5/20/2014 Crop protection chemicals http://www.essentialchemicalindustry.org/materials-and-applications/crop-protection-chemicals.html 1/13 The development of chemicals to protect agricultural crops is an important activity within the chemical indus Without them, many crops would suffer dramatic losses. Some of these chemicals, the insecticides, are also important in combating human and animal diseases. The environmental and toxicological properties of thes chemicals have improved considerably over the last six decades. Research aims to produce chemicals that not just potent but are specific for the required purpose, whilst not affecting the environment in any other wa Because pests may develop resistance to crop protection chemicals there is a continual need for new prod be developed. Materials and applications Crop protection chemicals Crop protection chemicals Three groups of chemicals dominate this part of the chemical industry (Figure 1). They are: Herbicides : substances that kill or inhibit growth of unwanted plants (weeds) Insecticides : substances that kill arthropod pests, i.e. insects and mites Fungicides : substances that destroy or prevent the growth of pathogenic fungi All three are pesticides . Figure 1 Global sales of crop protection chemicals (2008). The effectiveness of a pesticide is the result of the proper 3D-assembly of specific groups in the chemical structure of active ingredient. If several compounds of a given chemical class have related efficacies, they include a set of groups common minimum basic feature responsible for the best fit for the specific molecules in the biochemical target molec a protein) of the pest. This set of groups is called the toxophore. Where possible, the toxophores described in this u indicated by shading. Development of new chemicals It is estimated that it costs about œ150-200 million to discover a new product, test it thoroughly for its action and its for the environment, and develop manufacturing techniques for its synthesis. It takes an average of 10 to 15 years to so it is small wonder that, worldwide, only about 12 chemicals are introduced each year. However, these chemicals to the efficient production of food (Figure 2). CONTENTS Home Introduction Industrial processes Materials and applications Biofuels Biorefineries Biotechnology in the chemical industry Colorants Composites Crop protection chemicals Edible fats and oils Fertilizers Nanomaterials Paints Soaps Surfactants Basic chemicals Polymers Metals search...
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The development of chemicals to protect agricultural crops is an important activity within the chemical industry.
Without them, many crops would suffer dramatic losses. Some of these chemicals, the insecticides, are also very
important in combating human and animal diseases. The environmental and toxicological properties of these
chemicals have improved considerably over the last six decades. Research aims to produce chemicals that are
not just potent but are specific for the required purpose, whilst not affecting the environment in any other way.
Because pests may develop resistance to crop protection chemicals there is a continual need for new products to
be developed.
Materials and applications Crop protection chemicals
Crop protection chemicals
Three groups of chemicals dominate this part of the chemical industry (Figure 1). They are:
Herbicides: substances that kill or inhibit growth of unwanted plants (weeds)
Insecticides: substances that kill arthropod pests, i.e. insects and mites
Fungicides: substances that destroy or prevent the growth of pathogenic fungi
All three are pesticides.
Figure 1 Global sales of crop protection chemicals (2008).
The effectiveness of a pesticide is the result of the proper 3D-assembly of specific groups in the chemical structure of its
active ingredient. If several compounds of a given chemical class have related efficacies, they include a set of groups as a
common minimum basic feature responsible for the best fit for the specific molecules in the biochemical target molecule (e.g.
a protein) of the pest. This set of groups is called the toxophore. Where possible, the toxophores described in this unit are
indicated by shading.
Development of new chemicalsIt is estimated that it costs about œ150-200 million to discover a new product, test it thoroughly for its action and its safety
for the environment, and develop manufacturing techniques for its synthesis. It takes an average of 10 to 15 years to do this
so it is small wonder that, worldwide, only about 12 chemicals are introduced each year. However, these chemicals are key
They are selective systemic herbicides which are absorbed by the foliage before translocating through the plant. They are
particularly useful in controlling annual grass and dicot weeds in crops such as corn (maize) and are rapidly degraded in
soils.
A well known example is nicosulfuron which is marketed, for example, as Accent which is used to control weeds in corn.
(e) TriketonesThe triketones act by inhibiting an enzyme that would have led to the formation of compounds needed in the biosynthesis of
carotenoids. These, in turn, play a key role in absorbing light energy for photosynthesis and protecting chlorophyll from
photochemical damage. Triketones act through application on the foliage or via the roots. In either case there is translocation
through the plant. They are particularly effective in controlling a wide range of weeds. Degradation in soils is fairly rapid.
Tembotrione is an example that allows the control of grasses and broad leaves in corn (Figure 4) and is sold under the trade
name Laudis.
Figure 4 The weeds
in this field of corn
(maize) have been
controlled with
tembotrione. It has
been applied in
combination with a
compound that
triggers degradation
of tembotrione in the
corn but not in the
weeds. Such
compounds are
known as safeners.
By kind permission of Bayer
CropScience.
Mesotrione is another example of this class of herbicide and is sold under various trade names, including Callisto.
(f) Inhibitors of acetyl-CoA carboxylaseSome herbicides act by inhibiting the action of the enzyme, acetyl-CoA carboxylase, essential for the biosynthesis of
carboxylic (fatty) acids, needed by the plant for the production of cell membranes. An example is pinoxaden which is
particularly useful in dealing with grass in fields of barley and wheat.
