near main veins. Leaves are distorted and deformed, similar to leaf curl.
  • Hickory, maple, elm and others host specific species as well.

Gall wasp on flowers

  • Rose gall waspmossy rose gall and spiny rose gall are both due to gallflies that infect rose shrubs.
  • Waxflower gall wasp – gall wasp eggs on the stems induce a swelling and impact the branching of this flower shrub of the myrtle plant family.

Gall wasp on fruits & vegetables

  • Raspberry gall wasp – grows on stems, produces roundish galls like a donut around the stem
  • Blueberry gall wasp – produces a kidney-like gall around the twig that consumes all the sap to the branch
  • Citrus gall – galls are swollen portions of stems and sometimes main veins on the underside of leaves

Gall wasp on succulents

  • Currently, no significant cases of gall wasp on succulents have been found.
  • Some succulents present strange growth patterns called carunculations that are genetic deformities. They’re not caused by gall wasp.
  • There are cases where galls appear because of other causes, such as midges or fungus. An example is aloe vera mite that forms galls.

How to control gall wasp

Natural control for gall wasp – predator wasp

Often, gall wasp species are themselves under attack by other predator wasp species. These predator wasps locate galls and lay their own eggs in the gall, near the original larvae. When the predator wasp eggs hatch, they start feeding off the gall wasp larva or pupa.

For example, the citrus gall wasp has two other wasp predators. They both find citrus galls and kill off the citrus gall wasp hatchlings.

  • Note: in many places where citrus gall wasp is expanding, predator wasps aren’t introduced yet. This is a problem since it places the burden of control on us humans.
  • When you’re transferring a tree (such as a citrus) to a new location, check it over diligently for galls. Only transfer trees in the season when galls are maturing and clearly visible. This makes checking easy and thorough.

Natural control for gall wasp – predator vine

In a surprising discovery in 2018, botanists discovered a vine that actually fed off galls formed by gall wasp on oak trees. When the vine was present, gall wasp pupae and larvae would only hatch in half the cases. Normally, gall wasps on oak nearly always mature to term.

This spectacular vine, called the “Love vine” or Cassytha filiformis, was capable of creeping up towards galls and attacking the gall wasp that was forming inside, depriving it of nutrients.

Artificial control for gall wasp – pruning

Small galls on a leaf laid by different gall wasps.In most cases, gall wasp doesn’t spin out of control to the point of being a serious concern. Except for citrus gall, there’s usually no need to act.

However, if ever you wish to keep gall wasp from spreading, the best solution is to prune galls off.

  • Check for the season when galls are visible but not yet hatched
  • With clean shears, cut the branch below the gall
  • Make the cut just above a bud so that the branch may direct growth to the bud (you don’t want to go about hat racking the tree, just prune it)
  • It’s useless to prune off dried and brown galls that have already hatched (check for holes)

In the case of leaf gall, simply pluck the leaf off.

  • Note: destroy the infected galls either by burning or by “baking” in a tied black bag set to warm up hot in the sun. Don’t compost the materials directly. Wasps can still mature and hatch from galls that are cut off from the tree.

Artificial control for gall wasp – traps & trapping

In some cases, it’s possible to set traps that attract hatched adult gall wasps. This can either interfere with the mating process or mislead them when they look for suitable egg-laying sites. In many cases it’s difficult to target only a specific gall wasp and other insects get caught, too.

Since they spend their “caterpillar stage” of larvae and pupae in the gall, you can’t pick them off. Size would make it impossible anyways.

Learn more about gall wasp

Over 1400 species of gall wasps have been identified and catalogued to date. This doesn’t include other types of galls, such as those due to fungus or mites.

Each wasp is usually specific to a single host plant species. This is because the chemical cocktail required to trigger gall formation is unique to each plant species.

In the course of time, the parasite wasp evolved to produce exactly the mix that would trigger the “ideal” gall for it. This would afford the hatched pupa protection, food and safety from predators.

Some gall wasp species only lay eggs where other gall wasps have laid them, in the gall. They then benefit from the gall without having had to trigger it. These are called inquiline gall wasp. Usually  they out-compete the original gall wasp for nutrients, much like a cuckoo replaces its siblings in the host bird nest.

Can a gall wasp sting?

Gall wasps won’t sting persons or animals. Their sting isn’t designed for attack, it’s actually simply a tube that lays eggs. It can pierce through woody tissue but the wasp won’t use it to defend itself.

Smart tip about gall wasp

A gall sliced open to reveal tiny gall wasp larvae.These are so tiny, bring a magnifying glass out when you are on the lookout for them!

Indeed, the largest are only a few millimeters long (¼ inch). Another fitting name for them is “gallfly” or “gallflies”.

A sharp blade can slice open galls, revealing parasites inside.

More about plant diseases and pests

Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Gall wasp lays eggs by Alex Wild, Insects Unlocked under Public Domain
Red and white gall on Chestnut by Manfred Richter under Pixabay license
Gall wasp dotted oak leaf by Gartenredakteurin under Pixabay license
Gall cut open to reveal wasp larvae by Frost Museum under © CC BY 2.0