Garden Pests Thrips Control
WHAT ARE THRIPS?
When growing your favorite fruits, flowers, or veggies in your garden, sooner or later, you’ll have to deal with thrips feasting on your plants. Usually, the first signs of Thrips on Marijuana will be their visual damage to your plants’ leaves. This damage will appear as small, slug-like, silvery trails accompanied by patches of white/yellow spots on the leaves. This is the result of thrips piercing the leaf tissue and sucking out the contents of the cells. You may also see tiny black spots and thrips’ droppings. So, not only do they eat your plants, but they poop all over them too. Don’t you love them?
There’s a wide variety of thrips species that feed on cultivated plants, including onion thrips (Thrips tabaci), melon thrips (Thrips palm), and tobacco thrips (Frankliniella fusca). Still, worldwide, the most common pests of this species are the western flower thrips (Frankliniella occodentalis). There are subtle differences in color and size among the species, but seeing as western flower thrips are the most common, we’ll focus on them.
Useless Factoid: You can have many thrips, but there is no such thing as a single trip. Like deer, sheep, and pants, the word thrips is used for singular and plural references.
Adult western flower thrips are light to dark brown. They are tiny, rarely larger than 1.2mm in length, and, when stationary, they look a bit like a tiny splinter of wood. Adult thrips are very elusive and can be challenging to spot. Vegetative plants spend most of their time on the underside of leaves and often hide along the edges of the leaf veins. On flowering plants, thrips will assemble in the flowers, making them difficult to see. Adult thrips have wings, but they are not excellent fliers. Although they only fly short trips between leaves or plants, they make many short flights a day, making them relatively fast at moving around even the largest of crops. Once outdoors and airborne, adult thrips can be caught by winds and easily cover longer distances.
The rate at which thrips develop and move through their life cycle depends largely on their environment—particularly temperature, humidity, and food quality. Adult western flower thrips can live for 30-35 days. During this time, the female can lay 2-10 eggs per day, with a total of 150-300 eggs during her lifetime. Rather than the eggs being laid on the surface of a plant, thrips insert their eggs into soft plant tissue, mainly that of leaves, flowers, and fruits. One egg at a time is inserted into its cut in the plant tissue; these eggs are often identified on plants’ leaves as small hard bumps. Eggs take around 4-7 days to hatch at 68-77˚F (20-25˚C). Once hatched, they enter their first of two feeding stages as larvae. The larvae are white to orange in color and feed for around 6-10 days at 68-77˚F (20-25˚C) before entering a non-feeding pre-pupa stage, which lasts for 1-2 days. After this fasting, they enter the pupa stage to complete their development into adulthood. Pupation mainly occurs on the ground or growing media, but it is not uncommon for it to appear on the plant, and it takes around 3-4 days at 68-77˚F (20-25˚C). After pupation, they emerge as adults and move quickly up the plant to begin feeding, mating, and laying more eggs. The total time from egg to adult is 22-23 days at 68˚F (20˚C) and 14-15 days at 77˚F (25˚C); this shows how integral temperature is to a thrips’ development.
As well as damaging plant tissue through feeding, thrips also harbor and spread viruses, most notably the tomato spotted wilt virus. Therefore, once thrips are found within a crop, appropriate action is taken to control them. A very effective way of controlling thrips populations is through biological controls: predators. Arguably, one of the most effective control measures for thrips is the introduction of the predatory mite Amblyseius cucumeris.
Control With AMBLYSEIUS CUCUMERIS?
How to Kill Thrips,
Also called Neoseilus cucumeris, this tiny predatory mite is beige-brown and has a pear-shaped body less than 1mm long. Cucumeris live on plant foliage and eat various food, mainly small arthropods like thrips larvae. Cucumeris even eat pollen in the absence of prey! Generally, they are not fussy eaters and will also eat spider mite nymphs and eggs, but they offer the best control when introduced to deal with thrips.
Close up of Cucumeris – Image courtesy of Nigel Cattlin/FLPA
Cucumeris is a tiny mite and works for you by feeding on the first-instar thrips larvae soon after they have hatched. Adult cucumeris are hungry little warriors and can eat 1-5 thrips larvae daily. Female cucumeris can lay 20-30 eggs at the rate of 1-3 a day, which are laid mainly on the underside of leaves. After 2-3 days, the cucumeris larvae hatch from the egg and develop through two nymphal stages, the protonymph, and deutonymph, before they mature as adults. The life cycle takes 11 days at 68˚F (20˚C) and just eight days at 77˚F (25˚C), which is considerably faster than a thrips’.
As well as temperature, relative humidity (RH) is a significant factor in cucumeris development. Eggs require an RH greater than 60% to hatch, and nymphs and adults are happier when the RH is between 65-75%. Growers maintaining an RH lower than 60% can still successfully use cucumeris as thrips control. This is because cucumeris are mostly found on the underside of leaves around the leaf veins, and these areas provide tiny microclimates where humidity is higher. Unlike some predators, cucumeris can function in long and short day lengths, making them suitable for year-round greenhouses and indoor gardens. As long as the relative humidity is adequate and the temperature stays above 50˚F (10˚C), cucumeris will remain active.
Cucumeris are best introduced as a preventative or when thrips numbers are low. As they are generalist predators, they can sustain themselves in crops when pest numbers are meager by feeding on pollen. Cucumeris is an excellent tool for thrips control because they can lurk in the shadows when thrips numbers are low and yet devour many thrips larvae when necessary.
