In view of the adverse effects of synthetic organic pesticides on non-target organisms and the environment, efforts have been made during the last three decades to develop safer and more selective pesticides. Accordingly, many conventional pesticides have been replaced by low-risk or ‘biorational pesticides’ which have been defined as those chemicals that affect the growth, development, biology and ecology of pest species, differently from that of beneficial species.
In other words, biorational or ‘reduced risk’ pesticides are synthetic or natural compounds, that effectively control insect pests, but have low toxicity to non-target organisms (such as humans, animals and natural enemies and the environment. Most of the biorational pesticides are preferable to the conventional pesticides because of their specificity to the targeted pests, their effectiveness at lower rates and their non-persistent characteristics in the environment.
Semiochemicals:
Semiochemicals are chemicals that are able to modify the behaviour of a perceiving organism at sub-micro/nanogram levels. The term is derived from the Greek word simeone, meaning a mark or a signal. Semiochemicals are divided into intraspecific and interspecific communication chemicals.
Such behaviour-modifying chemicals (BMCs) include pheromones, kairomones, allomones, attractants, deterrents and other parapheromones. Sometimes, a single chemical secreted may have both intra- and inter-species activities. Semiochemicals are becoming increasingly valuable in insect pest management and offer enormous potential for controlling many insect pests through a range of environmentally and ecologically sound pest management technologies.
The chemicals used for intraspecific communication are called pheromones. The term is derived from the Greek pherein, meaning to transfer and hormone, meaning to excite. Thus, a pheromone can be defined as a chemical or a mixture of chemicals that is released to the exterior by an organism and that causes one or more specific reactions in a receiving organism of the same species. When a chemical not found in an insect has a pheromone-like action, it is often referred to as a parapheromone.
Chemicals involved in interspecific communication are known as allelochemicals or allelochemics. These are non-nutrient substances originating from an organism which affect the behaviour, physiological condition or ecological welfare of organisms of a different species.
These chemicals are further divided into following categories:
1. Allomone:
A compound released by one organism which evokes a reaction in an individual of a different species that is favourable to the emitter but not to the receiver.
2. Kairomone:
A compound released by one organism which evokes a response beneficial to a member of another species but not to the emitter.
3. Synomone:
A substance released by organism which benefits both the sender and the receiver.
4. Antimone:
A substance produced or acquired by an organism that when it contacts an individual of another species in the natural context, evokes in the receiver a behavioural or physiological reaction that is maladaptive to both the emitter and the receiver.
5. Apneumone:
A chemical released by non-living substances that is beneficial to the receiver but detrimental to other organism in the substance.
Practical uses of semiochemicals in pest management:
Monitoring:
i. Detect the presence of a species.
ii. Measure seasonal activity and provide decision support.
iii. Evaluate the effectiveness of mating disruption.
iv. Assess levels of insecticide resistance.
Direct Control:
i. Mass deployment of attractant-baited traps.
ii. Application of attract-and-kill formulations or devices.
iii. Pheromone-mediated mating disruption.
iv. Manipulation of natural enemies using allelochemicals.
v. Pheromone-based interference with host location or acceptance.
vi. Plant allomone based deterrence of feeding or oviposition.
vii. Application of pheromones to enhance pollination.
Pheromones:
A dazzling variety of behaviour, from sexual attraction and dispersal to caste determination, is controlled by pheromones. It is thus natural that pheromones have attracted the attention of applied entomologists for exploitation in the control of insect pests.
Pheromones may be divided into two categories, viz., releasers, which induce an immediate behavioural change, and primers, which initiate changes in development, such as sexual maturation, and so do not result in immediate behavioural changes, but predispose to them.
Pheromones are categorized according to the function they perform, viz., sex pheromones, aggregation pheromones, alarm pheromones, trail pheromones and host-marking pheromones.
1. Sex Pheromones:
Sex pheromones are often produced by the females to attract males for mating, but they may also be produced by males to attract females. They seem to be highly developed in Lepidoptera and are frequently produced by eversible glands at the tip of the abdomen.
The release of sex pheromones is a complex physiological process, often associated with sexual maturity and environmental stimuli such as photoperiod and light intensity. Female sex pheromones are usually received by sensory sensillae on male antennae, and males search up wind, following the odour corridor of the female.
