Thrips are one of the most common and elusive pests in the indoor garden. They can often live on your plants for months before being noticed at all and can be very hard to get rid of completely.
The reason thrips are so elusive, is that they are incredibly fast moving and can actually jump off of your plants leaves when you lean in to inspect them. Thrips generally live on the underside of leaves to remain out of sight and can move so fast that by the time you flip the leaf over they are already long gone. Their slender green bodies match the color of the leaves they feed on, which further adds to their stealthy nature.
Indications of thrip infestations are generally only noticed because of the damage they cause, rather than actually finding one of the insects themselves.
What exactly are thrips?
There are well over 6000 species of thrips in the world. They are very small (1 mm long) and very slim. Thrips can be either black or green, and although they are terrible flyers, they do have a set of wings that they use to travel to new areas. Their bodies are not well suited for flying, but often get caught in winds that can take them long distances away. This helps them to spread far and wide in order to attack new and unsuspecting gardens.
Thrips are part of the Exopterygota superorder of insects. This means that they are related to insects like lice, locusts, and termites. They reproduce by laying massive amounts of tiny eggs which they then place inside slits in the plant tissue. Hiding the eggs here gives them an extra level of protection and makes them very hard to eliminate completely with normal methods of pest control.
Why are Thrips a Threat to Gardens
Individual thrips will not cause much harm to your plants on their own. They are simply too small and insignificant. The problem lies in the fact that they multiply quickly, and are hard to eliminate from the garden completely. A large group of hungry thrips, can cause some serious damage to your crop in a relatively short amount of time if left unchecked for too long.
On top of this, thrips act as a vector for some plant viruses, which can wreak havoc on your crop long term. They have been found to host such viruses as Tospovirus, and up to 20 other plant viruses.
How Thrips Feed
Thrips eat by scraping away the surface of a leaf to suck out all of the sugary juices from the inside. This leaves a characteristic silvery appearance to the surfaces of your plants leaves, especially on the underside where they spend most of their time. You will often notice this before ever seeing the insect itself.
This small amount of damage is not usually a problem on its own, and a plant can easily cope with some leaf damage. In large numbers however, it is possible for entire leaves to be completely stripped of their leaf surface, and all of the nutritious juices sucked out.
In nature, thrip damage is usually kept in balance by all of the other insects that feed on thrips and thrip eggs. Unfortunately, inside a grow room these predatory insects are not usually present, and the issue can grow out of hand very quickly. They will leave many leaves drying out, curling over, and dying.
The Life Cycle of a Thrip
Thrips do not need to reproduce sexually, and many colonies have actually been found to contain only one sex. This means that with the entry just a single thrip insect, an entire colony can form.
The lifecycle of an average thrip is as follows:
- They begin their life as tiny eggs growing beneath the surface of plant tissue. This is usually underneath the leaves near the base of the stem.
- They then go through various nymphal stages where they may hide in the underside of a leaf or other dark spot where they await to reach adulthood. This can take anywhere from 1 to seven days.
- After this time, most species will drop off the plant and burrow into the root zone or surrounding leaf cover to pupate.
- Anywhere from a few days, to a few weeks later, once they have developed into adults, they'll fly to the nearest plant to begin feeding and repeat the cycle.
Knowing this lifecycle is important when considering the course of action best used to control thrips. Since they are so hard to kill at various stages in their development (especially eggs) it is important to understand how you may be able to cut off one or all life stages of this life cycle.
How to control thrips
1. Use a Spray and Be Persistent
The key to success with controlling thrips is to be persistent and use as many different techniques as possible. Using sprays such as pyrethrum, or neem are great for killing the adult thrips, and can even be useful at killing the pupils hiding in the root zone. These products will not reach the safely hidden eggs however since they are hidden beneath the surface of the plant tissue itself.
To get around this, it’s important to use repeated treatments to hit the thrips as they hatch and leave the safety of the eggs. Spray the entire crop, wait 2-3 days and spray again to kill all freshly hatched nymphs. Then repeat again over the course of about 2 weeks.
2. Use Barriers to Cut Off the Life Cycle
Another easy method that can be used for interrupting the life cycle of thrips is to cover the root zone with plastic. This can be done by cutting circles out of poly plastic sheeting.
As mentioned previously, thrips tend to drop off the plant during development in order to burrow into the surrounding leaf litter or roots to finish developing into adults. This helps thrips to survive the winter months while they hibernate in the leaf litter and root zone to wait out the cold, leafless winter months. By physically blocking the roots off, their life cycle becomes interrupted and will prevent them from being able to finish developing. An exposed thrips nymph will quickly die in the intense lighting heat if not able to burrow to safety in the roots of your plants.
This method is great for prevention and long-term treatment, but will not do anything to combat the existing adult thrips already munching away on your leaves.
3. Use Bugs to Kill Bugs
The most interesting option is to use predator insects or parasites. These can be fairly hard to use, and will not wipe out the thrip infestation completely. If used correctly, however, they will keep the pest in a manageable state of balance, and offers a great organic option for controlling thrips.
Predatory insects will not destroy all of the target insects. If they did, they too would die of starvation once there is no food left. Predator-prey relationships on any level will always find a balance at some point. When there is an abundance of prey (thrips) the predators will thrive and grow until enough of the prey has been eaten that it becomes hard for the predator to find food. At this point, many will die and a balance will be achieved. This balance keeps the prey (thrips) from ever reaching massive colony sizes and is perfectly manageable for your indoor grow to continue thriving, despite small amounts of leaf damage from the insect.
Some examples of predatory insects to use with thrips:
1. Thrips Predators (Neoseiulus cucumeris):
Thrips predators are a form of tiny, mast moving mite, very similar to spider mites. The difference is that they run around feeding on the eggs and larval stages of thrips instead of your plants themselves. They are fast enough to catch juvenile thrips, and small enough to reach the eggs hidden away under the surface of the plant tissue.
2. Pirate bugs (Orius insidiosus):
These are another useful predatory insect option. They are much larger than mites, but are capable of eating all stages of thrip development. They have the added benefit of eating mites and aphids as well.
3. Lady Bugs (Coccinellidae)
Ladybugs are not as effective at eating thrips as some of the other mentioned predator insects but are the cheapest and easiest to cultivate.
4. Lacewings (Chrysopidae)
Lacewings are a huge family of insect, and many different species will eat thrips but the generally preferred species is the green lacewing for its ability to be easily cultivated inside the grow room with nearly no effort. The larvae of these insects have a huge appetite and will eat everything from thrips to mealybugs and aphids.
For Optimal Results, Try Mixing These Methods Together
A good treatment regimen may be as follows:
- Spray the crop with natural short-lived pesticides such as neem or pyrethrum, and repeat a few days later. Making sure to get the undersides of all of the leaves.
Tip: Using a pump sprayer will make spraying the undersides of the leaves MUCH easier.
- Cover the root zone with plastic, and remove all severely infected leaves completely from the grow room.
- Then after a few days release predatory insects to keep them in check.
By following these steps and a high-quality pesticide, thrips will have a very hard time developing into anything more than a minor issue in your grow room.
Remember to be persistent and use a few different methods to hit this resistant pest where it hurts!