Fighting Spider Mites in the Growroom
If you want to keep your grow room free of mites and other nasty plant-killing pests, the key is early detection to vanquish them before they start reproducing and infesting your crops.
Mites in an indoor garden are one of the most challenging pests to conquer and can be easily overlooked in the early stages of an infestation. Those who have battled the twospotted mite (Tetranychus urticae) or other mite species know exactly what formidable enemies these pests are. Minuscule in size, rapid breeders, and capable of destroying a crop, mites are best prevented from the outset. However, infestations can be dealt with if caught early and treated correctly.
Mites, often called spider mites, are difficult-to-see eight-legged pests usually less than one millimetre in length that cause extensive damage to a wide range of greenhouse and grow room crops. The twospotted mite is the most common species, but the carmine mite, broad mite, tomato russet mite, and bulb mite also occur in protected cropping.
Adult mites are difficult to see without magnification. Those with good eyesight may be able to detect small, pale yellow, orange, brown, or black dots on the underside of infected leaves and the later stages of fine webbing. Adult mites that infect a new crop rapidly lay eggs on foliage. The newly hatched mites pass through a six-legged larval stage and two eight-legged nymph stages before going into a resting stage from which the adults develop. Mites are active under warm temperatures, prefer drier conditions, and go into a dormant or diapause stage once day lengths fall below 12 hours accompanied by dropping temperatures or a lack of food source. Mites are common and highly active outdoors under hot conditions. During winter, they’ll hibernate in cracks and crevices, on plant debris, or in soil. In a warm, protected growing environment, mites will be active throughout the year and don’t need to enter a dormant phase.
The Infestation Process – How do Mites Get In?
Infestations are typically started from the inadvertent transfer of a small number of mites from other plant material. They could be on new transplants brought into an indoor garden from other sources and, while a close inspection may not show the initial signs of pests, are extremely hard to detect in the early stages and their presence is often missed. If the adult mites have been controlled in the nursery, no sign of leaf damage or older mites may be seen. However, mite eggs, which are difficult to control and almost impossible to detect, can still be on the foliage, ready to hatch and start a fresh infestation. Quarantine of any new plant material is always good for small indoor gardens. Initially holding new plants in an area separate from the main hydroponic system allows any pests and diseases to develop to the point where they can be detected, identified, and treated. This process is particularly useful for pests and diseases which grow rapidly, however, mites developing from eggs may take some time to show symptoms and careful inspection with a magnification lens is required. Apart from new plant material, mites can make their way into an otherwise clean and pest-free hydroponic system through other methods. They are the ultimate hitchhiker; simply brushing against an infested tree will transfer enough mites onto clothing which could then be transferred into a clean indoor garden. Mites can travel on shoes, clothing, skin, hair, pets, equipment and tools, in composts and organic growing media, in dust, on soil debris, and may even be carried by other pests such as whitefly. While mites technically can’t fly, they do travel on parachutes made of the fine webbing they spin, carried on air currents through doors and ventilation systems.
Often greenhouse crops and indoor gardens that have persistent problems with mite reoccurrence have an outdoor source of the pests in the surrounding area. The source could be outdoor crops or ornamental gardens with species particularly attractive to mites. When outdoor conditions are favourable, massive population explosions of these pests occur. Under these conditions, mites are particularly persistent and will readily infest new areas, particularly those that are sheltered, warm, and dry. To help prevent infestations, commercial indoor hydroponic systems take precautions such as having employees change clothes, or wear coveralls, shoe coverings, hair nets, and gloves before entering the production area. Foot baths with sanitizer at the entrance and a double door entry system are also used. Smaller indoor gardeners may not be so keen to undertake these types of precautions. However, not entering the indoor area directly after working outside in the garden, washing hands before touching the plants, and not using outdoor tools, composts, or containers in the hydroponic garden will help prevent infestations.
Mites travelling through vents on air currents are a possibility if outdoor vegetation is harboring this pest. This is most common in late summer when infestations have built to the level that adults will spin webbing and float to a new home. Filtering of air intakes can help prevent this, but door entries may also be a point of infiltration, particularly when they lead directly outdoors.
