Thrips are one of the most serious pests within the commercial plant production industry, because once they have entered a glasshouse it is virtually impossible to eradicate them in the short or long term. The thrips life cycle is short, so resistance to insecticides, even new ones, is built up quickly over a relatively short period.
Indoor growers do not face problems on this scale, it is possible to eradicate thrips, and once this has been done insect screening material can be stretched over air intakes to filter out thrips, spider mites, and many other pests. If carefully screened in this way there is no reason why any more should enter and colonise your growroom, but, a larger extractor may be required to compensate for the air restriction.
Eradication may not be easy but the effort required is the only way to ensure long term peace of mind and high quality crops. These little buggers not only munch on your leaves and buds, but crap all over and in them as well.
The larval form of flower thrips appear worm like and range from pale yellow to light orange. Since thrips are quite small and their pale-coloured nymphs are difficult to detect, using magnification when looking for them is recommended. The presence of thrips gives a splotchy appearance to leaves. There can be thousands of them inside your buds if they are not controlled.
Female thrips deposit barely visible eggs under the surface of leaves and inside the buds/flowers once they start to form. The following are the life stages and their approximate duration:-
1st larval stage (1-2 days)
Pre-pupa (1-2 days) in the soil
Pupa (1-3 days) in the soil
Adult (can live up to 45 days) Cooler temperatures slow the rate of development and warmer temperatures speed development.
Damage They use rasping-sucking mouth parts, both larvae and adult thrips cause a distinct splotchy appearance on leaves by gouging deep grooves nearly through the upper surface of the leaf. Because they feed on leaves, the growing tips and flowering parts, they are very destructive. Typical thrips damage includes structural damage to foliage: rolling, blistering, discolouration, splotches, and occasionally leaf drop.
Thrips species are also known vectors of certain plant viruses, they are also the main carrier of leaf fungal infection. This makes it even more important to eradicate them as soon as you find them.
Control and Eradication:
The main Thrips Predator Mites (Amblyseius cucumeris) are only effective where humidity is between 70-90%, this does not really suit cannabis, If the humidity drops below 70% the predator mites stop feeding and die out. Reports have been very poor in low humidity environments, so the use of these predators in interior locations is only worthwhile with high humidity levels, so not really recommended for the indoor growroom.
Equipment and Insecticides:
You will need a good pump up garden sprayer such as a killer spray, get one with a hose and a wand. Not a hand held sprayer or worse still a bug gun. We are only going to consider low toxicity short life Insecticides as cannabis is a product that is eaten and smoked. I recommend that you use a combination of an insecticide and soft soap.
Only buy insecticides in concentrate forms.
Pyrethrum may be the safest option but it is very unlikely that will kill them all, as most thrips have resistance to it.
The newer types of synthesised pyrethroids like “Plant Rescue Bug Killer Fruit & Vegetable Concentrate”, active ingredient = lambda-cyhalothrin and Bayers greenfly killer containing Deltamethrin are both said to be effective in eradicating thrips.
Insecticidal Soft/horticultural soaps are only capable of killing a few adults used on its own but when used mixed with an insecticide it thoroughly wets the insect and helps keep it wet for 2 to 3 times normal, this gives more time for the insecticide to work and for the soap to block the insects breathing tubes suffocating them.
There are several soft soaps on the market, safers soft soap is no longer marketed in this country, there others one made by Savona you can buy on amazon and uncle tom sells soft soap.
An idea of the equipment needed, the picture is out of date relating to spray product
Block your air input, turn off extraction and all circulation fans. Mix soft soap to its normal recommended concentration then add the insecticide at its maximum recommended concentration, mix well.. Pump the sprayer to maximum pressure, set the spray nozzle to its finest spray pattern. When you spray, cover every leaf to run off, this includes the underside of the leaves, stems and soil surface. Also mist surrounding surface area ie floors and wall etc.Turn off your lights and seal the room for the night. This keeps the plants wet for longer, these products are most effective while wet also light breaks down the active ingredients in the pyrethroids. Wash out the sprayer with a little washing up liquid and water then rinse well. Do not keep any mix left over it looses potency and goes off quickly. Always mix a fresh batch and use immediately for best effect. In the morning open the input duct, turn ventilation and circulation fans back on and wait half an hour before you switch your main lights back on. Repeat the process a week later whether you see any thrips or not. If any thrips are evident after the first spray you need another type of insecticide as your thrips have resistance to the first type you used.
Final Note: Good luck with your extermination.
I would like to thank Wilbur Nutsack for donating the feast and leaf damage pictures also his time helping me with this. I would also like to thank Mono and MrBenn for reading through and suggesting alterations to the text.
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