Inconvenient Truths You Need To Know About Chilli Thrips

by Rita Perwich, Consulting Rosarian, San Diego Rose Society

Author’s note: In 2020 I received an Award of Merit for my article entitled, Ten Inconvenient Truths About Chilli Thrips. I have revised the article. Inconvenient Truths numbers 1-9 remain inconvenient but true. But my concluding opinion, Inconvenient Truth #10 that spraying to combat this pest is necessary, has changed…and accordingly so too has #10.

Hot days and dry Santa Ana winds have been the kick-off for chilli thrips season for the last five years in many California rose gardens. The following are the inconvenient truths you need to know about chilli thrips. ABOVE: Chilli thrips’ signature scorched bloom and foliage

1. You are battling an invisible pest. We cannot see chilli thrips at any stage of their life cycle (egg, first and second instar larvae, prepupa, pupa and adult) with the naked eye. (Chilli thrips measure 0.016 to 0.024 inch in length, about one-quarter of the size of the western flower thrips.) By the time we see their first damage, they already have an established life cycle underway in the garden.

2. New chilli thrips’ damage is not easy to spot. Chilli thrips larvae and adults have caviar tastes and extract sap from new growth and tender buds with piercing and sucking mouthparts. The first sign of damage is quite innocuous and innocent and does not generate alarm. There is a slight marbling of red and green on the back of fresh new foliage.

Sometimes there is a slight crinkling, curling or puckering in the new foliage. This is followed by dirty brown streaks on the back of the new leaves and brown or bronzed-tinged buds. Next comes the chilli thrips’ unmistakable signature: deformed and scorched blooms. Severe infestation can defoliate a plant.

3. The speed of the life cycle and damage is horrifying. In hot weather the life cycle can repeat from egg to egg in 11 days, and, within that period, female chilli thrips can lay 60-200 eggs. Because of this rapid-fire reproduction and because chilli thrips can be transported 60 feet on a breeze, delay and hesitation with this pest means that damage can become horrific and spread to roses and other host plants all over the garden.

4. They hit us where it hurts the most. This pest takes advantage of a rosarian’s natural disinclination to sacrifice new growth and buds. The new growth we celebrate is the prime tender target of chilli thrips. Don’t neglect to check the new growth at the base of the bush.

5. When in doubt, you must cut it out. If it is the season for chilli thrips and your new foliage is looking remotely questionable, just take a deep breath and cut it out. The growth you decide to observe for a day or two because you are not sure whether it is chilli thrips’ damage can transform into visibly distorted growth overnight especially when it is hot. Bag up the cuttings and don’t put them in your green waste.

6. Chilli thrips’ predators are not in full force yet. I implement the cultural, biological and mechanical controls of Integrated Pest Management (IPM) in my garden so I am able to avoid the use of the fourth control which is identifying the pest before using the least toxic pesticide. But in the management of chilli thrips, we have some disadvantages. We are handicapped in the mechanical control as we cannot squish a pest we can’t see and we can blast with jets of water but we can’t see if this is effective. Presently, the biological control is inadequate because these pests immigrated from southeast Asia without their predators. Minute pirate bugs and the larvae of the lacewing and syrphid fly do attack them but in our gardens right now, chilli thrips are pests without sufficient enemies.

7. All colors of roses are susceptible. I never thought I would find anything good to say about the western flower thrips but here it is: at least they don’t attack foliage, and they favor only white and yellow and pale color blooms. Chilli thrips, on the other hand, love all colors.

8. The life cycle takes place in hard-to-control areas. Before chilli thrips, I had nothing good to say about spider mites and rose slugs, but now I have a new appreciation for them. We can visually see them on the underside of leaves, and we can target them when we blast them with jets of water. During the course of their life cycle, chilli thrips are in the unopened bud, in the leaf, on the leaf, in the grooves where leaflets attach to the stems, in the bloom and in the soil or litter on the soil. And, at every stage, we cannot see them.

9. Fall pruning and fertilizing provides a feast of fresh growth. In San Diego, rosarians fall prune and fertilize their roses on Labor Day weekend to stimulate new growth. The continuing heat, Santa Ana winds and the feasting bonanza of tender growth promote this pest’s rapid reproduction and damage. Researchers have found that high nitrogen and phosphorus in plant leaves appear to contribute to higher numbers of chilli thrips.

10. The most inconvenient and disturbing truth of all. Some rosarians may decide to spray the new growth on their roses preventively in the hot months. Here is what you need to know: Ultra-fine horticultural oil smothers chilli thrips and other soft-bodied insects it contacts. Captain Jack’s Deadbug Brew with the active ingredient of Spinosad is an organic pesticide that targets chilli thrips and will not harm ladybugs, green lacewings, minute pirate bugs, and predatory mites. However, it is toxic to bees exposed to treatment for 3 hours following application so spraying must be done in the evening after the bees are done working for the day. Additionally inconvenient is the fact that chilli thrips can evade pesticide application as much of their life cycle and feeding is within the leaf, and within the terminal buds and unopened bloom. Hugely inconvenient is that systemic pesticides must be rotated with pesticides with a different Mode of Action to prevent pest resistance. Extremely disturbing is that many of these pesticides have active ingredients that are highly toxic to bees, and also to minute pirate bugs, lacewings and predatory mites or spiders that help us in our fight against this and other pests.

With this pest, in addition to cleaning up leaf and petal litter, we must each make a thoughtful analysis and adjustment of our summer and fall rose routine. I plan to stimulate less new growth on my roses by adding an organic fertilizer only once this summer, and I will not do a fall pruning. I have already re-smothered my soil with a thick blanket of worm castings and fresh organic mulch and I will be present daily with pruners ruthlessly at-the-ready.

The path to growing roses is not paved with just blooms. Chilli thrips are a challenge and I considered reaching for a spray bottle. But spraying pesticides is not a slam dunk for a garden with no pests and it could be a slam dunk for a garden with no beneficials. Besides, I am sure that there will soon be plenty of beneficials with a huge appetite for this pest.

All photos by Rita Perwich