Biological pest control

The current scope of this article does not include composting techniques. For these see Composting: Destroying pathogens, seeds, or unwanted plants or Mulch: Mulching (composting) over unwanted plants.
Syrphus fly larva feeding on aphids.
Parasitic wasp Cotesia congregata on tobacco hornworm Manduca sexta.

Biological control is a bioeffector-method of controlling pests (including insects, mites, weeds and plant diseases) using other living organisms.[1] It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also involves an active human management role. It can be an important component of integrated pest management (IPM) programs. There are three basic types of biological pest control strategies: importation (sometimes called classical biological control), augmentation and conservation.

Natural enemies of insect pests, also known as biological control agents, include predators, parasitoids, and pathogens. Biological control agents of plant diseases are most often referred to as antagonists. Biological control agents of weeds include seed predators, herbivores and plant pathogens.

Types of biological pest control

There are three basic types of biological pest control strategies: importation (sometimes called classical biological control), augmentation and conservation.[2]

Importation

Importation (or "classical biological control") involves the introduction of a pest's natural enemies to a new locale where they do not occur naturally. This is usually done by government authorities. In many instances the complex of natural enemies associated with a pest may be inadequate, a situation that can occur when a pest is accidentally introduced into a new geographic area, without its associated natural enemies. These introduced pests are referred to as exotic pests and comprise about 40% of the insect pests in the United States.

The process of importation involves determining the origin of the introduced pest and then collecting appropriate natural enemies associated with the pest or closely related species. Selected natural enemies are then passed through a rigorous assessment, testing and quarantine process, to ensure that they will work and that no unwanted organisms (such as hyperparasitoids) are introduced. If these procedures are passed, the selected natural enemies are mass-produced and then released. Follow-up studies are conducted to determine if the natural enemy becomes successfully established at the site of release, and to assess the long-term benefit of its presence.

To be most effective at controlling a pest, a biological control agent requires a colonizing ability which will allow it to keep pace with the spatial and temporal disruption of the habitat. Its control of the pest will also be greatest if it has temporal persistence, so that it can maintain its population even in the temporary absence of the target species, and if it is an opportunistic forager, enabling it to rapidly exploit a pest population.[3] However an agent with such attributes is likely to be non-host specific, which is not ideal when considering its overall ecological impact, as it may have unintended effects on non-target organisms.

There are many examples of successful importation programs, including:

Classical biological control is long lasting and inexpensive. Other than the initial costs of collection, importation, and rearing, little expense is incurred. When a natural enemy is successfully established it rarely requires additional input and it continues to kill the pest with no direct help from humans and at no cost. However importation does not always work. It is usually most effective against exotic pests and less so against native insect pests. The reasons for failure are not often known but may include the release of too few individuals, poor adaptation of the natural enemy to environmental conditions at the release location, and lack of synchrony between the life cycle of the natural enemy and host pest.

Augmentation

Augmentation involves the supplemental release of natural enemies, boosting the naturally occurring population. Relatively few natural enemies may be released at a critical time of the season (inoculative release) or millions may be released (inundative release). An example of inoculative release occurs in greenhouse production of several crops. Periodic releases of the parasitoid, Encarsia formosa, are used to control greenhouse whitefly, and the predatory mite Phytoseiulus persimilis is used for control of the two-spotted spider mite. Lady beetles, lacewings, or parasitoids such as those from the genus Trichogramma are frequently released in large numbers (inundative release). Recommended release rates for Trichogramma in vegetable or field crops range from 5,000 to 200,000 per acre (1 to 50 per square metre) per week depending on level of pest infestation. Similarly, entomopathogenic nematodes are released at rates of millions and even billions per acre for control of certain soil-dwelling insect pests.

Hippodamia convergens, the convergent lady beetle, is commonly sold for biological control of aphids.

The spraying of octopamine analogs (such as 3-FMC) has been suggested as a way to boost the effectiveness of augmentation. Octopamine, regarded as the invertebrate counterpart of dopamine plays a role in activating the insects' flight-or-fight response. The idea behind using octopamine analogues to augment biological control is that natural enemies will be more effective in their eradication of the pest, since the pest will be behaving in an unnatural way because its flight-or-fight mechanism has been activated. Octopamine analogues are purported to have two desirable characteristics for this type of application: (1) they affect insects at very low dosages (2) they do not have a physiological effect in humans (or other vertebrates).[4]

Conservation

The conservation of existing natural enemies in an environment is the third method of biological pest control. Natural enemies are already adapted to the habitat and to the target pest, and their conservation can be simple and cost-effective. An example that has shown such cost-effectiveness is growing nectar-producing crop plants in the borders of rice fields. These provide nectar to support parasitoids and predators of planthopper pests and have been demonstrated to be so effective (reducing pest densities by 10- or even 100-fold) that farmers sprayed 70% less insecticides, enjoyed yields boosted by 5%, and this led to an economic advantage of 7.5%.[5] Lacewings, lady beetles, hover fly larvae, and parasitized aphid mummies are almost always present in aphid colonies.

