Thrips and the Cannabis (Marijuana) Plant.

by Alex Robles

 When it comes to thrips, identification is key, because there are about 4,500 different species of them and they don’t all feed on cannabis.  The most dangerous threat to cannabis from thrips is the Frankliniella occidentalis, also known as the Western Flower Thrip.

Western flower thrip adult (UC Davis Department of Entomology)

What are they?

  • The adult thrips are going to be cigar shaped and are yellowish-white with some dark markings on them.  They have wings that let them glide or drift on the wind.
  • Adults can live for up to 30 days and the females can lay 2-10 eggs a day.
  • Females of most plant-feeding thrip species lay their elongate, cylindrical to kidney-shaped eggs on or into leaves, buds, or other locations where larvae feed.
  • When the weather is warm, the life cycle from egg to adult may be completed in as short a time as 2 weeks.
  • Thrips hatch from an egg and have two actively feeding larval stages and two nonfeeding stages, the prepupa and pupa, before becoming an adult.
  • The larvae or nymphs will look similar to adults but they’ll be pale or translucent in color and won’t have wings.  A some species of thrip larvae are going to be brightly colored, like the distinctive reddish-orange larvae of the predatory thrips, Franklinothrips orizabensis and F. vespiformis.
  • In 70-degree(F) weather the trips will go from egg to adult in 19 days.  If the temperature goes up by 5 degrees that growth period can go down to 13 days.

Environment

    • Thrips love the constantly warm environment of greenhouses and indoor grow rooms.
    • They also love to feed on buds and new leaves, so start looking for them at the top of the plant and work you way down.

Damage

  • Thrips will feed by puncturing the plants outer skin and sucking the sap out of it.  This is going to cause stippling (spotted appearance that can interfere with photosynthesis) on the leaf.
  • The edges of the leaves will look burnt when thrips are feeding on them.  Other things could also cause leaf edges to burn, like heat/drought stress. So make sure you identify the source before you jump to any conclusions.
  • Since thrips also suck out the chlorophyll, the leaves eventually get brown and brittle.   Before that the leaves can start to look bronze or silver, which will also interrupts photosynthesis.
  • Thrips usually leave behind frass (poop), that looks like coffee grounds, as they feed.
  • Western flower thrips have also been known to carry Impatiens necrotic spot virus and Tomato spotted wilt virus.
  • Before taking any kind of action, look carefully for the thrips themselves to be sure that’s what the problem is.  Below are some symptoms of thrips damage.

Symptoms

  1. Bud and young leaf damage
  2. Shiny, silvery spots on the stems and/or leaves
  3. Dry, brittle or brown leaves
  4. Bronze spots appearing on leaves and/or stems

How to fight them.

  • Keep a clean grow area, throw out any dead foliage that could give thrips a place to hide and multiply.
  • If you think thrips are the problem you can check your plant by branch beating or gently shaking the branch over a white sheet of paper.  You’ll be able to them moving around on the paper, but your going to need some magnification to a good look at them.
  • You can also use yellow sticky traps to get a better look at what you’ve got.
  • Feeding a plant too much nitrogen has also been known to attract thrips.

Neem oil:  Will strip the waxy outer protective layer (cuticle) of an insect’s body, which doesn’t allow them molt.  Neem doesn’t affect the eggs, so will have to reapply foliar spray every 5-8 days. Use as a  fungicide, antifeedant, mix 3 oz (6 Tbs)/gal water or 2 tsp/pint water.  You can use it as a foliar spray or soil drench.

CUCUMERIS [Amblyseius (=Neoseiulus)cucumeris]: Targets the Western flower thrips (Frankliniella occidentalis), Onion thrips and broad mites.  The ‘Cucumeris’ is a species of predatory mite that feeds on immature stages of thrips. It also feeds on pollen, two-spotted mites and other species of mites.

Hypoaspis miles are predatory mites that live in the top layer of the soil that target fungus gnat larvae, western flower thrips pupae (WFT) and account for up to 30% of WFT control.  Hypoaspis is a native species of soil-dwelling mite, which feeds on small insects and mites. It has a swarming behavior on all larval stages of pests and moves rapidly over the soil surface.

Green Lacewings.  Are  beneficial general predatory insect that attacks insects and insect eggs, such as aphids, small caterpillars, mites, whitefly, scale, mealybug, thrips, psyllids, and other soft-bodied insects.

I hope this helps you better understand what you’re up against if you have thrips.  Don’t forget that identification is key because what you might think is thrip damage could be something else.

Grow Learn Teach

 

Yellow sticky Trap
Neem Oil

Resources:

UC -IPM http://ipm.ucanr.edu/PMG/PESTNOTES/pn7429.html

UC Davis  http://entomology.ucdavis.edu/Faculty/Larry_D_Godfrey/Lab_Page_510/Ongoing_Projects/Cotton/Background/Shafter_Research_Laboratory/Thrips_Research/

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