Thrips ARRRRGHHHHHH!

Posted: November 26th, 2020 - In: Plant pests

There are pests and there are Thrips.  I think if I have a gardening nemesis it is them.  Tiny weeny little critters that just seem bullet proof.  Honestly, I think I have tried every remedy, oil and bug killer out there and only now am I starting to get on top of the problem.  Fortunately my thrips outbreak is restricted to only one large monstera deliciosa that lives in my upstairs bathroom, so I have been able to fully quarantine her so the rest of the plants in the house have not been infected (touch wood).

 

So let’s talk about Thrips

Thrips are tiny winged insects that’s are generally less than 2mm long as adults.  They feed on your beloved plants by sucking the juices out of individual cells  from the leaf blade or petiole. Entomologists have described over 6000 species globally. I think the ones I am hosting are the Western Flower Thrips (rankliniella occidentalis).  Although they have wings in their adult form, these are only suitable for short hops rather than long distance flying. Therefore, they are unlikely to spread too far if you are able to quarantine your plants quickly enough. Interestingly the singular name for this insect is Thrips as well as the plural.

The lifecycle of a Thrips is a complicated one and is made up of several stages, called instars. The female Thrips will cut a small slit in her chosen section of plant into which she will lay her eggs. From there the larvae will emerge and commence the attack on your precious plant!  The larvae which make up the first two stages are tiny wingless critters that you will see moving around on the leaves of your plants. At this stage they are already feeding on the tissue of your plants by piercing the surface layer and sucking out the plant’s juices (nice!). In the third, fourth and fifth instars the Thrips stop feeding altogether and go through a resting stage, similar to the pupae stage of a butterfly, during which their body metamorphose int0 adult form.  Eradicating Thrips is most easily done in the first three stages while they are eating and their body is more vulnerable to your chosen method of killing them.

First and second Larval Instars

Thrips are a serious problem in global agriculture, they are a vector for several viruses that have a huge impact on fruit and vegetable production.

 

Know your enemy

If only it was that easy! Unfortunately, what I have learnt over the last seven months of battling Thrips is its not a simple spray and walk away exercise.  You must take a considered approach over an extended period of time if you want to eradicate them altogether.  I started with a Neem oil mixed with insecticidal soap that appeared to work after it was  applied a few times, but after a few weeks they returned. I then tried a product called SB plant invigorator and bug killer which again worked quite well for a time but once again they returned.  It was at this point that I realised that I was missing something in the battle and it was part of the life cycle of the Thrips that was catching me out.  As I have described above, the female Thrips will lay her eggs under the surface of the leaf where they will stay nice and cosy in a state of dormancy until such time as the conditions are suitable for the Thrips to hatch out and begin its lifecycle.  The key thing to understand is that once the egg is protected under the surface of your plant there is nothing that you can spray on that will get to it unless you decide to go down the route of systemic pesticides and even then, there is some doubt as to whether this is effective at killing the eggs.  In effect the plant itself is shielding the egg from whatever your chosen method of eradication is. Clever Thrips! So the key to success is in fact not a single product or method but a considered long term approach to eliminating the little blighters.

My approach to Thrips eradication

Thrips, alongside may other typical pests such as spider mites don’t like high humidity but unless you are able to keep your grow area in a humidity of 75-90% constantly, I really don’t think humidity in the home is a useful method of control.  I don’t think misting helps at all either, I regularly give my plants a warm shower during which I rub down all of the leaves on both sides and the full petiole will be sprayed with the shower head.  Its surprising how much pressure is required to dislodge pests which is why I don’t think misting makes any difference at all. Doing this regularly also gives you to opportunity to closely inspect your plant for signs of pests.