The sex pheromone of an insect usually consists of a blend of different components, which are volatile, specific to one species or a small number of related species, and is very potent over considerable distances. This specificity allows targeted application against a specific pest and with minimal influence on the ecosystem.
Lepidopteran sex pheromones are usually simple molecules (e.g., long-chained aliphatic, lipophilic, acetates, aldehydes, or alcohols), often with one or two double bonds. Sex pheromones of Diptera, Coleoptera and other groups have usually more complex chemical structures, which are comparatively unstable and, therefore, much more difficult to synthesize and formulate.
2. Aggregation Pheromones:
These pheromones cause insects to aggregate or congregate at food sites, reproductive habitats, hibernation sites, etc., and are prominent in some species of beetles. They are particularly well understood in bark beetles, Ips spp. and Dendroctonus spp., which are involved in tree attacks.
They are attractive to both sexes and tend to operate over a long range and have the potential of attracting thousands of individuals. Generally, aggregation pheromones have more complex chemical structures, not very stable or amenable to synthesis and deployment, and elicit a much more complex behaviour that is less open to manipulation.
3. Alarm Pheromones:
Alarm pheromones are common in social insects such as ants and bees, and aphids, which are usually found to occur in aggregation. The function of this type of pheromone is to raise alert in conspecifics, to raise a defense response, and/or to initiate avoidance. For instance, the remains of the sting apparatus of a honey bee left in the victim’s body releases an alarm pheromone that attracts other bees and stimulates them to sting.
A dose response of attractancy and repellency has been demonstrated for several pure volatiles from the venom of common wasp, Vespa vulgaris Linnaeus and the German wasp, V. germanica Fabricius. The alarm pheromones are usually highly volatile (low molecular weight) compounds such as hexanal, 1- hexanol, sesquiterpenes [e.g., (E)-β-farnesene for aphids], spiroacetals or ketones.
(E)-β-farnesene is naturally occurring chemical found in about 400 plant species. It is synthesized from its precursor, farnesyl pyrophosphate by the action of the enzyme, (E)-β-farnesene synthase. (E)-β-farnesene repels the aphids, but attracts their natural enemies like the ladybird beetles and the parasitic wasps.
Recently, the scientists at the Rothamsted Research in Harpenden, Hertfordshire (UK), have developed genetically modified (GM) wheat by inserting the peppermint (Mentha piperata Linnaeus) into the DNA of a spring wheat strain Cadenza. The GM wheat would go in a long way to protect the crop against the grain aphid, Sitobion avenae (Fabricius); bird cherry-oat aphid, Rhopalosiphum maidis (Fitch); and rose grain aphid, Metopolophium dirhodium (Walker), which are three main aphid pests of wheat in U.K.
The applications of (E)-β-farnesene along with host plant volatiles caused the aphids to move away from feeding sites and may be utilized to prevent colonization of the host plants by the aphids. It also increased the mobility of aphids, which is useful for increasing the pick up of contact insecticides or microbial pathogens. (E)-β-farnesene is now commercially available for use against a number of aphid species including Aphis gossypii (Glover) and Lipaphis erysimi (Kaltenbach).
Farnesol and nerolidol are the alarm pheromones from the two-spotted mite, Tetranychus urticae Koch, and are released under natural conditions when the population is threatened or is being attacked by a mite predator. The result is an increase in activity of mites with consequent greater exposure to a co-applied miticide.
4. Trail Pheromones:
Trail pheromones are produced by foraging ants, termites and larvae of some lepidopteran insects. They are essentially used to indicate sources of requisites to other members of the colony. While trail pheromones are frequently associated with walking insects such as ants, they are also known from other insects.
Bees use trail pheromones during foraging for making attractive foraging sites as well as for scent marking of unproductive food sources. Trail pheromones are characteristically less volatile than alarm pheromones. The trails are continuously replenished through traffic, otherwise they dissipate.
Identification and synthesis of trail pheromone of bumble bees could lead to increased efficiency in their use for pollination. It is also possible to manipulate trail following and recruitment of tent caterpillars, Malacosoma americanum (Fabricius).
5. Host-Marking Pheromones:
Host marking, spacing or epidietic pheromones elicit dispersal away from potentially crowded food sources, thereby reducing numbers. These pheromones reduce intraspecific competition by disrupting landing, feeding or oviposition of pests on their host plants. They are thus one of the few pheromones that serve to repel rather than attract the insects.
