Spidermites

Controlling Spidermites in enclosed growing conditions such as greenhouses, hydroponics rooms and atriums is a balancing act between not enough humidity and too much. Too much moisture and you will fall into the danger of mildews and moulds. Not enough, and the Spidermites will explode into a severe infestation. Ideal humidity should be at least 62% to 70%.

Using chemicals will give you a quick knock down of the problems, but the eggs and young Spidermites who do not die from the chemical will be completely resistant to another treatment. Spidermites feed by sucking the sap out of the leaves, and like aphids, target young new growth on the plants. This causes discoloration. The plant suffers, as it cannot produce chlorophyll. It will therefore not be able to feed itself properly. Another sign of spidermite infestation is webbing on the plant leaves, usually concentrated on the newer growth. Proper monitoring and checking plants every few days is a good idea. Also, using yellow sticky cards to trap any flying insect pests is a great time saver.

If the plants in the area are to be consumed, you do not really want to add a chemical soup to the mix. Most persons who grow fruits, vegetables and other consumable crops in a greenhouse or hydroponics system want something that is safe and healthy to use.

One nice thing about using predator insects is that the pests do not become resistant to their natural enemies. If conditions are right, the predator will flourish and control the spidermite problem without the grower having to do much but monitor for “hotspots” and add more bugs if needed. Fortunately there are a few types of spidermite predators available to use. For humidity conditions at 60% or higher you are easily able to use Phytosieulus persimilis either on leaf or in vermiculite. Application is very easy and only takes a little while. The persimilis take four to five days to really get moving, but then they can be seen walking around. They move much faster than the spidermite and are usually orange yellow to quite a bright red. Persimilis are blind, but they do have some light sensitivity. What they target is the smell of the spidermite honeydew or excretment. They will crawl from plant to plant if the leaves are touching, but cannot fly or hop. If they do succeed in clearing up the spidermites to such a degree that the area is clean, they will cannibalize each other and die off.

For places that have problems keeping the humidity above 60%, Feltiella acarisuga can be another choice. It is a predatory midge that is shipped in a pupal form in a hanging container. When they hatch out they will be adults who will mate and fly around looking for spidermite colonies. They will lay their eggs and within a week the maggots or larva will hatch out and start consuming all of the spidermite they can find. They work very well in a lower humidity/higher heat situation and will work well with persimilis. They will not eat persimilis but will consume almost any other type of mite, not only spidermites. They are, however fairly expensive, but they do a great job. Once the spidermite drops to a low level, the Feltiella slow down on egg production and eventually die off.

A usual recommendation for proper spidermite control is 1000 persimilis per 10-15 average size plants, about 2-2.5 feet high. If there is webbing, which is always an indicator of a severe infestation, I would double that application and also check humidity levels. To boost humidity even for the hours of darkness is an effective stopgap to enable the persimilis to hatch their eggs or to lay new ones. An effective tool for a small space is a cool mist humidifier sold in most local drug stores. They are under $30.00 and will last for months.

Good luck and happy bugging!