How to Identify & Fight Grow Room Pests

It's getting warmer and the bugs are coming! By hook or by crook, they’ll try to sneak into your grow room and ravage your plants.

If they break past your first defences, all’s not lost. You can still win the war. Here’s how you prevent, identify and defeat common grow room pests.

Don’t wait until they've destroyed your plants. Instead, create an ongoing strategy for preventing, spotting and fighting pests.

This is called an IPM (Integrated Pest Management) strategy:

• Prevention: Spider mite sachets, bug blockers (more here)
• Inspection: Check for pests every 5 days using a magnifying glass
• Treatment: Biological control (natural predators) and pest sprays

Spider mites are very common, but easily treated.

Look out for small, yellow speckles on the top surface of leaves – a sure fire sign that mites have been feeding.

The undersides of leaves may also contain tiny white dots that are either the mites and/or their eggs. Webbing occurs in the latter stages of an infestation.

 You can use natural predators:

• Using sachets (preventative): Put 1 - 2 in a propagator, 1 per plant (veg) or 1 - 2 per plant (flowering)
• Via a bottle (treatment): Sprinkle the contents around your room & leave the opened bottle at the base of plants

You can also use sulphur:

• Using Hotbox Sulfume (treatment): A 2kg dose of sulphur will treat areas from 2m3 - 10m3 for about a year.
• Using a Fortefor P Fumer: This'll fumigate aread up to 80m3 (only use between crops)

After eradicating pests, clear off residue and dead bugs by spraying your plants once with water. Then remove damaged leaves and keep temperatures down to slow the rate of spider mite breeding. 


Extensive Damage on a Cucumber Plant


Infestation on a Cucumber plant


Early Damage on a Hop Plant


Extensive Damage on a Hop Plant


Close up of Spider Mite Adults, Nymphs and Eggs


Spider Mites and Eggs on Webbing


Close up of a Two Spotted Spider Mite, a Red Spider Mite and an Egg

Thrips are most harmful to plants at the larval stage. 

Search for silvery grey trails along the tops of leaves and small black deposits (their excretion) on the opposite side.

The larvae feed by rasping the leaf surface, leading to distorted growth and loss of colour.

Take off infected leaves and apply Pyrethrum 5 EC / Protector as a foliar spray. This should eliminate Thrip Larvae in vegetative and early flowering growth. You can also try using natural Thrip Predators or Nematodes.


Malformed Pepper Leaf Caused by Thrips


Severe Thrips Damage on Pepper Leaves


Thrip Damage on the Top of a Pepper Leaf


Adult Thrips, Larvae & Waste on a Pepper Leaf


Thrip Damage on the Underside of a Pepper Leaf


Early Signs of Damage Caused by Thrips on a Sunflower Leaf

Fungus Gnats are quite a common grow room problem. 

You know you’ve got them if you have small, slow moving black flies in your grow room. Yet it’s the larva that upset your plants, eating roots and slowing overall growth.

Lower your grow room humidity and growing media water level. That’s normally all you need to do.

If not, add Nematodes to the root zone – they’re 100% effective and are bound to do the trick.  

Don’t forget to place sticky yellow traps around your grow room to helps capture the flies and identify the problem early.

Adult Fungus Gnats

Fungus Gnat Larvae

Close up of Fungus Gnat Larvae

Close up of Adult Fungus Gnats

Leaf Miner Larva look similar to Thrip Larva, but they move far more slowly and are less of a risk.

These tricky little critters live within the leaf itself, so you’re only really likely to notice foliar damage – not the actual larva. Look out for colour loss and leaf imperfections caused by them feeding.

A good spray of Pyrethrum 5 EC / Protector promises to deal with the problem.

Don’t worry too much about harmed leaves – the vigorous growth rates of plants under HID lights means that they’ll often outgrow any damage.


Leaf Miner Damage


Leaf Miner Damage


Leaf Miners on a Plant