Insect Frass 101: The Aussie Gardener’s Secret Weapon 🌱

“Another great product from the Dr.Greenthumbs team.” – Jason Smith
“…noticed a massive decrease in gnat numbers! Happy days!” – Josh Van Limbeek 

What Exactly Is Insect Frass?

Insect frass is the nutrient-rich castings of black soldier fly larvae. Pack it full of beneficial microbes, a naturally high chitin content and a balanced N-P-K profile, and you’ve got an organic powerhouse that out-classes ordinary compost or worm castings for microbial diversity and plant-available nutrition. 

Pest frass vs insect frass fertiliser

One common point of confusion is the word frass itself.

In everyday gardening, frass can mean two very different things:

• Pest frass - droppings left behind by insects like caterpillars, borers, or beetles feeding on your plants or timber
• Insect frass fertiliser - a processed soil amendment made from beneficial insects (most commonly black soldier fly larvae)

Pest frass is usually found:

• On leaves, branches, or around entry holes
• As fine sawdust, pellets, or gritty debris
• Alongside visible plant damage

In this context, frass is a warning sign, not a soil input. It tells you insects are actively feeding and action may be needed.

Insect frass fertiliser, on the other hand:

• Is collected during controlled insect farming
• Is stabilised and prepared for soil use
• Contains nutrients, beneficial microbes, and chitin

Throughout this guide, when we talk about insect frass, we’re referring specifically to processed black soldier fly frass used to support soil biology, not pest droppings found on plants.

Understanding the difference helps avoid confusion — and ensures frass is used in the right way, for the right purpose.

Why insect frass can vary between products

Insect frass isn’t just one single ingredient.

Depending on how it’s produced, frass can contain:

• Insect castings (digested organic matter)
• Shed exoskeleton material (a natural source of chitin)
• Small amounts of residual feed material
• Beneficial bacteria and fungi

This means the exact nutrient profile and microbial activity of frass can vary between producers.

Factors that influence this include:

• What the insects were fed
• How long the frass was aged or composted
• Whether it was blended with other organic materials

Rather than thinking of frass as a fixed fertiliser, it’s best viewed as a biological soil input - one that works by supporting soil life first, and plant growth second.

That’s also why frass performs best when used regularly in living soils, alongside compost, mulch, and organic matter.

Why Gardeners Swear By Frass

• Explosive microbial life – up to 20 × more biology than worm castings means faster nutrient cycling and healthier soil.

• Chitin triggers plant immunity – roots detect chitin fragments and switch on natural defence pathways against pests and pathogens.

• Improved soil structure & water-holding – frass feeds the fungi and bacteria that create fluffy, well-aggregated soil.

• Eco-friendly & odour-free – produced from food-waste-fed larvae, it turns trash into garden treasure.

Pro Tip: Pair frass with a quality living-soil mix like TurboDirt for an instant biology boost.

How to Use Dr Greenthumbs Insect Frass

(1 heaped Tablespoon ≈ 3-4 g) 

Using insect frass for indoor plants

Insect frass works particularly well for indoor plants when used gently and consistently.

For potted plants:

• Apply a light top-dress (½–1 teaspoon for small pots, up to 1 tablespoon for large pots)
• Water in well to activate soil microbes
• Repeat every 4–6 weeks during active growth

Because frass supports microbial balance rather than forcing growth, it’s well suited to:

• Houseplants
• Herbs grown indoors
• Seedlings and young transplants

Many gardeners also find frass helpful when dealing with fungus gnats, as healthier soil biology makes conditions less favourable for larvae.

For best results indoors:

• Avoid over-application
• Ensure good drainage
• Let soil dry slightly between waterings

As with all organic inputs, less is more — frass works with your soil, not against it.

Frass vs Worm Castings: The Quick Low-Down

If you want to see how worm castings perform on their own—and where frass outshines them—our Worm Castings Australia: The Complete Gardener’s Guide breaks down the biology, nutrient release and real-world results.

Step-by-Step Application

1. Top-dress & water – Sprinkle the recommended rate and water immediately to activate microbes.
2. Brew a frass tea – Soak 1 cup of frass in 10 L of de-chlorinated water for 24 h, then drench soil or foliar-spray. And if you're stacking this tea with other organic inputs, the Kelp Meal: Australia’s Ocean-Powered Fertiliser (2025 Guide) shows how kelp hormones pair with frass to supercharge stress resistance and growth.
3. Charge your compost – Add a handful per 20 L of compost to accelerate decomposition.

Using insect frass with other soil amendments

Insect frass can be used on its own, but it also works well when combined with other organic inputs.

Some gardeners choose frass blends that include:

• Biochar – for long-term carbon storage and microbial housing
• Compost or humus – to increase organic matter
• Mineral amendments – to balance trace elements

You can also create your own “stack” by:

• Applying frass first to activate microbes
• Following with compost or mulch to feed them
• Using biochar as a long-term soil structure addition

Straight frass is ideal when:

• You want a simple biological boost
• Soil biology needs rebuilding
• You’re working with living soil systems

Blended products can be useful where soil structure or carbon levels need extra support.

Composted vs fresh insect frass: what gardeners should know

Not all insect frass products are exactly the same.

Fresh frass straight from insect production can be high in moisture and biological activity, which is why most frass sold for home gardening is stabilised or composted before use.

This extra processing step:

• Reduces excess moisture and odour
• Improves consistency and shelf life
• Helps ensure the product is safe and stable for soil application

When choosing an insect frass product, it’s worth checking:

• That it’s intended for soil use, not raw waste material
• How it has been processed or stabilised
• Storage guidance (dry, sealed containers are ideal)
  

Well-processed insect frass should smell earthy, not sour, and feel crumbly rather than wet or sticky.

At home, store frass in a cool, dry spot and keep the lid sealed to protect the beneficial microbes until you’re ready to use it.

What research says about insect frass

Interest in insect frass as a soil input has grown significantly in recent years.

Research into insect-derived fertilisers shows frass can:

• Support microbial diversity in soil
• Contribute slow-release nutrients
• Provide chitin, which may help stimulate natural plant defence responses

Studies also highlight that outcomes depend on:

• How the frass is produced
• Application rates
• Existing soil conditions

This reinforces a key principle of organic gardening: biology first. Insect frass isn’t about quick fixes — it’s about building resilient soil systems that support long-term plant health.

Frequently Asked Questions

Is insect frass safe for edibles?

Absolutely. It’s an organic input and leaves no harmful residues.

Will frass attract pests?

No. In fact, the chitin helps plants deter sap-sucking insects.

For growers chasing an even stronger phosphorus and calcium boost—especially in fruiting crops—our Seabird Guano: The Ultimate Aussie Grower Guide 2025 explains how to integrate guano without upsetting soil balance.

Can I overdo it?

It’s pretty forgiving, but stick to the rates above—less is more thanks to the dense biology.

Ready to Super-Charge Your Soil?

Grab a bag of Dr Greenthumbs Insect Frass (High Calcium & Chitin) today and see the difference for yourself. Your tomatoes, chilies and citrus will thank you!

Happy growing from the team at Dr Greenthumbs—proudly helping Aussie gardeners cultivate thriving, chemical-free gardens since 2012.

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