Article 5-3 Yield Predictions

I am working with a family which is raising hydroponic tomatoes (loose, not clusters or cherry). We need to be able to establish a “normal/expected” yield and we do not know where to obtain that sort of information.
Can you point us in the right direction?
Connie Skinner

Thanks for your question. If growing in a well-managed hydroponic environment, light duration and intensity will play a strong role in determining the yield potential of the crop. In a hydroponic greenhouse environment, 35 + kilograms of fruit harvested per meter square during a four-month harvest period is probably about par. I suggest contacting the Canadian Greenhouse Crop Processing and Research Center (GPCRC) in Leamington, Ontario. I haven’t taken note of what they have been up to lately, but in the past they have done some cutting edge research followed by public reports on growing many crops, including hydroponic tomatoes.
Sincerely, Erik Biksa

What are they? Life Cycle

Spider mites are so small that they are sometimes difficult to see without a magnifying glass, and individual spider mites are almost microscopic. Spider mites are not insects; they are closely related to spiders, harvestmen (daddy longlegs), and believe it or not ticks. There are many species of mites, yet the one most likely to attack the indoor garden is the 2 spotted spider mite. They can be identified by the two spots on their back, which can be seen under a magnifying glass.

However, unlike insects that have six legs and three body parts, spider mites have eight legs and a one-piece body. Mites are about 1/50″ long, approximately the size of a grain of salt or slightly larger. They may be red, black or brown or pale green. In the summer, the adults and nymphs are white with two greenish spots.

Mites are triggered mainly by the cooler weather and shortened daylight hours in the fall, some or most of the mites turn red in color, stop feeding and egg laying, and then crawl off to protected nooks and crannies to hide or diapause through the winter. These eggs or adults overwinter in growing mediums, crevices or in ground debris, or on equipment that hasn’t been properly cleaned, emerging when the climate conditions are again favorable to them and plant growth. If you had spider mites last year and didn’t perform any preventative maintenance at the end or beginning of the season then, it’s likely that they will return again.

A female lays about 100 eggs during her lifetime. Depending upon climate conditions, the eggs hatch in as little as 2-3 days, and the adult stage is reached in 7-10 days. The females reproduction is greatly affected by climate and humidity with a significant difference in the days till their maturity dependant upon the temperature i.e.) egg to adult. For instance at 60 degrees Fahrenheit, it takes 30 days for the egg to become an adult, at 70 degrees 14.5 days and here is the astounding number – 90 degrees Fahrenheit they reach adulthood in just 3.5 days! So now imagine those populations increases over a month when the offspring start to reproduce less than a week after hatching – at 70 degrees she and her offspring number 13,000; at 80 degrees she and her offspring represent a staggering potential of 13,000,000 individuals over a single month – huge population increases!

Mites evaporate large quantities of water from their bodies, so they must suck juices from the plants. This is easier for them to do in a dry environment. Humid environments (above 60% RH) slow down their metabolism, life span and reproductive rate. Mites may be controlled somewhat by lowering the temperatures (at about 50 degrees Fahrenheit they start to hibernate), thus slowing down their life process considerable. Even if you only decrease the temperature during the indoor dark cycle (when it is easier to lower temperatures), the progression rate of the infestation is slowed considerable.

Symptoms and Damage

Feeding marks are usually the first sign of a mite infestation and many confuse these as nutrient deficiencies. Spider mites lack chewing or piercing-sucking mouthparts. Instead they have a pair of needle-like structures that they use to rupture leaf cells. A feeding spider mite pushes its mouth into the torn tissue and draws up cell sap. Small patches of cells are killed, resulting in a stippling or fine flecking on the upper surface of leaves giving the leaves a “sandblasted” look, it also appears that the foliage is losing its green coloration. This injury generally becomes the most obvious first on the older established plant foliage. With heavy infestations the foliage become bronzed, bleached, yellow, or even gray.

