Most gardeners and many farmers have probably run into problems with plant-feeding mites at some point in time, and some face them as a recurring problem. Others may have just scratched their heads and wondered why the plant that was formerly healthy is now so sickly and not even known the cause of the problem.
There is a huge variety of mites, most of which cause no problem to crops. Many are predatory and are quite helpful, but a few types of mites regularly cause damage to susceptible plants. One of the worst culprits is the two-spotted spider mite; it can feed on many different vegetables (cucumbers, peppers, tomatoes, beans), fruits (strawberries, pears, raspberries) and flowers. Its been reported on over 180 plants so chances are if you grow something, the spider mite probably feeds on it. Their numbers can grow quickly, especially in warm weather when they can go from egg to adult in under a week.
The most obvious sign of a problem is the telltale webbing, similar to a spiders’ web, covering new shoots. When populations of spider mites get out of hand, the entire plant can be covered in fine webs. It is far more effective to control pest mites before they ever reach that level of infestation.
Early identification of mite damage can be done through the appearance of the leaves. Mites suck plant juices and cause white spotting on the surface of the leaf. If damage similar to this is also accompanied by black spots beneath the leaves, the cause may well be thrips and not mites. As the problem grows, the leaves start to lose colour and drop off. Sharply striking a branch over a sheet of white paper can help make mites more visible. To the naked eye, mites appear like moving specks the size of a grain of salt. Predatory mites may be slightly larger than plant-feeding mites and move much quicker. Under magnification, you can see that predatory mites have mouths that extend forward to pierce prey. The mouths of pest mites point downward to feed on plants.
Since early control is so important, detection of problems before they reach epidemic proportions is vital. If you are growing a crop that is prone to infection, a good scouting plan is important. For example, scouting for spider mites in strawberries may involve walking in a diagonal line across the plantings and collecting one mature leaf from every second row. If after gathering sixty leaves and more than 25% have at least five mites present, control may be warranted. Other sources have suggested that control might not be needed if one predatory mite is found for every ten pest mites. Different crops and regions may have different control thresholds, but establishing a scouting plan suitable for your own operation is essential if you have had problems with mites.
Mites first appear on stressed plants because these plants are more nutritious for mites and their plant defences are weakened. Reducing plant stress should be the first control method but that is easier said than done. Water stress is frequently associated with mite infestation, so keeping plants well watered can minimize mite problems.
Dustiness causes other mite control challenges leading some growers to water roads and try to reduce traffic speed next to fields. When dust collects on the webs, mites are protected from control sprays and the dust discourages predators.
Plants that are not exposed to predator populations are more at risk of developing a mite problem; this is why greenhouse plants are so susceptible to mites. Predators of mites may be vulnerable to chemical controls intended for other pests. Frequently, outbreaks of pest mites are a response to other pest controls used, including some organically approved substances, such as rotenone, which is known to kill many beneficial insects.
Re-establishing the populations of mite predators can be a highly effective control technique. These predators come in many shapes and sizes. A few types of predatory mites are available for purchase but different mites are suitable for different environmental conditions and infestation levels, so knowledge is essential. Mixes of different mite species each with their own niche are also available.
Maintaining high humidity is important for the success of biocontrol insects, but water has other uses in controlling mites. A blast of strong water to the foliage is enough to knock loose most mites and their webs. If the fall doesn’t hurt them, it makes them more prone to being eaten by a predator on the ground or just makes them have to climb all the way back into the foliage. Removing the webbing also interferes with egg laying. Some growers have found using water alone can prevent the problem from getting out of hand.
Once the problem has been noticed, stripping the most infected leaves can help reduce the mites from spreading. It is important to not compost the leaves since mites are hardy and can migrate to part of the compost where temperatures are lower, and later reinfect new plants. It is better to put the leaves in a plastic bag, tie it tightly, and throw the bag away. A small sacrifice to your garden’s fertility can be a boon to your pest management.
While mites can be problematic pests, the combination of management, biological and botanical controls can keep them in check. The best strategy is to minimize the problem before it grows too severe. That seems to be a strategy suited throughout the garden and beyond—the challenge is to achieve it.
Biocontrol Distributor Information: http:// highplainsipm.org/ HpIPMSearch/Docs/ BiologicalOrganisimSuppliers.htm