As cucumeris can only eat thrips larvae, it’s essential to try and maintain high numbers of cucumeris to ensure they don’t make it to adulthood. By targeting the young developing thrips larvae, cucumeris break the thrips’ life cycle, controlling their population.
You can introduce cucumeris mites into your garden and start the biological control of thrips using two different products and release methods. If you look into purchasing Amblyseius cucumeris from your grow store or a specialist online retailer, you will find they are available in tubes and sachets.
The tubes contain a carrier material, mostly a mixture of vermiculite and bran, with cucumeris mites throughout the material at all life stages. They are bred in controlled laboratory conditions using other tiny mites as a food source. Packaged and sold in cardboard tubes, the most widely used sizes are the 17-ounce (0.5 liters) line with 10,000 cucumeris and the 34-ounce (1 liter) tube containing 25,000 or 50,000. Some suppliers offer more giant buckets containing 1.3 gallons (5 liters) with 250,000 cucumeris predatory mites!
A couple of techniques can be employed when introducing cucumeris onto plants from cardboard tubes. For large short plants with a dense canopy, distributing the mites and carrier material over the tops of the plants works best. For taller plants, sprinkling the mites and carrier material over the lower larger leaves is fine. When plants are young and do not have much foliage to catch the carrier material, creating a small heap of cloth next to the young plants works well as the cucumeris mites can move from this base and walk up the plant’s stem. For an adequate level of thrips control, 50-100 Amblyseius cucumeris should be released per 3.3 square feet (1 square meter); this action should be repeated every 2-4 weeks to keep cucumeris numbers high.
Top Tip – When releasing cucumeris from the tube, a gentle rotation while sprinkling will ensure mites in the box get distributed evenly.
The sachets are made of non-porous paper and are essentially small bags containing a bran material and an active breeding colony of Amblyseius cucumeris. There are a few different sachet designs; the most popular are the larger sachet with a card hook and the smaller folding twin sachet. All sachets will have a small hole through which the adult cucumeris can venture out. Hanging the sachets on plant stems or petioles (leaf stems) provides the plant with a continual source of fresh cucumeris ready to eat the thrips. Depending on the environment and size, the sachets will release 200-400 cucumeris mites per week, lasting up to six weeks. The sachets provide a protected breeding ground for the cucumeris and enable the colony to thrive in lower humidity. Most sachets available can withstand a constant relative humidity as low as 50% in the growing environment and still work effectively. They should be introduced into the garden at one sachet per 3.3 square feet (1 square meter) or one per plant. They should be replaced every 4-6 weeks to maintain high cucumeris numbers and continuous plant protection.
Top Tip – Sachets should always be positioned in the shade within the crop to prevent direct light from drying them out, which will help maintain a long life for the breeding colony.
TUBES VS. SACHETS
If you have noticed small amounts of thrip damage on your foliage and want to introduce cucumeris before the thrips become an infestation, it’s best to start by distributing the contents of a tube. This will quickly introduce a healthy population of adult cucumeris to begin eating the thrips larvae straight away. A follow-up tube distribution two weeks later or the introduction of sachets soon after the first tube release will further support the cucumeris population. Sachets work best as a preventative measure because it takes time for the cucumeris to establish themselves within the crop. A popular method is introducing sachets into a newly planted crop or spraying a compatible pesticide shortly after. The spray will knock out many of the thrips population, and the cucumeris sachets will help maintain the reduced population and prevent outbreaks.
OTHER CONTROL MEASURES
A few other bio-controls work well in the battle against thrips. In the last issue, we focused on Hypoaspis miles, which eat soil organisms. Hypoaspis is very useful when combined with cucumeris, as they will consume the thrips that complete their life cycle pupating in growing media.
Another famous thrips predator is Orius, the ‘pirate bug.’ This bug preys on all the life stages of thrips, including adults, and can be used when thrips numbers are too high for cucumeris to cope with. One last measure you can use for thrips control is Steinerema feltiae, which is a beneficial nematode that attacks thrips pupae in/on the growing media when used as a soil drench and which also kills adult thrips when used as a foliar spray.
SPRAYING AND MONITORING
Thrips damage on Squash leaf – Image courtesy of Richard Becker/FLPA
When dealing with any pest problem in your indoor garden, it’s essential to take the appropriate first step. If you have a terrible thrips problem with lots of visible damage, you should use an insecticide spray. Introducing any predator to deal with a heavy infestation very rarely works. It would help if you considered introducing predators only after one or two sprays when the pest population is lower. Remember to check your chosen insecticide’s compatibility to ensure it will not kill your introduced predators. Sprays that kill on contact and have a short persistency on plant foliage are most applicable.
To keep an eye on the thrips population in your crop, it’s a good idea to hang blue or yellow sticky cards. These will catch the adult thrips and give you an idea of the size of your infestation. Replacing the sticky traps every 2-4 weeks will give you an even better picture.
IN A NUTSHELL:
AMBLYSEIUS CUCUMERIS EAT:
- Preferentially: first-instar thrips larvae.
- Secondarily: other arthropods such as spider mites, their eggs, and pollen.
- Control thrips populations when introduced early or preventatively.
- Eat thrips larvae, thereby preventing thrips from developing into adults.
- Eat young spider mite nymphs and eggs, but not as a significant control measure.
- Control large thrips populations.
- Prevent more damage occurring from existing or invading adults.
- Work well in temperatures below 50˚F (10˚C) or low humidity.