A turnaround flowerpot, filled with straw to attract Dermaptera-species

Cropping systems can be modified to favor the natural enemies, a practice sometimes referred to as habitat manipulation. Providing a suitable habitat, such as a shelterbelt, hedgerow, or beetle bank where beneficial insects can live and reproduce, can help ensure the survival of populations of natural enemies. Things as simple as leaving a layer of fallen down leaves or mulch in place provides a suitable food source for worms and provides a shelter for small insects, in turn also providing a food source for hedgehogs and shrew mice. Compost pile(s) and containers for making leaf compost also provide shelter, as long as they are accessible by the animals (not fully closed). A stack of wood may provide a shelter for voles, hedgehogs, shrew mice, some species of butterflies, ... Long grass and ponds provide shelters for frogs and toads (which themselves eat snails). Not cutting any annual or other non-hardy plants before winter (but instead in spring) allows many insects to make use of their hollow stems during winter.[6] In California prune trees are sometimes planted in grape vineyards to provide an improved overwintering habitat or refuge for a key grape pest parasitoid. The prune trees harbor an alternate host for the parasitoid, which could previously overwinter only at great distances from most vineyards. The provisioning of artificial shelters in the form of wooden caskets, boxes or flowerpots is also sometimes undertaken, particularly in gardens, to make a cropped area more attractive to natural enemies. For example, the stimulation of the natural predator Dermaptera is done in gardens by hanging upside-down flowerpots filled with straw or wood wool. Green lacewings are given housing by using plastic bottles with an open bottom and a roll of cardboard inside of it.[6] Birdhouses provide housing for birds, some of whom eat certain pests. Attracting the most useful birds can be done by using a correct diameter opening in the birdhouse (just large enough for the specific species of bird that needs to be attracted to fit through, but not other species of birds).

Besides the provisioning of natural or artificial housing, the providing of nectar-rich plants is also beneficial. Often, many species of plants are used so as to provide food for many natural predators, and this for a long period of time (this is done by using different types of plants as each species only blooms for a short period). It should be mentioned that many natural predators are nectivorous during the adult stage, but parasitic or predatory as larvae. A good example of this is the soldier beetle which is frequently found on flowers as an adult, but whose larvae eat aphids, caterpillars, grasshopper eggs, and other beetles. Letting certain plants (as Helianthus spp, Rudbeckia spp, Dipsacus spp, Echinacea spp) come into seed is also advised, to supply food for birds. Having some trees or shrubs in place that carry berries is also practiced and provide a source of food for birds. Often, trees/shrubs are used that do not produce berries fit for human consumption, avoiding food competition. Examples are Sorbus spp, Amelanchier spp, Crataegus spp, Sambucus nigra, Ilex aquifolium, Rhamnus frangula. Obviously for this to work, these trees can not be pruned/trimmed until after the birds and other animals have eaten all of the berries.

Also, the providing of host plants (plants on which organisms can lay their eggs) may also be necessary. These organisms for which host plants can be foreseen can be certain natural predators, caterpillars, and even a limited amount of host plants for pests can be tolerated. The latter ensures that natural predators remain in the vicinity and tolerating a certain amount of loss to pests would be needed anyhow since no chemical pesticides can be used (organic pesticides can be used but often can, on itself, not eliminate all pests during an infestation). This, as natural predators are susceptible to the same pesticides used to target pests. Plants for caterpillars are optional and only ensure that sufficient amounts of moths are produced which form a source of food to bats. Bats may be wanted as they also consume large amounts of mosquitoes, which despite not targeting any plants, can still be a nuisance to people in areas where there is much standing water nearby (i.e., pond, creek, ...).

Conservation strategies such as mixed plantings and the provision of flowering borders can be more difficult to accommodate in large-scale crop production. There may also be some conflict with pest control for the large producer, because of the difficulty of targeting the pest species, also refuges may be utilised by the pest insects as well as by natural enemies. Some plants that are attractive to natural enemies may also be hosts for certain plant diseases, especially plant viruses that could be vectored by insect pests to the crop.

Biological control agents

Predators

Lacewings are available from biocontrol dealers.

Predators are mainly free-living species that directly consume a large number of prey during their whole lifetime.

Ladybugs, and in particular their larvae which are active between May and July in the northern hemisphere, are voracious predators of aphids, and will also consume mites, scale insects and small caterpillars.

The larvae of many hoverfly species principally feed upon greenfly, one larva devouring up to fifty a day, or 1000 in its lifetime. They also eat fruit tree spider mites and small caterpillars. Adults feed on nectar and pollen, which they require for egg production.

