The control of plant pests has always been a contentious issue. and we would assume that the majority of serious growers would like to avoid the use of toxic chemical wherever possible. Like every other aspect of plant raising, these have been tested and fine-tuned by professional growers and we are now becoming available to the amateur gardener.
Amongst the the new technologies, the idea of biological pest control must take pride of place. Like all great concepts, it is simple yet effective and is causing major changes to the way we do things. Biological pest control is one of the most exciting developments in modern horticulture and it offers a vision of a pesticide free future when man can use nature's own weapons to grow his food in an uncontaminated atmosphere and a cleaner greener world.
Basically biological pest control involves the introduction of friendly creatures to combat the ones that do the damage. These creatures are known as 'predators' because they feed on the pest at some stage in its life cycle.
These little creatures, almost invisible to the naked eye, are undoubtedly the greatest menace to the grower. They are often unseen and unsuspected until their numbers are high and they can multiply at a devastating rate. If they find favourable conditions in the greenhouse or indoor grow room they can literally destroy a crop.
First signs of their presence are small dead spots that appear in clusters on the affected leaves. A general bronzing of the foliage follows this and as the infestation increases there will be visible deposits of fine webbing on the underside of the leaves. Old-fashioned methods of chemical control have never been successful for long as these creatures are very adept at developing resistance to each poison in turn. Man has responded by using ever more toxic chemicals to control them with ever-decreasing success rate. The side effects of this are the collateral destruction of hundreds of beneficial or neutral insects that would normally co-exist with the mites in some sort of balance.
The solution that biological pest control offers is the introduction of another mite called Phytoseiulus persimilis which lives exclusively on two-spotted mites. If the population of pests is at a high level, the predator will multiply in relation to its food supply. Once the pest is reduced then the predator will begin to die out as well. A balance should then be achieved which will maintain the pest population at low level, below the point at which they will cause visible damage.
If the pests do begin to multiply beyond the predators' capacity to consume them, the grower can then make small adjustment to the environmental conditions (temperature and humidity) that will favour the predators over the pests.
Most gardeners will be familiar with these little flies that fly up in clouds when you disturb the affected foliage. They are a major pest of the commercial greenhouse industry and they can cause as much damage in a hobby greenhouse. Once again the chemical approach has failed utterly to control them and breeder of biological agents have come to the rescue. The predator of whitefly is a tiny wasp called Encarsia formosa.
This minute wasp is totally harmless to everything but whitefly. It bears no resemblance to wasp as we normally think of them but resembles a fruit fly, only much smaller. Encarsia is totally dependent upon whitefly for reproduction as it lays an egg in the whitefly larvae which then becomes the food source for the developing larvae of the wasp. This process effectively replaces a whitefly with a wasp which can then go on and lay more eggs.
sciarid fly (fungus gnat)
Sciarids are tiny black flies that can occur and become numerous, in and around plants. They are a common reality for indoor growers and they may represent a much more serious problem than many growers realise It is now believed that these pesky little flies are actually vectors of various fungal diseases. In laboratory studies, adult Sciarids carried spores of Pythium, Botrytis, Verticillium, Fusarium and Thielavaiopsis as they moved from plant to plant. Fungal spores have also been found in their frass (droppings). It is not yet clear how serious this problem is but the potential is certainly there. The roots are damaged by the larvae and then exposed to infection by the oviposting adult females.
Adult Sciarids are delicate, grey, dark-grey, or black fly-like insects about 2 mm long. They are often seen running over the surface of rockwool slabs and cubes, especially around wet areas. The females lay up to 300 eggs on the growing medium. The eggs (barely visible to the naked eye) are oval, smooth, shiny-white, and semi-transparent. They are laid singly, or scattered in groups or strings of 3 to 10. In suitably warm and moist conditions the eggs will hatch in five to six days. The larvae will feed on any organic matter available for 10 to 14 days. The pupal stage takes place in a silky chamber below the surface.. Three to four weeks are necessary to complete the life cycle of the fungus gnat.
Adult Sciarids do not damage plant materials. The larva lives in the soil or in rockwool and is white, almost translucent with a shiny black head. These larvae feed on any organic matter and can attain a length of about 5mm. They will quickly damage plants by chewing or stripping the roots. Affected plants may lose vigour and the leaves turn yellow, without any visible injury showing on the above ground parts. Root hairs are eaten off, as are the small feeding roots. In severe infestations, all that is left is the centre part of the root.
The damage caused by Sciarids is often confused with seedling 'damping off', and other fungal diseases or even with nutritional problems.
However if adult flies are visible then it is highly likely that they are, at least in part, responsible for the declining plant performance.
The maggots may be found throughout the soil or rockwool or burrowing into plant stem near the soil line. Adults are attracted to moist media and rotting plant material acts like a magnet. The larvae feed primarily on fungi and other micro-organisms, but will also attack soft plant tissue such as root hairs seedling stems and the base of cuttings.