They are known from a number of insect orders, viz., Coleoptera, Lepidoptera, Diptera, Homoptera, Orthoptera and Hymenoptera. The females of apple maggot, Rhagoletis pomonella (Walsh) and cherry fruit fly, R. cerasi (Linnaeus), ovipositing in fruit, mark the surface to deter other females.
Mating deterrent pheromones are known from a number of insects including tsetse flies, house flies and other Diptera. There is also exploitation of prey host marking and sex pheromones by parasitoids, which use signal persistence of these intraspecific cues to find their hosts.
The host-marking pheromones of the bark beetles, Dendroctonus spp., such as verbenone for D. frontalis Zimmermann and 3, 2-methylcyclohexane (MCH) for D. pseudotsugae Hopkins, are well known. The beetles that have successfully colonized a host release these pheromones. These compounds, in combination with aggregation pheromones, tend to keep beetle densities within an optimal range (6-8 per 1000 cm2 of bark surface).
6. Recruitment Pheromones:
Recruitment pheromones are common in social insects, which are used to maintain order, and recruit members and coordinate the activities of the group. One example is a pheromone called the Nasonov pheromone in honey bees which is released by worker bees to orient returning forager bees back to the colony, as well as to recruit other workers outside the hive.
To broadcast this scent, bees raise their abdomens, which contain the Nasonov glands, and fan their wings vigorously. These pheromones are also produced by bark beetles when they find a suitable tree for colonization.
7. Recognition Pheromones:
The insects have sex and species recognition pheromones that act only at close range. In case of social insects like ants, bees and termites, these are used to distinguish colony members from non-colony members. These pheromones tend to be simple straight-or branched-chain hydrocarbons and are a blend of compounds.
The termite egg recognition pheromone (TERP) has been one of the most important pheromones to be identified, which strongly evokes the egg-carrying and grooming behaviour of workers. In five species of males of carpenter bees, the secretion has been from the mandibular glands.
8. Retinue Pheromones:
These chemicals were reported to be released by queen honey bee and, therefore, termed as queen retinue pheromones. These invoke the retinue attraction which encourages workers to feed and groom the queen and acquire and distribute her pheromone messages to other workers throughout the colony. Nine different compounds have been identified in queen retinue pheromone.
Allelochemicals:
Among the most effective weapons developed by plants against phytophagous insects are the various noxious phytochemicals which adversely affect the growth, survival, development and behaviour of these pests. Plants may also provide chemical cues for the parasitoids and predators of insects.
1. Allomones:
The primary defense of plants against insect pests is the possession of toxic or repugnant allomones. These substances repel, deter or harm many potential phytophagous insects. Plant allomones might be applied to crops to act as long or short distance insect repellents or contact feeding deterrents. Therefore, allomones which reduce pest injury by rendering plants unattractive or unpalatable offer a novel approach in vector and disease management.
There are a number of examples where allomones have been found to affect the growth and development of insects. The level of hydroxamic acid in a number of graminaceous plants including maize and wheat, affects the survival and development of Ostrinia nubilalis (Hubner), Schizaphis graminum (Rondani) and Sitobion avenae (Fabricius).
The toxic effects of gossypol present in cotton plants have been demonstrated against Spodoptera exigua (Hubner), Helicoverpa zea (Boddie), Heliothis virescens (Fabricius), Trichoplusia ni (Hubner), Spodoptera littoralis (Boisduval) and Pectinophora gossypiella (Saunders).
Pentadecanal in rice exhibits allomonal properties against Nilaparvata lugens (Stal), Sogatella furcifera (Horvath) and Chilo suppressalis (Walker). Another chemical, 2-tridecanone, which is found in the leaves of tomato plants, is toxic to the larvae of Manduca sexta (Johannsen) and H.zea, and the adults of Aphis gossypii Glover, whereas a derivative, tridecanyl acetate, is active against stored product beetles.
2. Kairomones:
Plant-produced substances that serve as attractants, arrestants, and opposition and feeding stimulants for herbivores are undoubtedly the best known kairomones. These plant-derived substances also influence organisms of the third trophic level when they are sequestered by herbivores and used by natural enemies to locate the herbivore.
