Medium 9781780642703

The Handbook of Naturally Occurring Insecticidal Toxins

By: Koul, O.
Views: 40
Ratings: (0)

Naturally occurring toxins are among the most complicated and lethal in existence. Plant species, microorganisms and marine flora and fauna produce hundreds of toxic compounds for defence and to promote their chances of survival, and these can be isolated and appropriated for our own use. Many of these toxins have yet to be thoroughly described, despite being studied for years. Focusing on the natural toxins that are purely toxic to insects, this book contains over 500 chemical structures. It discusses the concepts and mechanisms involved in toxicity, bioassay procedures for evaluation, structure-activity relationships, and the potential for future commercialization of these compounds. A comprehensive review of the subject, this book forms an important source of information for researchers and students of crop protection, pest control, phytochemistry and those dealing in insect-plant interactions.

List price: $295.00

Your Price: $236.00

You Save: 20%

Remix
Remove
 

7 Slices

Format Buy Remix

1 Introduction

PDF

1

Introduction

The study of naturally occurring toxins found in plants, animals and microorganisms in the field of toxicology is termed as toxinology.

These natural toxins range from simple to complex molecules and are lethal. Many have been studied for years but have yet to be thoroughly described. There are many plant species that produce toxic compounds for their own defence. Hundreds of microorganisms produce toxins that cause toxicity in other living organisms. There are hundreds of toxins produced by marine flora and fauna.

Overall, with the introduction of modern scientific methods of research, our knowledge of insecticidal plants, microorganisms and marine flora and fauna has expanded vastly.

Such compounds were documented in our earlier volume Insecticides of Natural Origin

(Dev and Koul, 1997), but since then there has been an enormous addition to our knowledge of this subject. Therefore, in this book

I describe the natural toxins that are purely toxic to insects, i.e. excluding feeding deterrents discussed in another volume (Koul,

 

2 Concepts and Mechanisms

PDF

2

Concepts and Mechanisms

The concept of botanically derived insecticides has gained favour in recent years, due in part to the perception that, because they originate from plant material, they are safer or ‘natural’.

These pesticides are often used for growing crops organically, according to guidelines set by certification programmes. However, it is important to be aware that they are pesticides, and that they fall under the same state and federal regulations as synthetic pesticides. In fact, plant-based insecticides have advantages as well as disadvantages. The advantages are that:

(i) they degrade rapidly and therefore are less persistent in the environment with reduced risks to non-target organisms, and may be applied shortly before harvest without leaving excessive residues; (ii) their action is rapid

(i.e. they may stop feeding by insects quickly or may cause paralysis); (iii) they have low mammalian toxicity; (iv) they are selective; and

(v) they are less phytotoxic than synthetic pesticides. However, there are disadvantages too, such as rapid degradation; possible toxicity in some cases for the user; cost and availability; and lack of toxicological data that are necessary for long-­term use and establishment of tolerances.

 

3 Bioassays

PDF

3

Bioassays

It is imperative to use standardized methods of testing for any bioactivity of a phytochemical, specifically when unknown new products are evaluated. According to Hurst

(1943) the fact that each component of a toxic compound may contribute towards gross toxicity restricts the usefulness of chemical assay as an index of toxicity. It is an invalid premise that the main purpose of any carrier solvent is to transmit the toxic compound to the insect in order to correlate chemical and biological tests, after which toxicant concentration is a limiting factor in biological activity. This assumption has been called a ‘standard’ protocol of insecticidal bioassays directed more towards arbitrary elimination of unknown variable factors than towards the fundamental causes of this variation. Bioassays against insects have been used for decades as a means of elucidating the activity of many chemical components.

The major goals achieved by using bioassay techniques are the determination of the roles of naturally occurring chemicals, the mechanism of resistance in crop plants and to find various insect control agents. As the aim of this book is to understand phytochemicals that are toxic to insects, it is imperative to know about certain fundamental requirements for such evaluations. The basic way of studying toxins is either to apply a product directly to the candidate insect body or

 

4 Structure–Activity Relationships

PDF

4

Structure–Activity Relationships

All major classes of secondary plant substances have biologically active compounds, especially among the higher oxidized metabolites. These compounds are structurally diverse and contribute to the defence of the plants. Insect toxins are examples of such biologically active substances which induce acute or chronic toxicity either temporarily or permanently; however, such compounds may or may not interfere with the behavioural physiology of an insect. One could expect specific quantitative structure–activity relationships (SAR) but there are several problems.