Heavy maggot infestations can cause seedling collapse through root or stem damage. Severe infestations of cuttings and established plants may lead to poor callus formation, inadequate root establishment, and subsequent wilting or death. Most importantly the maggots spread fungal pathogens when feeding and may greatly increase crop losses due to diseases caused by Pythium, Phytophthora, Chalara, Fulsarium, Rhizoctonia and Vertillium fungi.
lOnce Sciarids are observed on the plant the careful grower must act immediately. For soil crops it may be possible to allow the soil to thoroughly dry out between waterings. This will kill the larvae through desiccation as well as help future problems. If the plant is of a type that cannot be allowed to dry out, then another option is to drench the soil with an insecticide registered for this type of use.
It is always good practice to avoid over watering soil, or any other medium. A wet environment is an open invitation to fungus gnats as well as a host of other problems such as root rot or stem rot.
For plants in rockwool it is definitely not an option to allow the medium to become dry. This is to run the risk of losing the crop altogether.
The first step to control is monitoring the problem. It is highly important for the grower to be aware to be aware of how serious the problem is and if it is increasing.
The ideal way to monitor Sciarids is to make use is to make use of the old-fashioned yellow sticky trap. These traps should be placed about 10cm above the crop canopy near susceptible crops. Sciarids are attracted to the yellow surface in much the same way as white flies and will quickly and happily begin committing suicide. Traps should be inspected at least weekly and changed every two to four weeks. Numbers of less than 20 flies per trap/week may not present an economic problem but this threshold will vary with the sensitivity and value of the crop.
Learn to identify fungus gnats on sticky traps. A 10x hand lens in sufficient.
Infested pots, trays, media or root masses placed in a clear plastic bag or bottle for a few days will trap emerging adults to give a definitive idea of the size of a population. The more sticky traps that can be used the better, as the reduction in numbers of flying can be significant - and satisfying to the grower. However once the scale of the problem is known more serious control strategies will need to be adopted.
It is possible to monitor larval populations by pressing moist skinless slices of potato into the growing area. This may not be so easy with rockwool. Leave the slice for at least four days before quickly lifting and counting the larvae on and under the slice.
Adult Sciarids may be controlled with any general purpose insecticide. For the small hobby grower it will be adequate to purchase a ready to use spray from a garden centre. One that is labelled for whiteflies is likely to be suitable. treat daily or as needed. Dusting the surface of the soil or rockwool lightly with an insecticide dust will also control adults satisfactorily.
For larger areas a commercial spray product will be needed. The literature suggests that most knock down sprays (such as those containing Maldison or Permethrin) would be highly effective against Sciarids. Perhaps the most effective control of the adults would be achieved with insecticidal smokes which can also be obtained form garden centres.
Larvae are controlled most effectively by an insecticidal drench. There are a number of commercially available insecticides that have been reported as effective on Sciarid larvae though none that have approval for this use with food crops. Basically the technique is to make up a solution of product concerned, either at the same concentration recommended for spraying or lower. This solution can be poured slowly through the rockwool cube or onto the surface of the growing medium. It should then be allowed to stand for as long as possible before further irrigation or watering takes place.
It is crucially important with this, as with any chemical treatment, to experiment first on a single plant or on a small area of the crop. Some plant species are very sensitive to chemicals and can be affected badly, even killed. A few plants, an expendable portion of the crop, should be treated first and a few days allowed to elapse before proceeding further. If there is no damage to the treated plants it will then be okay to treat the main body of plants.
There are very few biological agents that are effective against Sciarids and the ones that are have a very limited availability. Perhaps the best known at the time of writing is a special strain of the well known product Bacillus thuringiensis (known universally as Bt). Various formulations of this bacterium are marketed commercially (for example for caterpillar control ) but the strain effective against fungus gnats, mushroom flies and mosquitoes (Bacillus thuringiensis israeliensis H-14) is not yet widely available.
For routine Sciarid control, the grower drenches the rockwool with Gnatol (Bacillus thuringiensis H-14) applying it weekly for two or three weeks. Results aren't immediate. The bacteria must be ingested by the larva, after which a toxic protein crystal is released into the insect's gut. The larvae stops feeding and dies. Repeat applications will be needed.
Bt doesn't kill egg-laying adults, so the applications need to be successful and persistent. It certainly takes time to get control with Bt, we would suggest it to prevent infestations rather than cure them. A rule of thumb with 'biologicals' is that you can't wait until you get an infestation. All these organisms seem to work best when pest populations are lower. Generally the recommendation would be to use these materials as a routine from day one.