The aphid parasitoid, Diaeretiella rapae (M’Intosh) is attracted by the allylisothiocyanate released by cruciferous plants harbouring various aphid species including Brevicoryne brassicae (Linnaeus), Lipaphis eiysimi (Kaltenbach) and Myzus persicae (Sulzer). Chrysoperla camea (Stephens) and Collops vittatus (Say) are attracted to caryophyllene, a terpenoid released by damaged cotton leaves. The spined soldier bug, Podisus maculiventris (Say), orients to soybean plants damaged by Trichoplusia ni (Hubner).
3. Synomones:
Some plants respond to feeding or tissue damage by emitting synomones attractive to insect’s natural enemies. Corn seedlings attacked by Spodoptera exigua (Hubner) release large amounts of terpenoid volatiles which serve as cues for females of the parasitoid wasp, Apanteles marginiventris (Cresson). The females of parasitoid, Eucelatoria bryani Sabrosky respond positively to the extracts of 19 plants which serve as food source for Helicoverpa spp.
Lima bean leaves, upon infestation by two spotted spider mites, Tetranychus urticae Koch, produce the terpenoids which attract predators of the herbivores. Corn plants contain the chemical tricosane and the corn earworm, Helicoverpa zea (Boddie) incorporates tricosane unchanged into its eggs. This chemical attracts egg parasitoid, Trichogramma evanescens Westwood, to find its host.
Interestingly, a number of chemicals released by phytophagous insects serve as cues for their natural enemies. Aphid odours are both an arrestant and an opposition stimulant for the syrphid fly, Syrphus corollae Fabricius and a short range attractant for Aphidoletes aphidimyza (Rondani). Larvae of Chrysoperla camea (Stephens) respond to chemicals emanating from the scales left behind by the ovipositing females. The odour of frass or webbing from herbivorous mites is highly stimulatory to predatory mites.
Some arthropod predators have evolved the ability to attract their prey chemically. The bolas spider, Mastophora sp. attracts males of two noctuid moth species by producing an allomone similar to the sex pheromone of the female moth. The assasin bug, Apiomerus pictipes Herrich-Schaeffer releases a substance attractive to the stingless bee, Trigona fulviventris Guerin-Meneville.
Although the majority of successful uses of semiochemicals are for monitoring pest activity, there are an increasing number of examples of direct control with pheromones and other behaviour- modifying compounds.
Push-Pull Strategy:
The push-pull strategy involves behavioural manipulation of insect pests and their natural enemies by integration of stimuli that act to make the protected resource unattractive or unsuitable to the pests (push), while luring them towards an attractive source (pull), from where the pests are subsequently removed.
The pests are repelled or deterred away from the resource (a crop or a farm animal) by using stimuli that mask host apparency or are repellent or deterrent. The pests are simultaneously attracted, using highly apparent and attractive stimuli, to other areas such as traps or trap crops, where they are concentrated, facilitating their elimination.
The term ‘push-pull’ was first conceived by Pyke et al. (1987) to explore the use of repellent and attractant stimuli, deployed in tandem, to manipulate the distribution of Helicoverpa spp. in cotton in Australia.
Subsequently, the concept was formalized and refined by Miller and Cowles (1990), who termed the strategy as stimulodeterrent diversion for management of onion maggot, Delia antiqua (Meigen). However, the original terminology has been favoured by the scientific community and has been well accepted.
The stimuli for push components include visual cues (host colour, shape or size), synthetic repellents, non-host volatiles, host-derived semiochemicals, antiaggregation pheromones, alarm pheromones, antifeedants, oviposition deterrents and oviposition deterring pheromones.
The stimuli for pull components include visual stimulants (traps or trap crops), host volatiles, sex and aggregation pheromones, and gustatory and oviposition stimulants. The principles of the push-pull strategy are to maximize control efficacy, efficiency, sustainability and output, while minimising negative environmental effects.
This strategy maximises the efficiency of behaviour-manipulating stimuli through the additive and synergistic effects of integrating their use. The efficacy and efficiency of population reducing methods can also be increased by concentrating the pests in a predetermined site.
Population reduction by biocontrol methods or highly selective botanical pesticides is preferred to broad-spectrum synthetic pesticides. The deployment of renewable sources, particularly plants, for the production of semiochemicals is encouraged.
