When discussing pesticides, the mode-of-action in which they enact their effects is important as it determines the cost, efficacy and safety of a given pesticide plan. For insecticides, they can be categorised in a few ways: Based on their chemical composition, on their mode of entry, on their biological mode of action and on their stage specificity (e.g. targets larvae or adults).

For the purposes of appreciating the diversity of how insecticides are delivered and function as well as their relevance to environmental pollution, we shall be focusing on mode-of-entry, on their biological-mode-of-action.

Insecticides may be delivered to the target though a few main methods:

• Contact poisons: These insecticides may enter the insect body through its spiracles (breathing holes) or cuticle. And can result in lethal effects just from physical contact. Eg: DDT (nerve poison) and suffocants (blocks or clogs spiracles)
• Stomach poisons: Insecticides applied on parts of plants, which when ingested, act on the digestive system of the insect and results in death. E.g.: Calcium arsenate, lead arsenate.
• Fumigants: Volatile chemical substances or gases are used to suffocate or poison the pests. Eg: Aluminium phosphide, Carbon disulphide.
• Systemic insecticides: Chemicals that can move through the vascular systems of plants irrespective of site of application and poisoning insects that feed on the plants. Eg: Methyl demeton, Phosphamidon , Acephate
• Of course, these categories may not always be so clear cut. For example, Bacillus thuringiensis (Bt) poison may come in liquid sprays (stomach poisons), but Bt poisons may also be genetically engineered into pest resistant plants (figure 1), making them systemic insecticides as well.

Even from methods of delivery we can begin to see where pollution problems might arise (e.g. fumigation gases being inhaled by anyone in the area and non-target organisms such as pollinating bees).

However, delivery is only part of the equation. Understanding the biological-mode-of-action allows us to understand the environmental and health risks associated with pesticides.

• Physical poisons: Kill insects by exerting physical effects. E.g.: oily substances cause death by asphyxiation by blocking spircles.
• Protoplasmic poisons: Substances that can damage or kill living cells. In the context of insecticides, a protoplasmic poison can bind to midgut epithelium resulting in destruction of cell lysis. Eg. Bt
• Respiratory poisons: Chemicals which block cellular respiration as in hydrogen cyanide (HCN), carbon monoxide etc.
• Nerve poisons: Chemicals which block Acetyl cholinesterase (AChE) and effect the nervous system. Eg. Organophosphorus, carbamates.
• Chitin inhibitors: Disrupts the process of synthesis of chitin, which makes up an insect exoskeleton. E.g. Novaluron, Lufenuron, Buprofezin
• General Poisons: Compounds which include neurotoxic symptoms after some period and do not belong to the above categories. Eg. Chlordane, Toxaphene.

While there are hundreds of compounds used as insecticides, the most prevalent are Nerve poisons such as organophosphates, carbamates and synthetic pyrethroids which make up 75% of the insecticide market (Merrington et al., 2005).

This is especially concerning because unlike poisons that only affect insects such as chitin inhibitors and Bt poison (only activates in alkaline insect gut), without going too much into the details (as this is a geography mod and the biological details can be quite dry and boring) our mammalian nervous systems have some similarity to those of insects, making nerve poisons dangerous to us as well!

In subsequent chapters we will elaborate on other types of pesticides and the effects of pesticides on the environment.

Sources:

• Hassall, K.A. (1990) The Biochemistry and Uses of Pesticides. (2nd Edition). Macmillan Press, London
• https://www.slideshare.net/gill0094/insecticide-classification-of-insecticide-insecticidal-act-and-spraying-techniques-davinder-gill-135021014
• Merrington, G., Winder, L., Parkinson, R., & Redman, M. (2005). Agricultural Pollution Environmental: Environmental problems and practical solutions. London: Spon Press.
• https://agresearchmag.ars.usda.gov/1999/nov/pest