There is plenty research into effective beneficials for Sciarid control and this, more than anything, underlines how seriously this problem is being considered now.
predatory mites (hypoapsis)
Hypoapsis (Stratioaelaps miles) is a soil dwelling predatory mite that feeds on fungus gnats, thrips, springtails and other small insects that inhabit the soil. Adult Hypoapsis are 05 - 1mm long and light brown. The mites live in the top 1-2cm of soil or media and move rapidly in search of eggs and young larvae. Mites will consume from 1-5 fungus gnat larvae per day. They can survive up to 4-5 months with food and for 7 weeks without live prey, feeding on organic debris and nemotodes.
Hypoapsis may be applied to rockwool blocks and slabs and to soil to control these pests. They do not harm plants or humans. Hyoapsis is widely available in the UK.
Entomopathogenic (insect eating) nematodes are microscopic worm-like creatures that actively seek out their hosts in moist soil or media, enter the insect body, killing the insect.
Nematodes prefer slightly moister and cooler conditions than the predatory mites. Several nurseries successfully use a combination of mites and nematodes for year round control.
A combination of mite and nematode may be the best option where they are available. Beneficials can't always be counted on for the long run, because once they've consumed their pest hosts, they'll die or depart, leaving the plan unprotected. However when Sciarid food resources are exhausted. Hypoapsis mites will turn to feed on the nematodes - and thus be present when Sciarid populations rise again.
Pythium is a recurring problem for the horticulture and it seems to be an increasing one. Pythium is a waterborne fungus and re-circulating hydroponic systems provide it with an ideal environment in which to live and breed. Plants can survive and grow with high levels of inoculum in the nutrient solution. The fungus however will restrict the root system. A sudden rise in temperature will find the plants unable to increase their uptake of water and they will wilt. For many growers this is the first sign that Pythium is active in their system.
Pythium rot rot can be caused by several different species of the fungus Pythium. There is also a number of similar pathogenic organisms that can attack the roots of plants in hydroponic system. When a plant is attacked by the colonising fungal spores, the root tips, which are very important in taking up nutrients and water, can be damaged and eventually killed. Pythium can also rot the base of un-rooted cuttings and attack small seedlings, the condition known as 'damping off'.
The hydroponic grower can encounter Pythium at any time and if he is unprepared he may well lose his crop. the fight against fungal root disease is ongoing and the careful grower will have it in mind from the day he starts to plan his system.
The first step, as in so many things, is prevention. By maintaining a clean growing environment and by treating the incoming raw water the grower can minimise the opportunities for fungal infection to enter the system. The second area of interest is in the optimisation of system design. A well-designed system will be an unattractive environment for fungal spores to develop in and this represents the most important defence against them. There is also a number of additives and treatments designed to eliminate pathogenic organisms from the actual operational system.
The most visible symptom of these fungal diseases is in the root systems. Roots will begin to go brown and lose their healthy white appearance. As the pathogen spreads the roots become soft and mushy and there is always a tendency for the plant wilt in the warmer part of the day.
Nitrophilic bacteria can become established in the root zone of hydroponically grown plants.
They are most often found in pot culture plants grown in a medium such as Perlite or granulated rockwool. These bacterial organisms feed on nitrate ions and excrete ammonia. They are especially flavoured by an excessive wetness in the root zone and by cold stagnant nutrient in reservoirs (saucers).
If they become established, the level of Nitrate (NO3) in the nutrient will drop very rapidly. This will result in decreased growth rates and a rapid yellowing of the leaves. Yellowing will affect the entire plant and will eventually lead to total necrosis.
The first symptom is often the yellowing of the foliage but this may be accompanied by the development of unpleasant ammonia smells from the reservoirs and the root zone.
The following course of action may be of assistance, it is pretty drastic and is aimed at recovering plants that might otherwise be lost. No guarantees are given. The grower might like to try this on a single plant before treating the entire crop.
Flush the pots very thoroughly with clean tepid water. Allow pots to drain completely. Clean the saucers and replace the pots in them.
Make up a solution of Liquid Oxygen at 5ml per litre. Water plants with this solution and allow it to stand in the reservoirs overnight.
Empty reservoirs and allow pots to drain again. Make up a solution of ammonium nitrate at 10g per 10 litres. Add 10ml Liquid Oxygen per 10 litres.
Water plants with this solution and allow to stand in reservoirs for 48 hours.
Drain pots again. Make up a standard batch of a high quality nutrient solution such as Optimum or Ionic. pH should be slightly under 6 and EC around 2mS. Add Liquid Oxygen at 10 ml per 10 litres.
From this point the grower should continue as normal. Yellowing should cease in a few days and plants should be okay. If yellowing re-occurs then ammonium nitrate can be added to the nutrient solution at a rate of 5g per 10 litres. Liquid Oxygen should be added to the tank every other day at the rate of 10 ml per 10 litres.
Don't allow reservoirs to be too deep. They should only hold about as much liquid as the plant will draw up in 24 hours. Keep the room warm and keep humidity low.