report
2.2.3.2 Disease and Pest Control in Crop Plants

Pesticides are used to kill organisms that are detrimental to agricultural production, including viruses, bacteria, fungi, parasites, weeds, and insects.  They have been used for millennia (sulphur being the first recorded), however, modern pesticides place their own burdens on farming systems, for example accumulation in soils and ecosystems which can have unexpected and often deleterious effects, as was the case with DTT.  This is not just historic; atrazine is widely used to control weeds, but is persistent in soil (with a half-life of anything between 41 and 231 days, dependent on depth of soil and water content[i]).  It has a permissible limit of 3 ppb, but this is often exceeded locally and it can also migrate far leading to contamination of rivers and drinking water.  The situation is further deteriorated when one considers that, depending on the environmental conditions and mode of application, as much as 90% of conventional pesticides are lost to the air during application, as run-off, or decompose; affecting both the environment and cost to the farmer[ii].  There is therefore an urgent need to a) find alternatives to current pesticide deployment and b) find ways of rapidly and locally detecting levels of the pesticide and either removing or degrading it.  Several systems based on nanotechnology are being developed for the purpose of detection (see section above) and degradation of pesticides (see environment report).

In the last few years there has been much activity by companies in the re-formulation of pesticides which are coming off patent, to protect market position.  This is largely in the area of new adjuvants or delivery systems (as is the case with pharmaceuticals).  For crop pesticides it is also driven by the need to resolve issues with current formulations including: use of organic solvents (many pesticides are poorly water soluble, and many of the organic solvents used in their formulations, such as benzene and toluene, are toxic); sensitivity to UV light (many have half-lives measured in hours or minutes, e.g. phoxim 40 minutes, avermectin 6 hours), bioavailability (crossing plant cell walls), deposition and drift (due to droplet size when spraying), foaming, rain-fastedness (sticking to leaves) and combining multiple pesticides into a single product.

Another issue is that of legislation.  In the EU there is a drive to limit the numbers of pesticides available.  Directive 91/414/EEC is the EU regulation covering the use of pesticides, and is essentially risk-based.  However recent amendments could decrease the number of pesticides available to farmers (15% of the 300 accepted chemicals have been estimated by the UK government’s Pesticides Safety Directorate), which various agrifood industry associations are concerned could affect yield and quality of produce[iii].

The solutions are then to move towards measured release of small (but sufficient) amounts of pesticide over a period of time, in response to environmental stimuli (e.g. UV, moisture, temperature) or to target the pest more effectively.

This involves formulating the pesticide with a carrier, most of which are lipid or polymer based (nano-emulsions), however other systems including silica and nano clays are also being developed.