If you spot Thrips you must first evaluate just how extensive the infestation is. If it’s significant then it may be worth going down the full chemical route. There is a product called Spinosad which is very effective apparently, but is not available in the UK so I can’t confirm that.  I have used Ultimate bug Killer once when I’d had enough of the Thrips coming back which worked very well to remove the visible Thrips but it left a horrible residue on the leaves and I am not really a fan in using insecticide in the home.   As I have eluded to, removing the visible Thrips is just the start of the battle.  You need to do regular repeat treatments to eradicate the emerging Thrips until you have managed to break the lifecycle.  If you don’t want to use chemicals then there are two options available to you. The first is the use of predatory mites. This method requires you to purchase a mite called Amblyseius Cucumeris.  These little guys are a great way to kill off your Thrips present in first three stages of life.  However, I have found that once the visible Thrips have disappeared, these mites will die off through lack of food.  When the next wave hatches you have to purchase another batch of mites. I think this is a great approach if your infestation is relatively small and isolated but if it is larger, this will prove to be an expensive solution.

The second option and my preferred method is the use of a Neem oil solution. My recipe is not revolutionary and it’s a formula that many users have tried in the past with varying degrees of success.  The difference is that I add Winter Savory essential oil to mine in a very low concentration.  I came across a really interesting paper  in The Journal Of Economic Entomology while researching Thrips where several different essential oils were shown to have a positive effect on repelling Western Flower Thrips.  Whilst I have not followed the methodology in my application I thought a low concentration was worth a try. I have absolutely no idea if its making any difference at all but its not doing any harm to my plants either.  The key for me has been the regular repeated application and ongoing use of this product even when you think the pasts are all gone.

Treatment of all plants in the vicinity of the infected plant is essential as these little blighters can and will hop/fly from plant to plant.  Even if it looks pest free you need to treat; a single 1mm insect is all it takes to start an infestation, due to them being asexual.

In order to work, the spray has to make contact with the Thrips.  The oil in the water will coat the insect and suffocate them by blocking the pores through which they breath.

 

General formula for Neem oil.

 

Once you have collected your ingredients use half of the required water and add it to your spray bottle. If you add all of the water when you shake it up you will just end up with a bottle full of foam.

Add the Neem oil, washing up liquid and essential oil to the water and shake the bottle until you are happy that the solution is well mixed.

Add the remaining water and gently shake the solution.

Now its time to spray your plants. This is a messy and to my nose quite a smelly process so I would highly recommend doing this in your shower or bath. Spray the plant from top to bottom ensuring that you get your solution on both sides of the leaves and into every nook and cranny. You need to have the solution running off the plant. Don’t skimp on this stage, use plenty of solution.  Once you are happy that you’re plants are well soaked in the Neem oil solution, leave them in the shower for five to ten minutes to allow the oil to do its work.  Its at this stage that most people will recommend washing off the residual oil but I do a second application because it doesn’t matter how thorough you were first time, you will have missed some areas. This is the opportunity to give them a second dose from top to bottom including the surface of the soil. The spray is cheap so don’t skimp on it. Now leave for a further five minutes.

After two thorough applications, any of the Thrips living on the surface of the plants will have had a good coat of your formula and it will be doing its thing.  Now turn on the shower and give the whole plant a good rinse making sure once again the both sides of the leaves are cleaned off. Use your hands to rub off the solution rather then just let the shower do the work, Its an oil and will not come off that easily.  I have left the solution on the plant previously but found that it left oily spotting on the leaves that was unsightly hard to remove later on.

Ta-Da you have a Thrips free plant – No you don’t!

Don’t forget the eggs that are cosy under the surface of the leaves and also that the latter stages of the Thrips larval instars are not so easily affected by this kind of method.  Now you need to make a note in your diary to repeat this process every week for at least five weeks.  I know it’s a royal pain in the backside but if you don’t repeat you will never break the lifecycle which in general lasts 30 to 45 days you must kill off each and every Thrips as it emerges from its egg and before it has the opportunity to lay more of them.  Development from egg to adult takes approximately 13-19 days depending on the ambient temperature, during this time you should have made two applications of Neem oil and the Thrips wont have a chance to lay any eggs.

Other things that you can do to help.

 

Here is a little reminder of what you’re up against…. Good luck