One is insecticidal soap, a suspension of sodium or potassium salts in processed coconut oil that penetrates the insects' bodies and dissolves the membranes around its cells. Use pesticides only as a last resort. Keep in mind that most of an applied pesticide eventually ends up in the environment. We are still living with the ravages of DDT, for instance, a quarter century after the chemical was banned. Many professional foresters think twice about the use of potent chemicals.
Some diseases are potent killers.
Fireblight, for instance, is a bacterial infection that blackens entire branches of a tree. Brown rot can destroy both buds and blossoms on many fruit trees. Not all tree diseases are so catastrophic, but all are cause for concern. In many cases, by the time symptoms appear, it is too late to fight the disease, so treatments focus on the coming winter and the next growing season. All dead leaves and branches as well as the mulch under the tree are collected and removed or burned.