A pesticide may be a compound or chemical used to target certain unwanted living organisms. They may exist in form of fungicides, herbicides, insecticides...

I will mostly focus on animals and plants. Most farmers use them for their efficient way of eradicating certain plant, insect or animal species that may hinder or reduce yields, also known as weeds (this increases food production and profits to the farmers) and killing disease spreading vectors.

However, they also have some negative effects such as;

-Threatening biodiversity. A good example of threatened species here are bees. These pesticides kill and negatively affect bees, one of the greatest plant pollinators, and without pollination, many plants are not able to bear fruits;

-Contamination of water as the chemicals may seep through the soil down to the water table or simply drop into any nearby water source;

-Cause illnesses among people such as; cancer, Alzheimer's disease, Diabetes (Yes!), Parkinson?s disease, Endocrine Disruption, Asthma and Birth defects, all through inhaling and consuming through food. No matter how much is done to make sure that crops are pesticide free, there are always some traces of the chemicals in food. This can be seen by some Samira Mebdoua and Co who carried out a research on the Evaluation of Pesticide residues in fruits and vegetables from Algeria. It A total of 160 samples of 13 types of fresh fruits and vegetables from domestic production and import were analysed to detect the presence of pesticide residues. Analysis was performed by multi-residual extraction followed by gas chromatography–mass spectrometry. In 42.5% of the tested samples, no residues were found and 12.5% of samples contained pesticide residues above maximum residue limits. Risk assessment for long-term exposure was done for all pesticides detected in this study. Except chlorpyrifos and lambda-cyhalothrin, exposure to pesticides from vegetables and fruits was below 1% of the acceptable daily intake. Short-term exposure assessment revealed that in seven pesticide/commodity combinations, including three pesticides (chlorpyrifos, deltamethrin and lambda-cyhalothrin), the acute reference dose had been exceeded.

All this said, some people have thought of replacing the use of pesticides with the use of genetically modified   foods but in by some studies, these were found to harm health and also increase weed growth. I would rather we try using drone technology like it's done in some developed countries like the Netherlands or try going back to the use of natural methods of weed control such us the use of trap crops planted around the main plant( these present similar characteristics as the main plant, and usually attract pests. When the pests are done breeding, the farmer usually cuts them down and burns them.), crop rotation, poly-culture, biological pest control depending on one's preference.

References:

http://www.gardendata.org/advantages-and-disadvantages-pesticides-and-environement

https://www.naturalhealth365.com/pesticides-toxic-chemicals-a868-html

https://www.southernstates.com/articles/pesticide-alternatives.aspx

http://www.tandfonline.com/doi/abs/10.1080/19393210.2016.1278047

http://sitn.hms.harvard.edu/flash/2015/gmos-and-pesticides/

https://www.npr.org/sections/thesalt/2016/09/01/492091546/how-gmos-cut-the-use-of-pesticides-and-perhaps-boosted-them-again