As the pests spread over a plant they spin a spider-like webbing over and between the leaves generally at the stems, enabling them to move about the plant (and from plant to plant) more quickly. Mites tend to congregate on the leaves rather than the flowers or fruits of most plants. However, as their population increases, they can start colonizing these as well.

Spider Mites will walk down stems, across medium or equipment and across dry spaces in search of new host plants in which to set up house! They are also great hitchhikers who cling to your pets’ fur, or your clothing, looking for a ride to more yummy plants. If left untreated these infected plants weaken to the point where they become susceptible to diseases such as viruses and bacteria, they can become sickly and may even eventually die.


Along with the plants symptoms, mites can be identified by shaking symptomatic leaves onto a sheet of white paper or by observing the underside of the leaf and stems infected leaf areas with a hand lens. In a hot dry climate, mites can cause plants to quickly drop leaves – in as short as a few weeks. Severely affected plants will yield poor quality fruiting and flowering.

Aphids or thrips can cause similar symptoms but they are usually clearly visible or else in the case of thrips leave dark specks of excrement behind right on the foliage as well as silvering.

Immediate Action

When an outbreak occurs you must act immediately:

  1. Carefully examine plants – separate the infested plants from the uninfested plants. Light and heavily infested/damaged plants should also be separated. Do not let the foliage of the plants touch, as the mites will easily walk from plant to plant via the foliage. Separation is very important !
  2. Install physical barriers such as sticky tape around the plant rims of the plant pots to prevent further migration of mites.
  3. Don’t forget to thoroughly wash your hands and change your clothing. Particularly if you have been working outdoors, visiting a nursery or fellow gardener (they will not be happy if you spread your problem to them), before nearing and tending to your own indoor garden. When tending and examining your plants, tend to your infested ones last – do not go back and forth between the uninfested and infested ones.
  4. Be sure to remove any fallen debris to a plastic bag and immediately dispose of to the outdoor trash.
  5. Wash down your plants with a forceful jet of water (but not so forceful it will damage the foliage), be sure to spray the underside of the leaves – sometimes the water will knock them off and kill them. Doing this in the shower works well for indoor, smaller plants.
  6. Raise the relative humidity above 60% (the wash will help do this). If you don’t have a automatic mister or humidifier, even misting the plants with a spray bottle several times a day will raise the humidity (be sure to do early in the day so that the water can evaporate under the lights. Hand misting is also an opportune time to foliar feed your plants !


  1. Biologically by naturally using “good bugs”
  2. Insecticidal Oils / Soaps or “Soft” Pesticides such as natural pyrethrum
  3. Chemical produced miticides

I will start with my least favorite:


The largest misconception is that people treat spider mites like insects and try to eradicate them the same way, using broad based insecticides. Unbelievably, I recently spoke with someone who tried using Raid and asked why it didn’t work! Most insecticides are not effective on mites and most will result in increased mite damage and disease by killing off any natural enemies and microorganisms. Resistance to chemicals has increased the difficulty of controlling of these pests. Spider mites can adapt to most chemical sprays within days if they are not directly hit with the initial spray. After that, they will just thumb their noses at you.

If you must use chemicals – use a miticide that has the least damaging effect on the environment, (natural pyrethrum and stronger synthetically produced permethrin have been shown to have the best miticide abilities) such as Doktor Dooms House and Garden Spray (note this product has a 60 day residual, which breaks down with sun exposure – and may only be used prior to fruit/flowering development, as well this product will kill all stages including eggs). Mite populations often are localized therefore spot spraying may be highly effective.


Because mites primarily occur on the undersides of leaves, applications of miticides need to be directed at both the lower and upper leaf surfaces, look for a product that allows for angle and upside down spraying. Mite eggs are resistant to some miticides, so repeated applications are often necessary to control infestations.

