Ticks are important vectors of disease-causing pathogens of humans, wildlife, and livestock. Reducing tick abundance is an important but elusive goal. Chemical pesticides applied to habitats occupied by ticks can be effective but appear to have significant negative effects on nontarget organisms.
Reducing tick abundance is likely to remain the most effective method for preventing tick-borne diseases. Several methods of biocontrol of ticks, including parasitoids and some bird predators, have been shown to reduce tick numbers in some situations.
Perhaps the most promising method of biocontrol is the targeted use of fungal pathogens, which has been shown to reduce tick numbers both directly (through mortality) and indirectly (through reductions in fitness). These preliminary successes demonstrate the importance and potential of rigorous research into novel and existing methods of biological control of ticks.
The most promising alternatives to chemical pesticides are biological control (biocontrol) agents, which are species that consume target pest organisms via predation, herbivory, or parasitism. Biocontrol agents typically are nontoxic to humans and to nontarget wildlife (for a few exceptions, see below). Moreover, biocontrol agents are expected to coevolve with their target organisms, reducing the likelihood that resistance will evolve.
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Biological control of ticks or mites means controlling them with natural organisms that are their natural enemies.
There are three major types of organisms that are natural enemies of those ticks and mites that affect livestock:-
- Predators: they just eat the ticks, either those still attached to the host or engorged females that have dropped to the ground: mainly birds, ants, and a few mite species.
- Parasitoids: these are wasps that deposit their eggs on ticks. The larvae of the wasps feed on the tissues of the ticks that are ultimately killed. They can be considered as “parasites” of the ticks.
- Pathogens: mainly bacteria, fungi and nematodes (roundworms) that infect and kill the ticks or mites or their larvae. They can be considered as “diseases” of the parasites.
So far there are no biological control methods against ticks and mites of dogs and cats. The biological control means ‘control of an organism by using another living organism’. Classical biological control covers the recognition, assessment, and introduction of a natural enemy from elsewhere, the maintenance of indigenous usual enemies and the expansion of the biocontrol agents. Biocontrol agents are generally slower-acting but cause longer-lasting biotic suppression of a specific pest population.
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Why biological control is preferred?
Pest biocontrol is becoming one of the most hopeful replacements to chemical pesticides.
This technique is used:-
- To minimize the chemical residues on our planet.
- To minimize the growing problem of arthropod resistance to pesticides.
- To balance rising prices of new chemical pesticides.
- To create a friendly environment(chemical free).
- Due to the longer effect of this technique as compared to other methods.
- To overcome the drawback of broad-spectrum insecticide.
The significance of ticks:-
Ticks are economically the most important pests of cattle and other domestic species in tropical and subtropical countries. They are the vectors of a number of pathogenic microorganisms including protozoans (babesiosis, theileriosis), rickettsiae (anaplasmosis, ehrlichiosis, typhus), viruses (e.g., Kyasanur Forest Disease reported from the Karnataka State of India; Crimean-Congo Hemorrhagic Fever reported time and again from Pakistan & South Africa), bacteria (e.g.Pasteurella, Brucella, Listeria, Staphylococcus) and spirochaetes.
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Bio-control of ticks:-
Ticks have numerous natural enemies, but only a few species have been evaluated as tick biocontrol agents. There is a long list of pathogens which are used to biologically control the ticks.
The BioPesticide Manual lists 96 commercial active ingredients based on microorganisms, which includes; Bacterial source = 33, Fungal source = 36, Entomopathogenic nematodes = 8
There are 73 bacterial isolates from field collected fromIxodesscapularis, including 11 species of Bacillus, mostly in the B. thuringiensisB. cereus, B. t. kurstaki, B. t. israelensis and B.t. thuringiensis species group. Few examples of specific bacterial attack to specific species of ticks includes are: Proteus mirabilis is pathogenic to Dermacentorandersoni, Amblyommahebraeum, HyalommamarginatumandRhipicephaluseverstieverstiand apparently cause the Blackening disease of Boophilusdecoloratus.
A bacterium Cedecealapageihas have been found to be pathogenic to Boophilusmicroplus. This bacterium infects ticks via the genital opening and under laboratory conditions can produce up to 100% mortality.
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How bacteria kill ticks?
Following ways used to kill ticks through the bacterial source.
- Ingestion of blood from the infected animals—————— successful entry of bacteria (B. thuringiensis) into the gut of the tick————–bacterial attack at the cells of the gut ——–pus in the gut———– mortality of the tick.
- Dipping of the Ixodesscapularis engorged larvae in B. t. kurstaki culture also kill them.
- The injection of crystalline D-endotoxin of B. thuringiensis in midgut also kills the ticks by producing toxemia
- Ten species of fungi have been currently developed for the control of ticks. The most promising fungi are from the class Deuteromycetes (mitosporic fungi).
