Controlling gall wasp on plants and trees

A strange swelling on a leaf or a stem usually means a gall wasp has coaxed the plant into building a nursery for its young. Only the citrus gall wasp really threatens a crop: elsewhere, leaving the galls where they are costs the garden nothing, and the reward is more biodiversity to watch.

• Read this: what is a gall wasp?

Strange growth on leaves and stems of your favorite plant?

This is possibly due to one of several species of gall wasps.

When to control gall wasp

Gall wasps are fascinating: they learned to coax host plants into building a safe haven for their offspring!

Should you destroy gall wasps or not?

Most gall wasps will only infect a few leaves or branches on any given tree.

It’s very seldom that gall wasps pose serious problems to the health of the plant.

You’ll find that leaving them alone won’t cause any problem to the garden; quite the opposite, it favors biodiversity!

• The only major exception is the citrus gall wasp: this insect is capable of devastating an entire harvest in a field of lemon or orange trees.

Ways to control gall wasp

There are two approaches to controlling gall wasp:

• First, you can find ways of letting nature do the work. This means finding natural predators that feed on gall wasps and reduce their numbers.
• Second, you can also do some of the work yourself to protect your plants. This gives you more control, but it’s also a bigger job.

There are at least two known natural predators of gall wasps: predator wasps on one hand, and, on the other hand, an interesting recent discovery: a vine that finds and feeds on galls!

As for manual control methods, the most effective one is pruning off and removing infected portions of plants. Increasingly, though, traps are being developed that keep the wasps of your plants by attracting and trapping them.

Let’s take a look at each method in more detail.

Gall wasp control methods compared

Before reaching for the shears, it helps to see the options side by side: two of them ask nothing of you at all, and only one is genuinely hard work.

Let’s take a look at each method in more detail.

Natural control for gall wasp with predator wasps

Often, gall wasp species are themselves under attack by other predator wasp species.

This predator wasps locates galls and lays its 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. Some countries and states have regulations to explain and enforce this.

How to attract predator wasps to the garden

As a rule, the more diversity in your garden, the higher the chance of attracting the insects you want.

• Set up insect hotels to attract beneficial animals.
• Beneficial insects love it when you leave a portion of your garden to develop naturally, like a wild prairie or even simply a garden bed that you don’t mulch or till.

A vine that protects oak from gall wasps

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.

From the oak tree’s point of view, letting the vine crawl around may seem like adding yet another parasite. But the trade-off for the oak tree makes it a winning combination, since gall wasps is much more damaging than losing a bit of sunlight to a vine.

Artificial control: prune those galls away!

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 (don’t hat rack 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.

Traps & trapping to control gall wasps

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 at this stage as you would with normal butterfly or moth caterpillars. Size would make it impossible anyways, they’re so small!

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