Soaps / Oils / Soft Pesticides

Most horticultural soaps are made from salts of “fatty acids” which kill on contact – homemade soap and water applications don’t contain these acids and are just a concentrated mixture of perfumes, chemicals and dyes that are harmful to plants. These soaps kill the mite by penetrating its protective outer shell, disrupting their respiratory system and causing damage, which in turn causes them to dehydrate and die. Insecticidal Soaps must be sprayed directly on the pest in order to kill it, and once evaporated are no longer effective. When applied properly, insecticidal soaps have very little to no toxicity to humans, animals, or most beneficial insects. Most horticultural soaps such as Safer’s can be used right up to the day of harvesting – just be sure to wash any fruits or foliage that you are going to consume. Residual is very short – once it has evaporated off the plant.

Oils such as horticultural oil and dormant oil, can also be used sparingly. These oils are a highly refined mineral oil that kills dormant adults and their eggs by suffocating them. Apply it early in the season to kill eggs, but not on very young plants – make sure you that they are diluted according to package directions. I know growers that actually spray the cracks in the floor of their greenhouse – places that they think that pests may be hiding. In my opinion this is quite messy and from experience know that dormant oil stains clothing. Caution: continued use may clog up the plant’s follicles causing further problems.

Neem Oil is a completely safe, non-toxic, biodegradable substance made by crushing the seeds of the Neem tree which has natural insecticidal properties. Different parts of the Neem tree are used in natural medicines, toothpastes, soaps, and spiritual food to millions in India. It is only in the past decade, that the pest control potential of Neem is being rediscovered. Neem oil makes the plant unpalatable to pests, it does not kill them but affects their behavior and physiology. Though subtle, Neem’s effects such as repellence, feeding and reproductive cycle, growth inhibition, mating disruption, chemo-sterilization etc. are now considered far more desirable than a quick knock-down in integrated pest management programs as they reduce the risk of exposing pests natural enemies to poisoned food or starvation.

If you must quickly “decimate” a large mite population, it is practical and efficient to use an organic Pyrethrum(in/oid) sprays or insecticidal soaps as this allows for quick plant regeneration without the possibility of toxic shock occurring due to the buildup of control agents in the plants’ tissues and surrounding soil.

“Pyrethrum” refers to the powder made with the dried flowers of the chrysanthemum, whereas the term “pyrethrins” refer to the six insecticide components occurring naturally in the powder.

The two most popular ways of using pyrethrum are dusting or spraying. It is important to note that cool temperatures enhance the effects of pyrethrum. It is therefore recommended to apply in late afternoon so that cooler evening temperatures will improve efficiency, and the sun won’t accelerate the degradation of the pyrethrins. The effect of pyrethrum is immediate. Insects are paralyzed on contact. If a very low dose is used you need to take care that the mites are dead and not simply stunned to recover several hours later.

One of the added benefits of Pyrethrum is that it has a much greater “flushing out” effect than any other insecticide; it disrupts pests from their hiding places forcing them to get out and to get exposed to the insecticide. Although pyrethrum is highly biodegradable and has a short-term effect, it is nonetheless a nonselective insecticide that affects a large number of beneficial insects and microorganisms.

Pyrethrum is effective against a large number of adult insects, but it is much less effective against larvae. It is not harmful to humans and animals, but is to fish and other aquatic organisms.

Insecticidal soaps acts as a surfactant or stickiness, allowing for better coverage and increasing pyrethrum efficiency. I have been told that some growers use a combination of Insecticidal Soap and Pyrethrin aerosol. One of the most widely used products is Safer’s Trounce? a combination of insecticidal soap and Pyrethrins. This product provides a quick one-two punch by having the insecticidal soap disrupt and weaken the insect’s outer protective shell also allowing for a more even coverage of the insect with pyrethrins. What happens is that when the insect is weakened the pyrethrin then “kicks in” to give the second punch and ensure the death of the pest. Doktor Doom Botanics? spray of Pyrethrum (aerosol) is another popular product, especially as the container can be sprayed upside down to get the underside of the leaves. You will have to reapply both of these products within about 5 days to get the current adults and then any eggs that hatch. There are becoming more and more of these products on the market, I only mentioned these ones as they are the most recognized – be sure to ask the “pesticide dispenser” at your favorite garden center for other varieties.