In agricultural systems, the goal is to maximise output from the whole system while minimising cost, and using harvestable trap crops or intercrops, rather than sacrificial crops, wherever feasible. The development of reliable and sustainable push- pull strategy requires a clear understanding of the pest’s biology and the behavioural/chemical ecology of the interactions with its hosts, conspecifics and natural enemies.
The push-pull strategies are under development or used in practice in the major areas of pest control. However, the most successful example currently used in practice, was developed in Africa for the control of lepidoptern stem borers, viz., Chilo partellus (Swinhoe), Eldana saccharina Walker, Busseola fusca (Fuller), and Sesamia calamistis Hampson in maize and sorghum. The strategy involves the combined use of intercrops and trap crops, using plants that are appropriate to the farmers and that also exploit natural enemies.
The stgm borers are repelled from the crops by repellent non-host intercrops, particularly molasses grass, Melinis minutiflora Beauv; silverleaf desmodium, Desmodium uncinatum (Jacq.) DC or greenleaf desmodium, D. intortum (Mill.) Urb. (push). These are concentrated on attractive trap plants, primarily Napier grass, Pennisetum purpureum (Linnaeus) or Sudan grass, Sorghum vulgare sudanense (Piper) Hitch (pull).
Molasses grass, when intercropped with maize, not only reduced stem borer infestation, but also increased parasitism by Cotesia sesamiae Cameron. A trap crop of Sudan grass also increased the efficiency of stem borer natural enemies. The push-pull strategy has contributed to increased crop yields and livestock production, resulting in a significant impact on food security in the region.
Molasses grass is also known to contain attractive compounds similar to those found from maize. In addition, five other compounds including (E)-b-ocemene and (E)-4, 8-diethyl-1, 3, 7-nonatriene, which were repellent to stem borers. Desmodium intercrops also produce these compounds, together with large amounts of sesquiterpenes.
When intercropped with maize or sorghum, desmodium suppresses the parasitic African witchweed, Striga hermonthica (Del.) Benth. Six host volatiles, viz. octanal, nonanal, naphthalene, 4-allylanisole, eugenol, and (R, S)-linalool, have been found to be attractive to gravid stem borers.
Recent studies have indicated that the differential preference of moths between maize and sorghum, and Napier grass trap crops is related to a large burst of four electrophysiologically active green leaf volatiles released from the trap crop plants within the first hour of the scotophase, the time at which most opposition occurs.
The deployment of push-pull strategies is advantageous over the use of individual components in isolation. Individual elements may not be able to provide effective control on their own. For example, trapping strategies using attractive baits may cause substantial effect on species with low reproductive rates, but not for species with high reproductive rates. By incorporating other element with negative effects on host selection, the preference differential is increased and the additive effects may reduce pests below economic threshold levels.
Moreover, the efficiency of push and pull components is often not only additive but also synergestic. The use of antifeedants and oviposition deterrents is often limited because of habituation, or host deprivation, in the absence of more suitable hosts. The pull stimuli provide a choice situation for alternative feeding or ovipositional outlets.
As the pest populations are concentrated in pre-determined areas (either traps or trap crops), less chemical or biological material is required to treat the pest population, thereby reducing costs. The semiochemicals in push-pull strategies are used in combination and do not select strongly for resistance. Moreover, the reduction in the use of conventional insecticides reduces the chances of pests to develop insecticide resistance.
Miscellaneous Approaches:
A number of other approaches have been evolved which hold a great potential for managing pest populations under certain situations.
1. Propesticides:
A propesticide is a compound which is inactive in its original form, but is transformed into a pesticidally active state by a plant, animal or microorganism. Classical pesticides can be modified to yield propesticides retaining their insecticidal activity but lowering mammalian toxicity and acquiring plant systemic properties.
The technique has led to the development of new derivatives of toxic methyl carbamate insecticides with improved toxicological properties. Carbosulfan is a derivative of carbofuran with a similar activity spectrum but is substantially less toxic to mammals.
This technique has also been applied for developing new organophosphorus compounds. Acephate, the acetylated product of methamidophos is 45-fold less toxic to rat than the parent compound but retains approximately the same insecticidal activity.