Why it should be preferred for the control of Ticks?
Fungi are the most reliable source of tick control due to the following reseasons.
1. The ability of entomopathogenic fungi to penetrate the cuticle of arthropods
2. The ability of a strain to kill several stages of the tick
3. The relatively specific virulence of a single strain to one or agents
Mechanism of action:-
Collected ticks were infected primarily with Verticillium spp. and Beauveriabassianaand for the complete control of ticks, spray of M. anisopliaeshould be done, which break up the scutum of tick and enters circulatory system of the tick. After reaching at different systems, it blocks them to function. These showed strong anti-tick activity.
Entomopathogenic nematodes (EPNs) of the families Heterorhabditidae and Steinernematidae are known to be obligatory parasites of ticks.
The only free-living stage of the nematode, the third/infective juvenile (IJ), actively locates and enters the host via natural openings, and then releases symbiotic bacteria that kill the host insect within 24–72 h. The nematodes then multiply within the host cadaver and6–18 days post infection thousands of IJs are released into the environment. The most common natural habitat of these nematodes is moist ground.
The DJs are well adapted to the changing conditions of moisture, temperature, texture, and chemical composition associated with different soil types.
The EPNs are known to be pathogenic to over 3000 insect species, whereas each strain may often be relatively specific to a small group of hosts and thus their effects on most beneficial insects have been found to be negligible.
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Mechanism of Action:-
EPNs penetrate engorged female B. annulatusticks almost solely via the anus or genital pore. Heterorhabditid nematodes killed engorged B. annulatus females in Petri dishes after less than 2.5 of exposure, whereas steinernematid nematodesneeded more than 4 h to penetrate into ticks. The injection of a single heterorhabditid nematode into a tick can cause mortality.
Most parasitoids used in the biological control of insect pests of plants belonging to the order Hymenoptera. Only a few species of hymenopteran parasites are known to affect ticks. It has been described that two species of chalcidoid wasps collected from ticks in Texas.
These are now both included in the genus Ixodiphagus of the family Encyrtidae, which includes seven species, all tick parasites. The most widespread species is I. hookeri which has been recorded in Asia, Africa, North America, and Europe.
Many tick bio-suppressors such as ants, beetles, and many bird species are general predators that feed occasionally on ticks, therefore their populations do not depend on the sizes of the tick populations. General predators can sometimes affect the size of a tick population in nature, but manipulating their populations to reduce tick numbers.
Some 50 bird species have been reported to eat ticks However, only a few species seem to feed specifically on ticks, and thus only a few would be expected to have a meaningful effect on tick populations.
Chickens (Gallus gallus) confined with cattle in Africa were reported to ingest an average of 338 ticks per bird during 5.5 h. Other experiments found that the birds ate from 9.7 to 81 ticks per bird per hour of foraging. At high tick concentrations, an average of 69% of the ticks was consumed by Chickens.
Chickens are neither tick-specific predators nor obligator predators; therefore their consumption of ticks depends largely on alternative food availability and the density of the tick population. Thus, chickens are unlikely to reduce tick densities below a certain level. Nevertheless, chickens maintained in any case on small mixed farms can help to reduce tick populations at nearly no cost.
Buphagusafricanus (the yellow-billed oxpecker) and B. erythrorhynchus (the red-billed oxpecker), both native to Africa, are the only birds known to feed specifically on ectoparasites, especially ticks.
Stomach contents of captured oxpeckers included 16 to 408 ticks per bird. The consumption by a young bird of larval, nymphal and adult Boophilus ticks averaged 1176, 1549 and 1293, respectively, during 6–7 days of exposure to tick-infested cattle.
Oxpeckers are visual predators, first plucking the engorged females, then searching large body areas and scissoring and eating the smaller tick stages. These prefer eating on buffaloes and white rhinos. Oxpeckers prefer feeding on weak mammals and will feed repeatedly on specific individuals within the same herd, with a preference for the hosts with most ticks.
Birds as predators of ticks:-
Numerous birds feed on ticks. Best-known diligent tick feeders are cattle egrets (Bubulcus ibis), oxpeckers(Buphagus spp. in Africa) and cattle tyrants (Machetornis rixosa, in America). But other chickens, guinea fowls, and many other domestic and wild birds eat ticks as well.
There are numerous studies investigating tick consumption of several bird species. Studying the stomach content of oxpeckers it was found that they actually eat large amounts of ticks, but on if they lived in contact with animals heavily infested with ticks. This means that they are not specific tick feeders, but eat what is available, as all investigated birds. For this reason they cannot be used for the purpose of biological control of ticks.