Now of course for my preferred method of control

Biological Control using “good bugs”.

In believe I did a rather good job of this in a previous article so I will just reiterate:

One of the best controls for spider mites is a predatory mite – Phytoseiulus Persimilis. Persimilis is a red pear-shaped mite with long legs. Be careful not to confuse summer spider mites that turn orange or deep when they hibernate, with persimilis.

An adult Persimilis will eat 5-20 prey (eggs or mites) per day. What makes them so effective against spider mites is that they reproduce more quickly at temperatures above 28°C (82°F) than the spider mite, and they feed on all stages of the pest spider mite. However persimilis must have high humidity temperatures – above 60% , which also affects the pest spider mite to reduce their egg laying.

Persimilis are very voracious, and have one of the highest consumption rates. Almost 75% of European greenhouse vegetable production relies on persimilis for spider mite control, and the California strawberry industry also use this species for control. Persimilis is also used in interior plantscapes and conservatories, and greenhouse ornamentals growers have long relied on their ability to control pest spider mites.

Because these mites are such efficient hunters and dispersers, they can cause extinction of their spider mite prey, which is extremely desirable where little or no spider mite damage can be tolerated, such as in ornamental plants. Typically, persimilis will eventually exhaust their food supply and starve, and so it must be reintroduced when new spider mite infestations occur.

Neoseiulus californicus is a predator almost identical to persimilis but will tolerate warmer temperatures from 55° – 90° F., and greater humidity fluctuations. As well, they can starve for a longer time than persimilis, therefore they don’t need to be reintroduced as often as persimilis would. This predator is often best for houseplants, and indoor gardening however it is very costly.

Another excellent predator for spider mite is the predatory midge, Feltiella Acarisuga. This midge larva is about 1/12th of an inch long and is a pinkish brown color. The female Feltiella will lay its eggs inside the spider mite colony and as soon as the young feltiella larvae hatch they begin their feast on spider mite eggs. The young larvae feed mainly on the eggs while the older larvae feed on all stages of spider mite. A larvae feltiella can feed on about 50 spider mites before pupating. The lifespan of the feltiella from egg to adult is 10-15 days and the female will lay about 12-14 eggs. Like Persimilis, when the food supply is exhausted they will also starve and thus must be reintroduced when new infestations occur. Feltiella will work more efficiently in much lower humidity and a broader temperature range.


Last but not least, I must reiterate the importance of preventative measures:

  1. Before planting or transplanting be sure disinfect pots with a 10 percent bleach solution, i.e., 1 part bleach to 9 parts water.
  2. Removal of debris and an early drench of insecticidal soap in areas where they were a particular problem last year can also help – but remember insecticidal soap will only kill on direct contact by breaking down the insects protective coating and causes them to dehydrate and die.
  3. If you have had continued problems in the past, it is very likely as soon as conditions are optimal that you will have problems again – then you may want to consider fumigation. I am not generally in favor with and believe that with some hard work – prevention and proper cleaning that you can significantly lower your chances of repeated incidence.
  4. Should you choose to fumigate before you start your indoor season, use products that are the least harmful to the environment such as “Doktor Doom’s High Pressure Fumigator”, this product contains natural pyrethrins, which break down with exposure to sunlight. Be sure if you do this that it is done prior to planting as it is non-selective and will kill naturally occurring beneficial organisms.

You must remember that certain insecticides, especially broad-spectrum insecticides, whether natural or synthetic, kill beneficial organisms as well as spider mites. This can sometimes allow the spider mites or secondary pests to reappear/appear and become an even worse problem because their natural controls are gone.

I will again reiterate that you should choose the safest controls / alternatives that will not harm the environment. Should you decide to use a commercial product, be sure to read and follow the manufactures dispensing instructions carefully. Only you can decide for yourself the safest way you want to grow.