How to Kill Thrips Organically on Rhododendrons and Other Plants

Thrips are a tiny sucking insect that pester Rhododendrons (particularly many older varieties) and Azaleas, some evergreen Viburnums, Photinia, and occasionally other plants in the coastal Pacific Northwest.

You can tell you have them because your ordinarily green leaves will develop a silvery sheen on them, while the undersides of the leaves will get little black spots from the thrips’ feces. Click here to see the silvery sheen caused by thrips.

While thrips can be a hard pest to get rid of, there are some very effective organic and biological controls you can use.

A note about thrip treatment:

You should know that the damage from thrips, that silvery sheen, won’t go away even after you kill the pest. All you can do is keep the thrips from damaging any other leaves.

So don’t judge the effectiveness of your treatment by whether the thrip damage goes away – rather pay attention to your undamaged leaves and make sure they are staying healthy.

Organic control for thrips:

Remove the infected plant:

Thrips are so tiny that once you can see the silvery leaf damage from them, you have enough of a problem that it can take some doing to get rid of them. If it’s a plant you don’t care for anyway, I’d take the excuse to get rid of it.

To keep them from icking up your next plant, take the cheap solution and put something there that isn’t susceptible to thrips.

Or, if you are-set on replanting the same darn thing, you could drop $30 on the biological control Hypoaspis miles, which works best in warm, moist soil, and will kill the pupal stages of thrips leftover in the soil. Then, follow the preventive care tips at the bottom of this article.

Take off infected leaves:

Thrips spend part of their lifecycle actually living inside of affected leaves, so if you see leaves that have severe damage, removing them can help control the population. The damage you see from thrips won’t heal – all you can do is keep new leaves from being hurt – so taking off the worst of the infected leaves is a good idea.

This won’t solve the problem on its own though, so do this in conjunction with another way of getting rid of thrips.

Pyrethrum spray from Chrysanthemums:

My local Rhododendron expert Don Wallace recommends using a Pyrethrum spray when the thrips are most active, in summer. He says the key to success with this kind of spray is doing three applications, three weeks apart: the first spray gets the adults, the second spray gets the recently hatched babies, and the third spray is the “cleanup” phase where you get anything you missed.

Now you should know that Pyrethrum sprays are indiscriminate in what they harm – they’ll harm thrips, they’ll harm the beneficial mites which might have otherwise eaten your thrips, and they’ll harm you in great enough quantity. But the advantage to them is that they stop being toxic very quickly after application, so beneficial bugs that wander in days later won’t be harmed.

Spinosad spray from soil microbes:

Spinosad (pronounced spin-OH-sid) is a somewhat new spray out on the market, brewed from, of all things, a soil bacteria found underneath an abandoned rum distillery in the Caribbean.

Spinosad is extremely effective, yet it’s only mildly toxic to humans and birds, and won’t kill the beneficial mites which are trying to eat your thrips for you.

Why isn’t it perfect? Well, in addition to stickin’ it to the thrips, it also kills caterpillars for about a month after spraying. I don’t know about you, but I like butterflies and I’ll happily tolerate some caterpillar damage to have butterflies in my garden later!

So if you have plants that are sheltering caterpillars nearby, consider using the Pyrethrum spray instead, since it’s only toxic for two days.

Spraying tips:

Whatever you spray, try to hit the thrips in summer while they’re most active, and be sure and coat the entire plant, starting with the undersides of the leaves at the center of the plant and moving outwards, then coating the tops of the leaves. Getting good coverage is with your spray is critical for it to work.

Happy honeybee on apple blossom

Both organic sprays are harmful to honeybees while wet, so spray in the early morning or at dusk so the spray can dry before bees are active, and don’t hit plants that bees really like.

(And of course, even though the sprays are organic, please suit up in whatever fashion the bottle tells you to – long sleeves, goggles, etc – organics generally have less impact on the environment, but they aren’t non-toxic.)

Biological control of thrips:

Biological control is when you bring in some beneficial bugs which either eat or lay eggs in (ouch!) the bugs you don’t like (see a gross photo here). For thrips, there’s only one that will really have an impact in our cool coastal climate – a predatory mite called Hypoaspis miles.

Hypoaspis miles mites live in the soil, and eat thrips when they drop to the soil to pupate. Since they only get thrips in that particular stage of life, they can’t be counted on totally eradicate an existing thrips problem – but they can have a big impact on next year’s population because they’ll interrupt the lifecycle.

Hypoaspis miles often come in a packet that you hang from the plant, or a canister you shake into the garden, and they work best when the soil is warm and moist. If you have an irrigation system or water regularly, then go ahead and use these little mites in late summer when the soil’s nice and toasty.

Hypoaspis miles are about $30 (you get a LOT of mites for that), which seems pricy until you consider that a bottle of Bayer Advanced for Shrubs, the least harmful synthetic control, can cost you $24.

If you have thrips in a greenhouse situation, or if you live someplace where soil temperatures can be 65-80 degrees F, you might try Neoseiulus cucumeris mites along with your Hypoaspis miles mites.

Neoseiulus cucumeris (let’s call them “Neo” mites) eat thrips when they’re in their larval stage of life, so by using both types of mite, you’re really cutting down the chances that your thrips will make it to adulthood.

Best thing about Neo mites? They are dirt cheap, like a couple bucks. If you’re unsure if your soil temperature is hot enough, adding Neo mites to your Hypoaspis order is a cheap thing to try, and it might just help.

My recommendation for getting rid of thrips?