In insects, acephate is converted to methamidophos which is believed to be responsible for high toxicity. Similarly, cartap is a proinsecticide which is rapidly converted to nereisotoxin in the insect body. Nereisotoxin is a dithiolane compound found naturally in the marine annelid, Lumbriconereis heteropoda Marenz.
2. Avermectins:
Avermectins are macrocyclic lactones which were originally isolated in 1976 by scientists at Merck & Co., Inc. (Rahway, New Jersey), from a culture of Streptomyces avermitilis from Japan. The avermectins seem to exert their toxicity by disrupting the action of both ligand-gated (i.e., GABA) and voltage-gated chloride channels. The end result is functional disruption of GABA-gated chloride channels.
These are among the most potent anthelminthic, acaricidal and insecticidal compounds. Among the eight analogues identified, avermectin B (commercialized as Abamectin) is insecticidally most active. Emamectin benzoate is a novel macrocyclic lactone insecticide derived from the avermectin family. Another group of related compounds called milemycins have been obtained from S. hygroscopicus aureolacrimosces.
3. Spinosyns:
A new class of insect control molecules, the spinosyns, was discovered in 1994 by Dow Elanco (Indianapolis, IN). Naturally derived from a new species of actinomycetes, Saccharopolyspora spinosa, they are very active against many pests of crops, ornamentals, forestry, greenhouse, garden and households.
Spinosad, a mixture of spinosyn A and spinosyn D, is the lead compound and it has shown contact and stomach activity against Coleoptera, Diptera, Hymenoptera, Isoptera, Lepidoptera, Siphonoptera and Thysanoptera. Spinosad causes persistent activation of nicotine acetylcholine receptors in the insect nervous system, a unique mode of action with no known cross resistance with other insecticides. Spinosad has been approved for registration in 1997 in India under the Insecticides Act, 1968.
4. Polynactins:
Polynactins are secondary metabolites from the actinomycete Streptomyces aureus strain S-3466. They are quite effective in controlling spider mites under wet conditions. It is believed that the mode of action is through a leakage of basic cations (such as potassium ions) through the lipid layer of the mitochondrion membrane. Water is considered essential to this toxic effect by either assisting penetration or accelerating ion leakage. They are particularly recommended for the control of spider mites on fruit trees.
5. Pyrrole Insecticides:
Pyrrole Insecticides have been derived from a natural product, dioxapyrrolomycin, isolated from a strain of Streptomyces. Chlorfenapyr is a promising pyrrole, which has been commercially developed because of its broad spectrum of activity against many species of Coleoptera, Lepidoptera, Thysanoptera and Acarina.
Chlorfenapyr acts at the mitochondrial level by uncoupling oxidative phosphorylation. It is mainly a stomach toxicant, but has some contact actively. Field trials have demonstrated that foliar applications of chlorfenapyr are effective in controlling more than 70 insect pests and mites on cotton, cereals, vegetables, orchard trees and ornamentals plants.
6. Phenylpyrazoles:
The phenylpyrazoles comprise a new class of biorational pesticides, which exhibit insecticidal and herbicidal activities. The first highly successful member of this class is fipronil, which is active at the neuroinhibitory GABA-gated chloride channels Fipronil exhibits broad activity against various insect pests including soil insects, foliar feeding pests such as Plutella xylostella (Linnaeus), Helicoverpa armigera (Hubner) and Spodoptera spp; sucking pests such as thrips (but not aphids or whiteflies) and household pests.
7. Pyridine Insecticides:
The most important chemical of this group is pymetrozine, which affects the nerves controlling the salivary pump and causes irreversible cessation of feeding due to an obstruction of stylet penetration, followed by starvation and insect death. It is highly specific against sucking insects such as aphids, whiteflies and planthoppers.
8. Oxadiazines:
Indoxocarb is the first commercialized insecticide of the oxadiazine group. It acts by inhibiting sodium ion entry into nerve cells, resulting in paralysis and death of target pest species. This insecticide is active against lepidopteran as well as certain homopteran and coleopteran pests on vegetables, cotton and other field and orchard crops.
Efficacy of this product has been demonstrated against important pests such as Heliothis sp., Helicoverpa sp., Spodoptera sp., Plutella sp. and Trichoplusia sp. (Lepidoptera) Lygus sp. and Empoasca sp. (Hemiptera) and also the Cpjprado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera).
