Thrips are one of the most widespread pests in ornamental and vegetable plants. There are thousands of species, and just as many opinions on the best way to treat them.

When browsing discussion groups and content on the subject, thrips are usually addressed as a generic problem. Solutions are often suggested for "thrips" in general. As soon as they are spotted, you might be advised to apply a certain product, change the soil entirely, etc.

We believe this is a flawed approach, and that this lack of nuance is the root cause of many failures and discouragements when treating thrips. Let us explain why.

An important distinction

We have chosen to divide the different species of thrips into two distinct categories: indigenous thrips and tropical thrips. The first group includes species that are naturally present in North America and adapted to our climate. In other words, species that survive our winter and are likely to attack plants outdoors. The second group consists of species native to other climates that are not naturally present here and that thrive in the warmth of indoor environments. They are spread from an indoor culture to another and are usually introduced with an infested plant.

Why does it matter, you ask? Well, it turns out that the individuals of these two groups have slightly different life cycles, and this detail changes everything.

Soil pupation

The pupae is the developmental stage of thrips between the larva and the adult. When preparing to pupate, indigenous thrips drop to the ground. They undergo this transformation in the top inch of the soil before emerging back into the foliage once they mature. For tropical thrips, this phenomenon does not occur. These thrips remain in the foliage at all times.

Understanding the strategy

For pest control to be effective, it must target all growth stages simultaneously. If you remove the adult individuals without ensuring the elimination of the pupae, you’ll only see a temporary improvement. Inversely, if you treat the pupae without targeting the other stages, new eggs will be laid, and the infestation will persist.

You’ve got it. Since indigenous and tropical thrips develop partly in different parts of the plant, they are not treated the same way.

The surface battle

For tropical thrips, all life stages occur on the foliage. You can dislodge most of them with a sufficiently powerful water jet. Green lacewings will be very effective against both the adults and the remaining pupae, while Amblyseius swirskii will target the larvae.

For indigenous thrips, only the larvae and adults can be treated on the plant. You can still perform a good cleaning with water. Neoseiulus cucumeris will be effective against the larvae, and green lacewings will again take care of the adults. However, you will also need to treat the pupae in the soil.

The underground battle

To treat the pupae of indigenous thrips, you can use soil predators such as Stratiolaelaps scimitus or nematodes. If you're particularly concerned and want to hit hard, combining these two soil predators is especially effective.

How to differentiate them

Now that we understand the differences between these categories of thrips and their treatment, we only need to differentiate them.

The type of plant being targeted can provide clues as to which category of thrips you're dealing with, as the origin of plants and their pests are usually linked. Indoor plants, like the thrips that affect them, are usually of tropical origin. The thrips that target garden plants, such as vegetables and herbs, are typically indigenous species.

You can also diagnose them by their color. While the larvae of all thrips species look similar, the color of the adults is more revealing. Simply ask yourself: are some of my thrips black or dark brown? If the answer is "yes", you are most likely dealing with tropical thrips. Otherwise, you are probably facing indigenous thrips.

Conclusion

In summary, if you are dealing with dark or black adults (tropical thrips), you can treat the plant by introducing lacewings and Swirskii. If you are dealing with light beige or pale brown adults (indigenous thrips), you will need to treat both the plant and the soil. You can use Cucumeris and lacewings, as well as nematodes and/or Stratiolaelaps.

In all cases, identify the culprits before investing time and money in treatment, and beware of solutions that don't take these differences into account.

You are now in a better position to win your battle against thrips. For more information, check out our pest fact sheets on native and tropical thrips.

Good luck with your fight!