Pesticides cost money, although they may be cheaper than the labour involved in other control methods. The pests that survive may develop a resistance to the pesticide, so that it may take more and more poison to kill them, and eventually the poison may no longer work at all. The pesticide may kill more things than just the pest, such as the pest s natural enemies. If the enemies or biological controls have disappeared, then the pest may come back in even larger numbers than before. Other pests may also become a problem because the controls limiting their populations may be affected too.
Most pesticides are dangerous poisons for people too.
They must be stored, handled and used with great care, following exactly the directions for their application, and using as little as possible. If pesticides are misused or an accident occurs, they can easily contaminate food or the environment, which can be particularly serious on a small island. If people are exposed to pesticides or become contaminated while using them, they may become sick or die. It is therefore best to keep chemical pesticides as a last resort in case everything else fails, especially since islands are very vulnerable to pollution by chemicals. Since pest control is basically population management, it is important to understand the population factors for each pest, such as its life cycle and reproductive strategy, in order to know when control may be easiest and most effective.