9. Antifeedants:
The antifeedants are chemicals which inhibit or deter the feeding of insects due to their presence on the natural food of the species concerned. In their absence, the species would otherwise feed normally. An antifeedant acts by suppressing the gustatory receptors. It is a type of feeding deterrent which deprives an insect from continued feeding on the host. Death in the end is due to starvation.
These compounds belong to five major categories:
(i) Triazines- Compound 24,055,
(ii) Organotins- Stenous chloride,
(iii) Carbamates-Baygon,
(iv) Botanical extracts like pyrethrum, Neem oil, and
(v) Miscellaneous-Copper stearate, Phosphon, Cycocel, etc.
The compounds like 4′- (dimethyl triazene) acetanilide, when used against surface feeders on cabbage and cotton, and Phosphon or Cycocel against leaf eaters on pepper and cotton, have been found to be very effective. A fungicide, Brestan is found to be a potent antifeedant against potato tuber moth larvae and the larvae of cutworms and cotton leaf worms. Naturally occurring antifeedants exist for many insects and play a major role in host selection and specificity.
10. Repellents:
The repellents elicit avoidance and thus, lead the insect pests to move away from the source. The plants are rendered unattractive, unpalatable or offensive. Except for the naturally occurring feeding repellents, the artificial use of such chemicals has not been successful. When used as foliage repellents these chemicals need a thorough coverage of the crop to be effective.
Since the growing points remain uncovered the chemicals do not show their full effectiveness. However, the repellents have been used successfully against mosquitoes (dimethyl phthalate), flies, fleas, mosquitoes (2-ethyl-1, 3-hexanediol and N, N-diethyl m-toluamide), mites (benzyl benzoate, benzil and dibutyl phthalate) and flies on cattle (dibutyl succinate).
Potential and Constraints:
The use of selective, biorational approaches in place of broad spectrum conventional insecticides offers several advantages in IPM programmes.
Some of the advantages are as follows:
i. Most of these chemicals are more or less non-toxic to man and domestic animals. Methoprene, for example, has an acute oral LD50 (rat) of > 34600 mg/kg.
ii. These compounds do not persist or accumulate in the environment and are degraded to simple molecules that are unlikely to cause problems of environmental contamination.
iii. Many of the semiochemicals are species specific and have no adverse effect on parasitoids and predators. Among the IGRs, studies conducted so far indicate that these are also comparatively safe to natural enemies.
iv. Most of these compounds are active at very low concentrations. In case of pheromones, it has been shown that even a single molecule landing on the receptor of a perceiving individual is capable of generating a response.
Biorational pesticides also suffer from several disadvantages which limit their large scale utilization.
Some of the disadvantages are:
i. Each of these compounds is effective against a single pest or a closely related group of pests. Therefore, the market potential of these chemicals is very limited.
ii. These chemicals disrupt the development and bahaviour of insects and do not provide immediate control of pests. In case of IGRs, the treated larvae/grubs continue to cause damage for a number of days.
iii. Many of these compounds are photodegradable and, therefore, they rapidly loose their effect following their application to the crop. This problem has been solved in a number of cases by developing suitable protected, controlled release formulations which retain their effectiveness for a considerable length of time.
iv. As with other techniques, insects are capable of developing resistance to these chemicals. In many cases, insects already possess the ability to metabolize these compounds in the course of their normal development. Any selection pressure would, therefore, result in rapid development of resistance to such compound.
v. Semiochemicals have to be used on an area-wide basis in order to achieve desired results.
vi. In many cases products available in the market are not of uniform quality and give inconsistent results. This problem could be overcome by establishing suitable standards for quality and performance.
These problems are not insurmountable and could be solved by undertaking further research. There is a need for fundamental research on controlled release systems which are cheap, non-toxic and biodegradable. Suitable application technology also needs to be developed for these formulations. On a long term basis, the process of regulation of insect development needs to be understood in a sequential manner.
The relative importance of different communication systems in the life of insect needs investigation. The behavioural responses of insects and effects of various meteorological and physicochemical factors on these responses need to be elucidated.
Registration procedures for these products need to be simplified. Last decade has witnessed significant advances in most of these areas. The biorational products are becoming more reliable and look set for a promising future in IPM programmes.