Pesticides And The Weather

Thursday September 27, 2012


Mr Pest Control


How do organic and inorganic pesticides react to temperature and relative humidity?

Marcus F., FL

Mr Pest Control


I'm going to fish around a little bit here with the word "organic", but from your question I am assuming you are referring to the chemical make up of pesticides rather than their use for Organic Food Products. These are, of course, two very different topics and ones that likely cause some confusion. Organic chemicals are those that contain carbon in their molecules, and these comprise the vast majority of the pesticides in use in pest management. They often are synthetic molecules that were created in a lab, but also include the great many natural pesticides that are derived from plants, such as pyrethrum and essential plant oils. Organic pesticides also include the pyrethroids, organophosphates, carbamates, and the many new families of pesticides that are synthetically created. 

Inorganic pesticides are those that are found naturally but usually are mineral in nature. These include active ingredients such as boric acid, diatomaceous earth, silica gel, aluminum phosphide, copper compounds, sulfur, and others. Because of the mineral nature of many of them they are very long lasting, and are essentially unaffected by environmental conditions such as heat or UV light. The boric acid placed within a wall or along a crevice in a bait product will last for many years as long is it is not somehow covered or tied up by other dusts or grease. Boric acid dust products are effective for as long as they stay dry, so I suppose that in a region with very high humidity this could affect the dust over time, perhaps causing it to cake or clump so that it no longer can adhere to a passing insect. 

But, for all of those organic pesticides, whether synthetic or extracted from plants, heat, water, UV light, and alkalinity are the enemies. Some families of chemistry are far more susceptible to these effects than others. Organophosphates like diazinon and malathion were quite susceptible, and in direct sunlight, in highly alkaline water, or on very hot days the active ingredients could break down rapidly to molecules that no longer killed the pest insects. Sometimes this could be a benefit, as with the aerial spraying of malathion for Med Fly. The malathion degraded so rapidly that within just a couple of days the treated areas had, essentially, no toxic malathion remaining on the leaves to worry homeowners. But, the rapid loss of the active ingredient also meant that future pests were not going to be affected, so repeated applications were necessary. The chlordane that protected homes from termites for 35+ years has been replaced by active ingredients that may provide protection for 5 years or less, meaning new applications are needed to maintain that protection. 

Synthetic pyrethroids, which include so many of our current insecticides, are less affected by temperature, water, pH, and UV light, so they will last longer when exposed to the elements but still degrade relatively fast in the environment. Again, this is a double edged sword. Rapid degradation means a lower chance of off-site movement but also a loss of control of new pests that enter a treated area. Temperature definitely has a direct effect, and the hotter it is the faster the molecules break apart. Relative humidity is probably less of an issue, but since water is one of the factors I suppose a dry environment would be more likely to sustain the molecule than one with high levels of moisture in the air. 

Mr. Pest Control

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Please note, Mr. Pest Control is answering questions supplied by PMP customers across North America. His answers are generated from industry and manufacturer-provided information. The answer may not be specific to the laws and regulations for your State, Province, Territory or Country. In addition, products mentioned may not be registered and or available in all areas. Always check with your local Univar office for specific information to your area. Always read and follow label directions.