The overall picture from various evaluations shows that small structural variations can produce drastic changes in the activity profile of compounds. Various functional groups present in the active molecules, when examined critically, provide crucial information about the optimal relative stereochemistry required to stimulate the toxic action. However, analysis based on functionality and skeletal types suggests it is difficult to produce any generalization, albeit one can certainly discuss activity variations within a skeletal type. Analyses of structure–activity information within specific skeletal systems are discussed in this chapter to allow rational modification of readily available toxins to be made into potential insect toxins. However, specific insect antifeedants have not been considered in this chapter as the SARs of

 

5 Impact on Insect Natural Enemies

PDF

5

Impact on Insect Natural Enemies

It is well known that, in any agricultural pest control, use of insecticides is common but they also cause several problems, such as the development of resistance in pest populations, environmental health hazards and even toxicity to the natural enemies of the pest. The reduction in natural enemy numbers by such hazardous insecticides may have serious implications for global crop production. An especially challenging issue is the emergence of new pests and the eruption of secondary pests. In fact, increase in secondary pests is generally due to the depletion of the natural enemies that are responsible for keeping pests below the level of economic loss (Fernandes et al., 2008). One strategy used to avoid such deleterious effects is applying selective insecticides with the lowest possible impact on the other components of the ecosystem; that is, those that have a low impact on natural enemies (Degrande et al.,

2002). One such approach in the recent past has been the use of plant toxins (Koul and

 

6 Commercialization Prospects

PDF

6

Commercialization Prospects

Many extracts and individual allelochemicals from plant sources have given excellent results in laboratory conditions. In field situations, only a few are satisfactory alternatives to traditional pest management.

Chemical control usually involves broad-­ spectrum insecticides, and they have to be broad-spectrum by necessity, for they must sell in large enough amounts to accommodate financial development, research and marketing. The class of plant products is tested against one or a small group of insects attacking a specific crop. As a compound, if toxic, it could inhibit the feeding of one species, but for another it may be ineffective, or just an attractant or a growth inhibitor. Thus, replacement of a traditional chemical with a specific allelochemical will make pest management more expensive (Koul, 2008).

Over the past 20 years, domestic Chinese enterprises have invested considerable manpower and resources in developing botanical pesticides and they have achieved remarkable results. Compounds such as rotenone, martine, nicotine, toosendanin, veratridine, limonin, eucalyptol, and azadirachtin from the neem tree, have become registered products in China. Available statistics suggest that various plant-based pesticides are manufactured in 13 Chinese provinces and, overall, 43,000 varieties of plants are available

 

7 Bioefficacy

PDF

7

Bioefficacy

There has been continued interest in plant-­ derived chemicals to control pest insects because chemical pesticide products have substantial environmental hazards and health impacts. Although many natural products show lower mammalian toxicity than organochlorine, carbamate or other synthetic insecticides, it is a misconception that they are given a free licence for use just because they are natural products. One has to be cautious when searching for new insect control agents because target insect species may become resistant to them and it is important that non-target invertebrates and natural ­enemies are not harmed. There has been an emphasis on obtaining either the compounds that are direct toxins for insect pests or are compounds that modify the behaviour of target species and have a primarily nontoxic mode of action that may, in the long term, provide the most dependable and environmentally safe method of chemical control.

Insect antifeedants are one of the major categories of behaviour-modifying compounds that have been extensively documented in my earlier volume about insect antifeedants (Koul,

 

Details

Print Book
E-Books
Slices

Format name
PDF
Encrypted
No
Sku
BPP0000198534
Isbn
9781780642710
File size
13.5 MB
Printing
Allowed
Copying
Allowed
Read aloud
Allowed
Format name
PDF
Encrypted
No
Printing
Allowed
Copying
Allowed
Read aloud
Allowed
Sku
In metadata
Isbn
In metadata
File size
In metadata