Pesticides are used to protect crops from insects and other pests. but sometimes pesticides can be as harmful as they are useful. how can pesticides be harmful? a) pesticides are usually mild and most insects are not affected by it. b) pesticides are very expensive and few farmers can afford to use them. c) insects become resistant to pesticides after being exposed to it for some time. d) pesticides kill all the crops affected by insects, causing huge losses to farmers.