The next line of defence is regular scouting of the plants and knowing exactly what the early signs of damage are. Mites need to be controlled before they get to the stage when they spin vast quantities of super-fine webbing which is impervious to spray control compounds and provides an unsightly home for more rapid population growth.
Early Signs and Symptoms of Mite Infestations
Inexperienced growers often miss the early, and sometimes even the later stages, of a mite infestation. This is because the adults are tiny and difficult to see, thus many growers who are new to mite infestations will miss these signs until the damage becomes severe. Those who have battled mites before become well-accustomed to the first signs that include small white or yellow flecking, often on older foliage. This occurs from mites feeding on the leaf epidermis, sucking out cell contents. As the mite population grows, the entire leaf becomes stippled or light in color and may eventually develop a bronze appearance under heavy infestations. Mites then produce the characteristic fine webbing, slung between leaves, over buds/flowers, or the growing points of plants. Mites use this webbing as protection from predators. If left untreated, mites will eventually weaken plants to the point of defoliation. Even milder infestations will significantly reduce yields, quality, and overall productivity.
Control Options for Mites
Control of mites has become increasingly difficult over recent years, largely due to the development of pesticide resistance which occurs rapidly in these pests. It’s also likely there have been other genetic changes in mite populations which have led to their persistence. There is a range of cultural, environmental, and natural control options for long-term successful control. An integrated approach is often required since there is no one easy fix. Greenhouse growers have had some success in retarding population growth via misting, fogging, or damping down to increase humidity levels. However, increasing humidity to levels which deter mites opens the door for fungal diseases if taken too far. Weekly scouting for the initial signs of foliage damage and mites on leaf undersides should become habit.
When mites are found, there are a few different courses of action which may be taken. These include insecticide sprays, oils and other smothering agents, biological control with predators, and physical removal of infested plants.
Insecticide sprays need to be used with caution. While they can be highly effective in the early stages, mites can build up genetic resistance if the same spray is used repeatedly on the crop. There are several spray compounds and products used for mite control (acaricides) in commercial horticulture, however, on a smaller scale, the main miticide compounds contain the active ingredients abamectin (a derivative of a toxin originally found in soil bacteria) or synthetic pyrethroids. These may only be used for a limited number of applications per year and chemical control compounds should be rotated within a spray program to prevent pest-resistance development. Azadiractin (derived from the Neem tree) is a botanical insect growth regulator available in a number of products that can work with other control options to help reduce mite numbers, but it does not destroy mite eggs. Various horticultural oil and soap sprays are often used as a smothering agent for mites. Excessive or repeated use of oil or soap sprays can cause crop damage, particularly on sensitive plants and those grown in highly protected conditions. Oil and soap spray damage typically shows as irregular brown spots with a darker border or water-soaked appearance that is often mistaken for a foliar disease.
Biological Warfare Against Mites
Biological control options include the predatory mite Phytoseiulus persimilis, Amblyseius californicus, which has more tolerance to lower humidity, and Amblyseius fallacis, which is resistant to some pesticides. Predators need to be introduced when there are already some mites present for them to feed on. If there are no mites present, the predators will rapidly die out, so there always needs to be a carefully controlled balance between predator and prey maintained for biological agents to be successful. P Persimilis also requires specific conditions to control mites, including temperatures between 68-79˚F. Once temperatures reach more than 86˚F and humidity is less than 60 per cent, the predators become ineffective, but mites will thrive under these conditions.
If a mite infestation becomes uncontrollable, removal of all infested plant material is essential, with cleaning and sanitation of all surfaces. Mites can survive on non-plant surfaces such as walls, floors, around heating pipes, and inside vents, so a thorough cleaning is required to prevent infection. Mites may hibernate in an empty indoor garden area for many months, only to re-emerge when warmth and new plant material is available again, so regular scouting needs to be carried out once a new crop has been established.
Mites in an indoor garden may seem like a formidable foe, but some knowledge of their life cycle and infestation methods, as well as simple prevention measures and effective sprays, make this pest controllable in many cases. Scouting and early identification, as soon as the first signs of damage is visible, are essential for successful control. A wide variety of control options, combined with careful application of spray compounds to prevent pest resistance, is vitally important when dealing with mites.
Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants
Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.