Diatomaceous Earth (DE) - Organic Insect Control

Diatomaceous earth (DE) is a non-chemical pest control agent which uses a 'mechanical' action to which no resistance can ever be developed by the pests it works on.  DE is a fine powder, derived from the sediment of ancient freshwater lakes which contain the remains of simple organisms known as diatoms, which have a silica based 'skeleton'.  It is these fossilised, skeletal remains that provide the pest control as these microscopic shards of silca are deadly to the pest, but completely harmless to humans.  To the insect or mite, it is akin to crawling across a surface covered with broken glass, the shards pierce or damage the skin of the insect, even those with hardened carapaces as they all have weak spots at the joints etc.  Once pierced or otherwise damaged, the 'victim' dehydrates quickly, assisted by the absorbant nature of the earth, and dies. 

Red Spider Controlled by DE on Alocasia leaf

It is effective against insects, arachnids and acarids (mites), including their larva (but importantly not their eggs, so repeat applications will be needed to control a population that has already started the breeding cycle).  It is not harmful to earthworms as it only works when dry. It should not affect larger winged insects (Bees etc) as they would have to fly through clouds of it to have any adverse effect. It works at any temperature, unlike organic biological controls, and can be applied before any pests are spotted so as to reduce the chances of damage occuring before you spot the pests.

It is probably the only reliable method of controlling the 2-spotted mite (aka Red Spider Mite), but works on almost every single garden or glasshouse pest. Other pests that can be controlled are aphids, thrips, whitefly (adults), mealy bug, Lily beetle, earwigs, all other mites, cockroaches, sciarid fly (and larvae) and many others. 


For horticulture, there seem to be 3 methods to apply your DE.

1. Puffer bottle or dispenser to literally puff a fine layer of DE on to the target area - which should be dry as this will only work when dry - try to avoid large clumps or thick layers on plants as this is wasteful and can slow down photosynthesis (which would weaken the plant)
2. Make up or soft paint brush, which can be dipped in the DE and then brush it or flick it on to the target area - this method is wonderful where aphids are clustering on stems as it allows you to target the groups of pests whilst keeping the DE away from beneficial insects.
3. Dissolve 15 grams of DE in one litre of water (add the DE to water and shake well).  Spray on the affected plants so as to wet them, but not so that it runs off.  Allow to dry and it will give a very thin but even coat.

Whichever application method you use, bear in mind that it only works when dry - so if you wash it off when watering or the rain removes it, you will need to re-apply it.  If you can see the DE on the plant, then it is still working, if you cannot see it, then you will need to re-apply.

Once you have killed off the pests (can take 2 or 3 applications to catch all generations and prevent re-population), you can then discontinue its use until you notice pests again.  
Because the DE is food grade there is no minimum interval between application and harvest.  Just wash fruit and veg as you normally would.

If you apply a lot of DE or have any respiratory issues then you should wear a face mask when using it dry. 

There are hundreds of uses for DE, here we are just describing methods to control pest in horticulture.  A rule of thumb application rate for control would be around 2 - 4 grams per square metre of leaf surface being covered at each application - this amount will remain effective until it is removed by rain or watering.