Precision Pesticides

November 10, 2018 11:15 PM
Blog Post

Pesticides are substances that are designed to control pests. Every year, nearly six billion pounds of pesticides are applied worldwide. In agricultural fields, they can be sprayed, injected into the soil, or added to the seed.

There are different kinds of pesticides, named after the pests they attack: insecticides act on insects, herbicides on plants, fungicides on fungi, rodenticides on rodents, etc. Older pesticides had broad targets — they could harm insects, plants, birds, and mammals. However, newer pesticides are largely effective only against some species. For example, neonicotinoids, the most commonly used insecticides in the world, are quite safe to mammals. However, they are unable to distinguish between insect pests that damage crops and beneficial insects like bees that pollinate crops. Hence, modern-day pesticide manufacturers must increasingly find ways to harm the pests while keeping the rest safe. This can be achieved in two ways:

1. The pesticide should be toxic only to pest species


2. Non-pest species should not be exposed to the pesticide

Described below are three emerging pest management technologies trying to achieve this goal: CRISPR/Cas 9, RNAi, and nanopesticides.

1. CRISPR/Cas9 system is found naturally in bacteria. Bacteria use it to defend themselves against invading viruses. Scientists now harness this defense to modify the properties of an organism, either by shutting down the organism’s genes or introducing foreign genes into it (Fig. 1). As genes are unique to every species, this technology can be species specific. It also causes the altered genes to be passed down to future generations.

Scientists recently used CRISPR/Cas9 to disrupt a gene necessary for female development in the spotted wing Drosophila, a fruit pest. This caused the female flies to die early. However, such modified organisms are currently not released into the wild because it is not known if any unintended effects could occur. For example, other organisms of the species could gain resistance to this technology by slightly changing their DNA. Or CRISPR/Cas9 could shut down the wrong gene or act on non-pest species with similar genes.

A CRISPR-Cas9 system.
Figure 1. A CRISPR-Cas9 system can be used to either a) prevent an important protein from being formed or b) create a protein which is deadly to the organism.

2. RNA interference (RNAi) system has been observed in many non-bacterial species, including plants and animals. It is also used as a defense against viruses. RNAi shuts down RNA; the DNA and genes are unaltered. When an organism’s RNA is shut down, it can no longer form protein and could kill an organism. For example, scientists developed corn which contain RNAi that targets western corn rootworm. When the pest ingests the corn, they can no longer form a critical protein and most die within 12 days.

As RNA is formed from genes, this technology is also species specific. It has similar drawbacks to CRISPR/Cas 9 but its effects are limited to one generation as modified RNA cannot be passed down to the offspring. Its persistence and movement in the environment are largely unknown.

3. Nanopesticides are pesticides which are incorporated within nanoparticles. Nanoparticles are very small molecules (billionth of a meter) that are usually made from metals. As smaller-sized particles cover a larger surface area, lower rates of nanopesticides would need to be sprayed in the field. Also, a greater surface area increases contact with pests. Nanopesticides do not break down easily. Thus, unlike traditional pesticides, they need not be applied frequently. And potentially, they could be encased in capsules that penetrate and enter pests more effectively than non-pest species. These properties could lower overall amounts of pesticides in the environment and/or reduce exposure to non-pests.

Nanosilver, the first nanopesticide, has been applied in plants to reduce rates of fungal root diseases. However, there are several unresolved issues. Since nanopesticides are more stable, they could persist in the environment for longer than required. And we do not know how they move - for example, do they stick to crops or do they slide off it and enter the soil or water and affect other organisms?