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Ants as predators of ticks:-
Several studies have shown that various ant species predate on eggs and larvae of ticks and other arthropods, particularly Pheidole megacephala(bigheaded ant in the US, coastal brown ant in Australia), Solenopsis spp (fire ants) and Camponotus spp (carpenter ants).
It has been found in India that pastures with abundant ant colonies have fewer ticks than those with scarce ant colonies. And if ticks are released on such pastures, they are not found afterward because the ants eat them. But regarding their feeding behavior ants behave like birds: they eat what is available, i.e. they are not specific tick feeders.
There are also studies that show that formic acid released by the ants has a repellent effect on ticks. Rabbits kept on ant-rich pastures carried fewer ticks than rabbits kept on ant-poor pastures.
For these and other ecologic reasons, it is not a good idea to try to eliminate ants from pastures using insecticides, unless they are excessively annoying as is the case for fire ants.
Summarizing, although ants can certainly contribute to eliminate some ticks on pastures, usually it is not enough to bring the tick populations below the economic threshold level.
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Mites as predators of ticks
Anystis baccarum is a mite species that predates on other mites and is used as a biological control weapon in crop protection. It also predates on tick larvae, particularly those that climb onto shrubs or grass blades questing for potential hosts. But results of trials run in Australia with these mites to control cattle ticks were disappointing.
Parasitoids of ticks
All parasitoid species of ticks are small hymenopteran wasps of the genus Ixodiphagus, particularly Ixodiphagus hookeri.
Some studies on the potential of these wasps for controlling ticks have been run in the US against Ixodes scapularis and other ticks that are vectors of human borreliosis. Ixodiphagus wasps are very efficient parasites of ticks that achieve 25 to 50% natural parasitization rates. Preferred hosts were found to be engorged larvae. Each wasp deposits 6 or more eggs inside an engorged tick.
Field studies in Kenya showed a parasitization rate of about 50% against Amblyomma variegatum, but no parasitization at all against Rhipicephalus appendiculatus. There are also studies in Brazil on Amblyomma cajennense and Rhipicephalus sanguineus. To our knowledge Ixodiphagus wasps are not yet commercially available in most countries where ticks are a problem for livestock.
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Pathogenic bacteria against ticks and mites:-
Bacillus thuringiensis Berliner (Bt), a particular strain of Bt produces a thuringiensin, a toxin that destroys the gut cells of ticks that ingest it. Bacteria themselves are also pathogenic for numerous arthropod species. Most commercial products used against crop pests are a mixture of the toxin and bacterial spores. If the arthropods or their larvae eat the spores or come otherwise in contact with them the bacteria will multiply in their organism and kill them within a few days.
There are numerous commercial products based on Bt, mainly for crop protection and for large-area mosquito control, but also for controlling other insects that develop in water, (e.g. black flies, midges, etc). Such products contain basically Bt spores. If the flies or their larvae eat the spores or come otherwise in contact with them the bacteria will multiply in their organism and kill them within a few days.
Ticks have to ingest bacteria or their toxin to be killed. But this is very difficult to achieve because ticks are bloodsuckers and do not eat anything else, neither the stages on the animals nor the free-living stages (larvae, nymphs, etc). Nevertheless, laboratory studies have shown that several tick species (Argas persicus, Boophilusannulatus, etc.) immersed in Bt suspensions showed considerably mortalities and a significant reduction of egg hatching.
There are also reports of a good efficacy of thuringiensin suspensions against northern fowl mites when directly applied on infested chicken.
However, to our knowledge, there are still no commercial products based on Bacillus thuringiensis approved for use on livestock or pets.
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Pathogenic fungi against ticks and mites:-
The use of pathogenic fungi against crop and livestock pests has been intensively investigated. There are already numerous commercial productscontaining spores of these fungi that are used against crop pests. There use against livestock pests is much less developed and little is known so far on their field efficacy against ticks and mites that affect livestock and/or pets.
When the spores of these fungi come in contact with the ticks or insects they stick to their cuticle, where they germinate and produce hyphae, thread-like filamentous structures. These hyphae diffrentiate into appressoria, structures capable of penetrating into the host’s body, which they often do by piercing the membranes between the body or limb segments, or through the mouth pieces. Once inside the host’s body they proliferate in the body cavity (=hemocoel) irreversibly damaging the body organs and ultimately killing the host within a few days. Some of these fungi produce also toxins.
The best investigated species of fungi pathogenic for ticks belong to the genera Beauveria, Metarhizium, Paecilomyces and Verticillium. Most species parasitize both insects and ticks, develop in the soil and are found worldwide.
