Citrus trees have few insect predators so threatening that they cannot be tolerated for a short season. Their damage tends to be superficial. So when a pest problem does crop up, don't reach for insecticides right away. Keep in mind that spraying your trees with some insecticides may mean your fruit is no longer safe to eat.
Powdery mildew is a pest of many garden plants and orchards, but is rarely a noteworthy pest on citrus trees. It can be kept at bay with applications of fungicide, but it won't harm the tree much unless the leaves are covered so thoroughly that they can't photosynthesize. Black mold, which looks like mildew, is a fungus that grows on the excretions of sucking insects such as aphids. While the black mold doesn't do much harm, the insects that are attacking your tree and excreting the honeydew that the mold grows on are the real problem.
Most bug infestations are indications of other problems, such as drought, planting too many susceptible plants in the same area, lack of predator bugs or an unhealthy citrus tree. If you can correct any of these problems before infestation begins, the severity will likely be reduced. Preventing problems is better for the health of your citrus tree than any treatment after an infestation has begun.
There are few effective citrus tree bug sprays labeled for home use, and they aren't generally recommended. Instead, plant herbs and flowers that attract predatory insects (such as lady beetles, hoverflies and lacewings), since they will do a far better job of controlling your insect problems year-round than trying to maintain a pesticide program. In addition, lady beetles are often available for purchase from garden centers during the spring, and may be available online through the growing season.
Armored Body Bugs
In general, sprays that are able to wipe out scaled insect infestations are fairly toxic. For these, predatory insects are a better investment, since they'll keep working at the problem until it's managed. For especially young trees, oil and neem sprays might help get the problem under control long enough for your citrus tree to get established.
Mature trees should be able to survive insect infestations. If you fertilize often, this encourages the tree to flush out. The new leaves are like a beacon to sucking insects, and fall under attack far more often. If it comes down to a choice between fewer oranges and loquats from less fertilizer or fewer insects attracted to the new growth, consider dropping your fertilization rates.