Cabbage Moth: How to Identify, Control, and Prevent

Cabbage moth, scientifically known as Mamestra brassicae, is a significant pest for Australian gardeners, particularly those growing brassicas. If left unchecked, these insects can wreak havoc on veggie patches, damaging crops like cabbage, broccoli, cauliflower and more. 

In this article, I will guide you through understanding, preventing, and controlling cabbage moth infestations, helping you safeguard your garden from these pesky infestations, and ensuring your veggie patches remain healthy and pest-free.

What are Cabbage Moths?

Cabbage moths are small, nocturnal moths belonging to the family Noctuidae. Although they may appear harmless, their larvae (commonly referred to as cabbage moth caterpillars) can cause extensive damage to vegetable gardens, especially brassicas such as cabbage, cauliflower, kale, and broccoli. 

Native to Europe and Asia, this pest has adapted to Australia's varied climates, thriving in both temperate and subtropical regions.

Not to be confused with the Diamondback moth (Plutella xylostella), which has a narrower, arrow-shaped body, Mamestra brassicae looks more like a typical moth and is much more active in the evenings. 

Cabbage Moths' Natural Habitat

Cabbage moths tend to inhabit areas where brassica plants are grown, as these are their preferred hosts. They thrive in gardens, farms, and even wild spaces where cruciferous plants grow.

In Australia, they can be found across all states, particularly in regions where vegetable farming is common.

Cabbage Moths' Life Cycle

The cabbage moth undergoes complete metamorphosis, passing through four stages:

1. Egg Stage: Females lay their eggs on the undersides of leaves, usually in clusters of 20 to 50 eggs.
2. Caterpillar Stage: The eggs hatch into small larvae (caterpillars) which begin feeding immediately on the leaves. Over a few weeks, they grow and moult several times.
3. Pupal Stage: The caterpillars pupate in the soil or plant debris, where they form a hard cocoon.
4. Adult Moth Stage: After pupating, they emerge as adult moths, ready to repeat the cycle.

In warm Australian climates, cabbage moths can complete several life cycles in a single growing season, making them a persistent threat.

Plants Attacked by the Cabbage Moth in Australia

Cabbage moth caterpillars mainly feed on plants in the Brassica family, such as:

• Cabbage
• Broccoli
• Kale
• Cauliflower
• Brussels sprouts

They can also occasionally attack other vegetables like spinach and lettuce. With brassicas being a common feature in Australian veggie patches, the cabbage moth is a frequent adversary for local gardeners.

How to Identify the Cabbage Moth

Correct identification is crucial to managing cabbage moth infestations effectively. Here's how you can spot them in their various life stages:

Eggs

Cabbage moth eggs are tiny, round, and yellowish, laid in clusters on the underside of leaves. They can be challenging to spot but are a tell-tale sign of a developing infestation.

Caterpillar

The cabbage moth caterpillar is green and measures around 30 mm in length when fully grown. It has faint white stripes along its body and tends to blend in with the leaves it feeds on, making it tricky to spot.

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These caterpillars are voracious feeders, creating holes in the leaves of the plants.

Adult Moth

The adult cabbage moth is grey-brown with a wingspan of about 40 mm. Though they are nocturnal and difficult to see during the day, they can often be found resting on plants or flying around at dusk.

Their wings have subtle patterns, helping them camouflage in garden environments.

Where You Can Find Them on Plants?

Cabbage moth caterpillars typically hide on the undersides of leaves during the day, feeding primarily in the early morning or late afternoon. Eggs are also laid on the undersides of leaves.

Adults are less frequently seen during the day but may be spotted resting on plant stems or leaves.

Signs of Damage to Look For

Cabbage moth caterpillars cause a range of visible damage, including:

• Holes in leaves: The caterpillars chew irregularly shaped holes in the leaves.
• Skeletonisation: They can eat entire leaves, leaving only the veins behind.
• Damage to vegetable heads: In crops like cabbage and cauliflower, they burrow into the heads, destroying the inner leaves and rendering the vegetables inedible.