If your plants only have a mild problem, like only a small percentage of leaves are affected, you’ll probably do fine by just removing the leaves that have the pest problem, raking up leaf litter and weeding under the plant to reduce hiding places, and spraying using either organic option I mentioned.

If you have a moderate problem, remove the leaves that are most affected, rake and remove leaf litter/ weeds under the plant, and spray using Spinosad, which has a minimal impact on predatory mites. Wait two weeks (so Spinosad residue won’t harm mite reproduction), then release Hypoaspis miles mites into your garden if your soil is somewhat warm and moist.

If you have a severe problem to where your plant is covered and you look at it and go, “ick!”, then give it up and get a new plant (see above), then follow my guidelines below for preventing a new problem.

A note about non-organic control:

Most synthetic treatments for thrips are really quite nasty, which makes sense, as thrips can be hard to get rid of. I wouldn’t spray Orthene (acephate) or any other organophosphate pesticide in any place humans go. Seriously, chronic nerve damage is not cute.

There is one synthetic control that isn’t quite as bad as the others, called Bayer Insect Control for Trees and Shrubs. Its main ingredient is imidacloprid, and there’s a liquid concentrate you can water into the soil. The plant takes up the pesticide from its roots and holds it in its leaves so that any insect that eats your plant will die, but it won’t harm people that touch the leaves or birds that fly onto the plant.

Three problems with it: one is that it kills earthworms and beneficial soil microbes, which are like your best friends when it comes to keeping plants healthy – they break down the organic and mineral matter in your soil into usable nutrients for your plants, and keep soil fluffy and aerated.

The second problem is the whole food chain thing. You get a couple of caterpillars eating your treated plant, a bird comes by and eats the caterpillars, and you’ve just exposed your friendly neighborhood bird to poison. I don’t know how many bugs a bird has to eat to get sick, but imidacloprid is very toxic to birds, so I wouldn’t imagine many.

Third problem? Honeybees who eat the pollen from a treated plant may experience disorientation or death. I love the honeybees, and we need them for pollination.

Imidacloprid lasts for six weeks, as compared with Pyrethrum which kills pests then breaks down in a day or two, or Spinosad which is less toxic to birds.

Imidacloprid is probably the least harmful synthetic to use for thrips, but my preference would be to use the organic or biological controls available, because their impact is so much less profound on soil, wildlife, and beneficial bugs.

Preventing thrips in future:

Take care of your soil, water regularly, and apply mulch. As a rule of thumb, pests find it easiest to attack plants that are already sickly or unhealthy in some way, and I most often see thrips on plants that are in hard, dead soil, with no mulch or regular water, so if that’s your garden, a little preventive work can go a long way towards preventing future pests.

Rake up leaves and litter and remove spent blossoms on susceptible plants. Thrips do live some of their lifecycle actually inside of plants’ leaves, and they also shelter in weeds and leaf litter under plants, so if you rake up the leaf litter from affected plants and remove the spent flowers, it can go a long way towards keeping things free of pests.

(If you rake up litter that you suspect has pests in it, take it to your county’s green waste facility or burn it. Home composting setups usually don’t get hot enough to kill garden pests.)

Don’t shear plants that are susceptible to thrips. Shearing can encourage lots of soft leafy growth, which is very attractive to pests.

Don’t over-fertilize. Similar to above, if you fertilize more than is recommended, it encourages soft fleshy growth, and all kinds of sucking insects – aphids, thrips, etc, will love you for it.

I hope this gives you some options the next time you’re faced with a thrips problem in your garden. Thrips can be hard to get rid of, but these are the safest and most effective controls out there for tackling it.

What plants have you faced a thrips issue with? What worked and didn’t work for you? Let me know in the comments below.

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Comments

  1. says

    I’m glad I don’t have anything to add on thrips because I don’t have them (knock wood). The only bugs that ever do any damage in my garden are Japanese beetles which like my (wait for it…) pussy willow! (I don’t have roses.) They’re never too bad, though, and I don’t do anything to control them.

    Monica’s last blog post..Out on the Streets: June 2009

  2. says

    Just figured you would want to know. That damage is not caused by thrips. Thrips are typically only an issue indoors in the Pac Northwest. Way too wet up here for the little buggers to be outside, typically. Anyway. the damage you described is caused by lace bugs, Stephanitis Rhododendri. And there is no way to rid your plants of them “organically”. The bugs will feed on any stressed plants, i.e. drought or heavy sun exposure. they appear just after rhodie flowers drop in spring/summer as the new succulent leaves break out from there buds. So your best bet is to plant healthy varieties and remove susceptible species and maintain health and vigor of established plantings.

    Nels Tronsen’s last blog post..Bee’s in the borage

  3. Annette says

    This is a really helpful post! Thanks so much. I don’t like to use any toxins in my garden, so I’m going to remove a badly infested bergenia bed and start over with something there that is not susceptible to thrips. There is a lot of really useful information in your post!

  4. Jeanne says

    My side yard, which has cement, soil with gravel on top, and a small area of soil with some ivy and a small shrub, has hundreds (maybe thousands) of thrips. The pest control service has sprayed it 2-4 times a year for the last three years. I didn’t notice the thrips until after the gravel was brought in. The side yard is a dog run. I am concerned that the spray is toxic to my dog. I’ve recently seen thrips in the backyard too. I worry about my dog, lizards and good insects that inhabit the yard. What can I do? Would treating the area with a mixture of Clorox and water help? The thought that these pests may be crawling on my dog is really upsetting!

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