Garden Pests Thrips Control
WHAT ARE THRIPS?
When growing your favourite 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 the visual damage they do 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 accompanying tiny black spots, which are thrips’ droppings. So, not only do they eat your plants but they poop all over them too. Don’t you just love ‘em?
There’s a huge variety of thrips species that feed on cultivated plants, including onion thrips (Thrips tabaci), melon thrips (Thrips palmi) and tobacco thrips (Frankliniella fusca), but 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 thrip. Like deer, sheep and pants, the word thrips is used for both singular and plural references.
Adult western flower thrips are light to dark-brown in color. They are very small, rarely larger than 1.2mm in length, and, when stationary, they look a bit like a tiny splinter of wood. Adult thrips are very evasive and can be difficult to spot. On vegetative plants, they like to spend most of their time on the underside of leaves and will often hide along the edges of the leaf veins. On flowering plants thrips will congregate in the flowers, making them difficult to see. Adult thrips have wings, but they are not great fliers. Although they only fly short trips between leaves or plants, they make many of these short flights a day, which makes 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 the quality of their food. Adult western flower thrips can live for 30-35 days. During this time the female is capable of laying 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, mostly that of leaves, flowers and fruits. One egg at a time is inserted into its own 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 mostly occurs on the ground or growing media, but it is not uncommon for it to occur 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. It is therefore extremely important that once thrips are found within a crop, appropriate action is taken to control them. A very effective way of controlling thrips populations is through the use of biological controls: aka predators. Arguably, one of the most effective control measures for thrips is through 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 in color and has a pear shaped body less than 1mm long. Cucumeris live on plant foliage and eat a variety of 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 very small 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 per day. Female cucumeris can lay 20-30 eggs at the rate of 1-3 a day, and these 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 8 days at 77˚F (25˚C), which is considerably faster than a thrips’.
As well as temperature, relative humidity (RH) is a very important factor in cucumeris development. Eggs require a RH greater than 60% in order to hatch, and nymphs and adults are happier when the RH is between 65-75%. Growers maintaining an RH lower than 60% can still be successful using 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 are able to function in both 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 very low by feeding on pollen. Cucumeris are a great tool for thrips control because they are able to 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 important to try and maintain high numbers of cucumeris to make sure that the thrips larvae 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, most often a mixture of vermiculite and/or 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) tube with 10,000 cucumeris and the 34 ounce (1 liter) tube containing 25,000 or 50,000. Some suppliers offer larger buckets containing 1.3 gallons (5 liters) with 250,000 cucumeris predatory mites!
When introducing cucumeris onto plants from cardboard tubes, a couple of techniques can be employed. 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 material 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 tube 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 deigns; 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 environment and size, the sachets will release 200-400 cucumeris mites per week and can last for 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 the rate of 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 drying them out. This will help maintain a long life for the breeding colony.
TUBES VS. SACHETS
If you have noticed small amounts of thrips 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 start eating the thrips larvae straight away. A follow-up tube distribution two weeks later, or alternatively the introduction of sachets soon after the first tube release will further support the cucumeris population. Sachets work best as a preventative measure, mainly because it takes time for the cucumeris to establish themselves within the crop. A popular method is to introduce sachets into a newly planted crop or shortly after spraying a compatible pesticide. The spray will knock out a lot of the thrips population and the cucumeris sachets will help maintain the reduced population and prevent outbreaks.
OTHER CONTROL MEASURES
There are a few other bio-controls that work well in the battle against thrips. Last issue we focused on Hypoaspis miles, which eats soil organisms. Hypoaspis are very useful when used in combination with cucumeris, as they will consume the thrips that complete their life cycle pupating in growing media.
Another popular thrips predator is Orius, the ‘pirate bug.’ This bug preys on all the life stages of thrips, including adults, and can be used in situations where thrips numbers are too high for cucumeris to cope. 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
Thirps damage on Squash leaf – Image courtesy of Richard Becker/FLPA
When dealing with any pest problem in your indoor garden, it’s important to take the appropriate first step. If you have quite a bad thrips problem with lots of visible damage, you should use an insecticide spray. Introducing any type of predator to deal with a heavy infestation very rarely works. Only after one or two sprays when the pest population is lower should you consider introducing predators. Remember to check the compatibility of your chosen insecticide to make sure 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 NUT SHELL:
AMBLYSEIUS CUCUMERIS EAT:
- Preferentially: first-instar thrips larvae.
- Secondarily: other arthropods such as spider mites and their eggs, and pollen.
- Control thrips populations, when introduced early or preventatively.
- Eat thrips larvae thereby preventing thrips 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 in low humidity.