Four species have been particularly investigated regarding their efficacy against livestock ticks: Metarhizium flavoviridae, Metarhizium anisopliae, Beauveria bassiana and Verticillium lecanii. Laboratory trials showed up to 100% mortality of cattle ticks two weeks after treatment, but without completely suppressing oviposition of engorged females. High mortality (50% to 90%) was also achieved against other tick species (e.g. Amblyomma spp, Rhipicephalus spp). Excellent laboratory results have been obtained against sheep scab mites (Psoroptes ovis) and fowl mites (Ornithonyssus spp, Dermanyssus spp) as well. unfortunately, these promising results are difficult to reproduce under field conditions.
It is relatively easy to produce large amounts of spores of these fungi and they can be usually administered to livestock in the form of water or oil suspensions using the same spraying equipment that is used for classic pesticides.
Distribution logistics of such products is also relatively easy because they do not require a cold chain or other usual precautions for biological materials. But when sprayed onto animals, the high concentration of spores required might result in too viscous suspensions that clog the standard application equipment. And at such high concentrations, it is difficult to keep the spores suspended in the carrier liquid. Such complications may lead to inadequate administration in the field trials that explain their failure.
Other factors can explain the difficulties found to reproduce the laboratory results under field conditions. It seems that efficacy strongly depends on the body temperature of the ticks, which is usually 26°-28°C in the laboratory, but 34°-37°C in the field. It is known that fungal vitality peaks at about 30°C and diminishes quickly at higher or lower temperatures. Changing climatic conditions in the field can also negatively impact fungal development: cold weather may slow down germination of fungal spores, heavy rains may wash the spores away, excessive sunlight can kill a certain number of spores, etc.
There are also studies on the effect of fungal spores directly applied to tick-infested pastures around places where livestock congregates (shade trees, salt licks, drinking troughs), where the density of tick eggs, larvae, and nymphs is likely to be high. It was shown that populations of cattle ticks could be reduced but not eliminated. But such treatments can also be detrimental for the beneficial fauna on the pastures.
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Pathogenic nematodes against ticks:-
Numerous helminth species that are pathogenic for insects and ticks are already used in commercial products against crop pests. They are called entomopathogenic worms. The best-known genera are Steinermena andHeterorhabditis. These roundworms penetrate into the insect body where they release symbiotic bacteria that they carry inside. These bacteria multiply inside the insect and kill it in a few days. The decaying tissues serve as nutrients for both worms and bacteria.
Promising results have been achieved in laboratory studies with Steinermena carpocapsae against Boophilusannulatus. Other nematodes showed efficacy against Amblyomma and Rhipicephalus ticks, but not against cattle ticks, Rhipicephalus (=Boophilus) microplus. Unfortunately, very few field trials have been conducted with these nematodes against ticks. Preliminary studies indicate that the type of soil plays a decisive role in the efficacy, as well as the body temperature of the ticks. Efficacy is highest between 22°C and 26°C and drops strongly below 18°C and over 30°C.
An additional difficulty for the use of such nematodes in the field is the fact that most tick species are not viable hosts for the worms, i.e. the worms do not complete their life cycle on the ticks. This means that pastures would need to be treated periodically to maintain a high density of infective worms. But very little is known about the survivall conditions of the worms on pastures, the influence of climatic conditions on worm survival and dissemination, etc.
What the end result is?
The development of anti-tick biological control agents (BCAs) is still in its infancy. Furthermore, the various steps required for commercialization of these products, including adaptation by companies (production, storage and delivery) and education of consumers (storage, application and evaluation of results), are still in the future. Nevertheless, we believe that the need to develop alternative control methodswill yield useful results.
The fact that some BCAs and particular strains are far more specific in their selection of target pests than are acaricides and that many strains are effective only under specific ecological conditions, provide considerable advantages over pesticides, because harmful ecological effects are minimized. Partial or total replacement of chemical acaricides with extra use of tick pathogens and/or parasitoids would require considerable changes in the techniques of producers and suppliers.
Biological control of plant pests, by means of parasitoids, predatory mites, viruses, B. thuringiensis, bugs, beetles, and others, has had several striking successes. These include the use of several enemies/ pathogens simultaneously or in a pre-determined order. However, only about 5% of all pest problems are treated with biological control methods and many problems have to be solved in order to increase their use.
Relatively fewstudies have been performed on the existence of promising natural enemies of ticks, or on their use against ticks in most parts of the world. Collaboration between biocontrol experts who have experience in managing plant pests and tick experts could lead to valuable developments in tickbiocontrol.
In India, there are naturally gifted biological agents, which can be used for the control of ticks. These include sparrows, crows, chickens, and parrots. But for this, we have to move towards the nature so that the balance of nature cannot be disturbed. Similarly, we can purchase birds like oxpecker, can develop anti-tick bacterial and fungal sources. A lot of research is still needed until reliable products based on these nematodes become commercially available for the control of ticks or mites of veterinary importance.
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