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If cultural controls fail or are impossible to apply, the second level is to use biological controls. Use predator mites, lady bugs to control aphids, etc. BT is now available for a wide range of pests.
If biological controls fail or are nonexistent for the pest, the last level is to use chemical controls in increasing order of toxicity. The least toxic used first.
The premise of IPM is that we never 'get rid of' anything. Pests and diseases are at best controlled, kept at minimum levels where the damage is acceptable. Pests and diseases are a fact of life, and we will always have to deal with them. The question is how will we deal with them.
The key to successful use of IPM is monitoring. Without monitoring the pest levels, one is doomed to blasting full blown infestations with the strongest stuff possible. Monitoring requires one to weekly check representative samples of the crop for pests and diseases. In my nursery this takes between one and two hours, but it is a fun job and a good excuse to get out of real work.
The essential tool for monitoring is a good 5X hand lens. Several insects are too small to see with the naked eye if you are over 40 like me. Two-spotted mites are one of my biggest problems and they are nearly invisible without the lens. I look for speckled discolored leaves, traces of yellow and red, then look at the undersides for traces of debris. A healthy leaf will be completely smooth with the exception of hairs for some species. An infected leaf will be covered with 'dirt', little bits of debris, webbing and eggs. The eggs are translucent spheres just visible even with the lens. Red spider mites are similar but are larger and a little easier to see with the naked eye and leave visible webbing. Red spider mites have a reddish color and are fairly easy to control. Two-spotted mites are translucent with two dark spots on their back and are the devil to control. Both have life cycles of about 5 to 7 days so follow up spraying is always necessary.
There are fungal diseases for which there is no cure, only cultural control, level one. In the case of some root rot problems cultural changes may be all that is necessary. In the process of instituting such cultural changes, I was recently forced to look at how I water and how wet the plants were staying. As a result I discovered other problems related to over wet conditions and have now acted to correct the problem by hand watering only the plants that are dry. This is the kind of thing that IPM is really good at, it forces one to observe and to act at the lowest level of toxicity, and often this is the very best solution, far better than the chemical warfare to which we have been raised to expect.
On the other hand I have a retail store where things are spaced evenly and attractive, a good diversity of plants and much hand watering which has a good washing action. I stock plants at my store with plants from the growing grounds. Now I never try to intentionally infest my store with pests so I am always looking for clean stock to bring in, but in over seven years of operation I have never sprayed for mites in my store. It amazes me, but I attribute it to good cultural practice, probably some natural predators in the area, and the hand watering which tends to limit mite infestations.
This is not to say mites have never gotten in there. I find them occasionally, but they never reach levels that endanger or disfigure my plants. I find them on their favorite targets but they never seem to spread. I monitor them all the time to make sure the damage is nonexistent or minimal. There is a natural balance going on in the store that is obviously not going on in my growing grounds. The natural balance has never been upset and I don't intend to upset it by spraying unless I really have to. I think that once this symmetry of natural control and balance is upset it is extremely hard to reacquire.
This is an incredibly complex topic and I think there is room for systemic insecticides for aphids without upsetting the balance, but I also believe that natural controls for even this pest if carefully managed would obviate a need for the chemicals.
The problem is that spraying for pests will usually upset any natural balance of predator organisms which may be present. Predators are not always other insects or mites. There are beneficial fungi and bacteria as well, possibly even beneficial viruses as yet undiscovered. There are even minerals in water that can favor or diminish the pests or the predator (beneficial) organisms. Very little is known about any of this but we see it all the time. My advice is: If it ain't broke, don't fix it. If insects, mites and fungal diseases are not a serious problem leave them alone or use the least toxic method possible first. If your trees are in danger then the balance probably has already been upset and using more toxic methods will probably do little more damage. This is the essence of IPM.
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