What types of pests are treated by insecticides?

Insecticides are specifically formulated to target insects that pose various threats either to agricultural crops, human health, or the environment. This includes insects that may directly damage plants, thereby affecting food production, as well as those that can transmit diseases to humans, animals, or plants. Furthermore, certain insects, even if not directly harmful, may be considered nuisances because of their presence or behavior, such as mosquitoes. This makes option B the correct response, as it comprehensively covers the various roles insects can play in relation to their impact on agriculture, health, and quality of life.

The other options refer to categories of pests that are treated with different classes of pesticides or do not fall under the purview of insecticides. For instance, harmful weeds are managed with herbicides, soil-borne pathogens are typically addressed by fungicides or nematicides, and fungi and algae are likewise treated with fungicides and algaecides, respectively. Thus, while those categories are important in pest management, they are not relevant to the specific action of insecticides.