(c) Macrocyclic lactones (avermectins and milbemycins)All natural and semi-synthetic 16-membered macrocyclic lactones disturb the systems which control the flow of chloride
ions. This shuts down the electrical impulses in the nerve cells of their target organisms.
X Y R
avermectin B1 OH H>80% C2H5
<20% CH3
emamectin benzoate H NH-CH3
>90% C2H5
<10% CH3
The naturally occurring avermectins and emamectin benzoate are based on the above structure. The avermectins are
produced by fermentation from the soil microorganism Actinomycetes (from the genus Streptomyces) and emamectin
benzoate is prepared from abamectin in a series of chemical reactions. Another naturally occurring example is milbemycin.
The whole family of macrocyclic lactones displays unprecedented potency against mites and insects as well as nematodes
(e.g. parasitic worms). Avermectin is used in various crops such as citrus, pome fruits (for example, apples and pears),
vegetables and cotton. Milbemectin is mainly used to combat the many different mites in tea and pome fruits (e.g.apples),
and also against pine wood nematode which devastates pine trees in Japan and parts of the US.
The products are non-systemic and are removed rapidly from the environment after application. Photolysis on plant surfaces
is fast, and they bind tightly to soil, where they are rapidly degraded by soil microorganisms. Thus, no leaching or
bioaccumulation occurs. Because of the rapid uptake into sprayed foliage combined with fast degradation of surface
residues, this family of insecticides is safe to use.
(d) PhenylpyrazolesA well-known member of this class of insecticides is fipronil, which can be applied to the foliage, soil and seeds. However, it
has a limited ability to translocate through the plant. Fipronil acts by disturbing the chloride ion concentrations in cells in
pest species such as the Lepidoptera (moths), Coleoptera (beetles) and Diptera (flies, mosquitoes). For example, it is widely
used as a foliar spray for the control of leaf- and planthoppers in rice in south-east Asia.
Other applications of the phenylpyrazoles include the control of urban pests such as ants and cockroaches indoors and on
lawns. They are also very effective in controlling termites. In animal health care, they are used on cats and dogs to combat
ticks and fleas.
(e) Nereistoxin analogues and neonicotinoidsMuch work has been devoted to finding chemicals which will prevent the nicotinic acetylcholine receptor (nAChR) from
functioning correctly. The receptor is important in allowing the passage of sodium and potassium ions in the nerve cells and
thus affects the central nervous system. First examples of these insecticides are cartap, thiosultap, bensultap and
thiocyclam.
They break down either by action of water or light to produce the toxin, nereistoxin, a substance that was first isolated from
the naturally occurring marine nereid worm Lumbrineris heteropoda Marenz, which acts by effectively paralyzing the pest.
Another group of insecticides which affect the nicotinic acetylcholine receptor is the neonicotinoids which are now the fastest
growing and fourth major class of insecticides in crop protection. They are active against a broad range of insect pests, and
exhibit activity through both oral (ingestion) and contact routes of application. They have a high level of efficacy, and a
favourable environmental and toxicological profile. This has led to their rapid adoption in numerous agricultural areas for quick
control of a broad range of chewing and sucking pests with minimum impact on beneficial insects.
Like the naturally occurring alkaloid nicotine (used for a long time in the form of aqueous tobacco extract),
neonicotinoids act selectively on and overstimulate the insect's central nervous system. The following are examples of this
Strobilurins interfere with the production of adenosine-5'-triphosphate (ATP), the nucleotide that transports energy within cells
for metabolism, thus preventing germination and growth. The strobilurins can combat most major fungal diseases found on
grasses (turf), vines, fruit and particularly cereals.
Like the triazoles, they are systemic, and they offer protection and also cure the problems caused by the fungi.
Examples of commercially available strobilurins are given below:
(c) CarboxamidesCarboxamides are so named as they contain the carboxylic acid amide (-CO-NH2) or related group. As with the strobilurins,
they interfere with the production of ATP.
An example of a carboxamide fungicide is boscalid used in particular to control powdery mildew on fruit and vegetables.
It is expected that several other compounds in this class will be commercially available in the near future examples of which
are bixafen and isopyrazam.
(d) Fungicides with a 'multi site action'Besides fungicides which act mainly on one target enzyme, several fungicides with a 'multi site action' are commercially
available. They act unspecifically on a number of enzymes and are therefore not very sensitive to resistance problems. They
are all protectant fungicides, acting on contact with the fungal spores on the surface of the leaves.
Some examples are inorganic compounds such as elemental sulfur and some copper salts of which the 'Bordeaux mixture' (a
mixture of copper(II) sulfate and calcium hydroxide) is the most famous. Both are allowed to be used in the production of
organic food. Examples of organic compounds with a multi site action are salts of dithiocarbamates such as chlorothalonil
and propineb. They are very widely used on a wide range of plant pathogens in fruits and vegetables such as grapes, apples,
(e) Specific fungicides with activity against downy mildewsSince downy mildews are a special class of plant pathogens causing severe damage mainly in grapes and potatoes a
number of specific fungicides active only against this kind of plant diseases are available. Most of them act systemically.
Since they belong to different chemical classes they exhibit several modes of action.