How Quickly Do They Destroy Veggies?

Cabbage moth caterpillars can quickly devastate crops. A small population can skeletonise plants within days, leaving gardeners with wilted, damaged vegetables. Early detection is essential to prevent severe infestations.

What Causes Cabbage Moth Infestations?

Several factors can contribute to cabbage moth infestations in Australian gardens:

• Warm Weather: Mild winters and warm springs allow the moths to thrive and reproduce rapidly.
• Monocultures: Large, uninterrupted plantings of brassicas create an attractive habitat for cabbage moths.
• Poor Garden Hygiene: Leaving plant debris in the garden provides ideal conditions for pupating larvae.
• Lack of Natural Predators: In some areas, the absence of beneficial insects can lead to an unchecked cabbage moth population.

Preventing Cabbage Moth Infestations

Prevention is always better than cure. Luckily, very similar methods can be used to prevent this species, much like its distant relative, the diamondback moth.

Here are several strategies Australian gardeners can use to prevent cabbage moth infestations:

Crop Rotation

Rotating crops from season to season helps break the lifecycle of cabbage moths. Avoid planting brassicas in the same location year after year, as this can lead to repeated infestations.

Netting

Using fine mesh netting to cover brassicas is one of the most effective ways to prevent adult moths from laying eggs on plants. Ensure that the netting is secured tightly to prevent moths from sneaking in.

Companion Planting

Planting aromatic herbs like dill, mint, or sage near brassicas can deter cabbage moths. These strong-smelling plants mask the scent of brassicas, making it harder for the moths to locate them.

Encourage Beneficial Insects

Attracting natural predators like parasitic wasps and predatory beetles can help keep cabbage moth populations under control. Planting nectar-rich flowers like alyssum or marigold nearby can encourage these beneficial insects to your garden.

Good Garden Hygiene

After harvesting, clear away any plant debris and till the soil to disturb pupating larvae. This reduces the chances of overwintering cabbage moths.

How to Get Rid of Cabbage Moths

If you've noticed an infestation, here are some practical and organic methods for controlling cabbage moths:

Handpicking them Off Plants

Regularly inspect your plants for eggs and caterpillars. Handpick them off the plants and either squish them or drop them into a bucket of soapy water.

Biological Control

Introducing natural predators, such as parasitic wasps, can help reduce cabbage moth populations. You can also purchase beneficial insects from garden centres to release in your garden.

Organic Sprays

Bacillus thuringiensis (Bt) is a naturally occurring bacterium that targets caterpillars without harming other insects. It's a safe and effective way to manage infestations. Neem oil is another organic option that can disrupt the growth and feeding habits of cabbage moth caterpillars.

Traps

Pheromone traps can be used to capture male moths, reducing breeding and the overall population. While not a complete solution, they can be a helpful addition to your pest management strategy.

Cabbage Moth Frequently Asked Questions

How long do cabbage moths live?

Cabbage moths have a short lifespan, typically living for around two to three weeks. However, in warm climates, they can complete several generations in one growing season.

Are cabbage moths harmful to humans?

No, cabbage moths are not harmful to humans. However, their caterpillars can cause extensive damage to crops, affecting vegetable production.

Can cabbage moth caterpillars survive in winter?

In cooler regions, cabbage moth caterpillars overwinter as pupae in the soil. In warmer climates, they may continue to feed and reproduce throughout the year.

What are the best plants to deter cabbage moths?

Companion plants like dill, mint, and sage can help deter cabbage moths due to their strong aromas. Planting these herbs near brassicas may reduce the likelihood of infestation.

Wrapping Up Our Cabbage Moth Guide

The cabbage moth is a formidable pest for Australian gardeners, but with the right preventative measures and control methods, you can protect your crops from damage and future infestations. 

By employing crop rotation, companion planting, and organic pest control strategies, you can successfully manage and prevent cabbage moth infestations, ensuring a healthy and productive veggie patch all season long!