Predatory Polistes wasp looking for bollworms or other caterpillars on a cotton plant

Dragonflies are important predators of mosquitoes, both in the water, where the dragonfly naiads eat mosquito larvae, and in the air, where adult dragonflies capture and eat adult mosquitoes. Community-wide mosquito control programs that spray adult mosquitoes also kill dragonflies, thus reducing an important biocontrol agent.

Several species of entomopathogenic nematode are important predators of insect pests.[7] Phasmarhabditis hermaphrodita is a microscopic nematode that kills slugs, thereafter feeding and reproducing inside. The nematode is applied by watering onto moist soil, and gives protection for up to six weeks in optimum conditions.

Other useful garden predators include lacewings, pirate bugs, rove and ground beetles, aphid midge, centipedes, spiders, predatory mites, as well as larger fauna such as frogs, toads, lizards, hedgehogs, slow-worms and birds. Cats and rat terriers kill field mice, rats, June bugs, and birds. Dachshunds are bred specifically to fit inside tunnels underground to kill badgers.

More examples:

Parasitoid insects

Parasitoids lay their eggs on or in the body of an insect host, which is then used as a food for developing larvae. The host is ultimately killed. Most insect parasitoids are wasps or flies, and usually have a very narrow host range.

Four of the most important groups are:

Examples of parasitoids:

Encarsia formosa was one of the first biological control agents developed.
Diagram illustrating the life cycles of Greenhouse whitefly and its parasitoid wasp Encarsia formosa

Parasitoids are one of the most widely used biological control agents. Commercially there are two types of rearing systems: short-term daily output with high production of parasitoids per day, and long-term low daily output with a range in production of 4-1000million female parasitoids per week.[11] Larger production facilities produce on a yearlong basis, whereas some facilities will produce only seasonally.

Rearing facilities are usually a significant distance from where the agents will be used in the field, and transporting the parasitoids from the point of production to the point of use can pose problems.[12] Shipping conditions can be too hot, and even vibrations from planes or trucks can disrupt the parasitoids.[11]

Micro-organisms

Further information: biopesticide

Pathogenic micro-organisms include bacteria, fungi, and viruses. They kill or debilitate their host and are relatively host-specific. Various microbial insect diseases occur naturally, but may also be used as biological pesticides. When naturally occurring, these outbreaks are density-dependent in that they generally only occur as insect populations become denser.

Bacteria

Bacteria used for biological control infect insects via their digestive tracts, so insects with sucking mouth parts like aphids and scale insects are difficult to control with bacterial biological control.[13] Bacillus thuringiensis is the most widely applied species of bacteria used for biological control, with at least four sub-species used to control Lepidopteran (moth, butterfly), Coleopteran (beetle) and Dipteran (true flies) insect pests. The bacteria is available in sachets of dried spores which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees. Bacillus thuringiensis has also been incorporated into crops, making them resistant to these pests and thus reducing the use of pesticides.

Fungi

Fungi that cause disease in insects are known as entomopathogenic fungi, including at least fourteen species that attack aphids.[14] Beauveria bassiana is used to manage a wide variety of insect pests including: whiteflies, thrips, aphids and weevils. A remarkable additional feature of some fungi is their effect on plant fitness. Trichoderma species may enhance biomass production promoting root development, dissolving insoluble phosphate containing minerals.

Examples of entomopathogenic fungi that can be mass-produced for biological control include:

In addition, certain genera are highly entomopathogenic, but difficult to manipulate including:

Several members of Chytridiomycota and Blastocladiomycota have been explored as agents of biological control. From Chytridiomycota, Synchytrium solstitiale is being considered as a control agent of the yellow star thistle (Centaurea solstitialis) in the United States.[15] Synchytrium minutum occasionally parasitizes kudzu and was considered as a control agent against this weed outside of its native range, but S. minutum parasitizes agricultural crop plants more frequently than it parasitizes kudzu.[16] Batrachochytrium dendrobatidis was briefly considered and soundly rejected as a means of controlling invasive frog populations in Hawaii.[17] From Blastocladiomycota, certain members of Coelomomyces were explored as possible agents of biological control of mosquitoes.[18]

Viruses

The European Rabbit (Oryctolagus cuniculus) is seen as a major pest in Australia and New Zealand.

Combined use of parasitoids and pathogens

In cases of massive and severe infection of invasive pests, techniques of pest control are often used in combination. An example being, that of the emerald ash borer (Agrilus planipennis Fairmaire, family Buprestidae), an invasive beetle from China, which has destroyed tens of millions of ash trees in its introduced range in North America. As part of the campaign against the emerald ash borer (EAB), American scientists in conjunction with the Chinese Academy of Forestry searched since 2003 for its natural enemies in the wild leading to the discovery of several parasitoid wasps, namely Tetrastichus planipennisi, a gregarious larval endoparasitoid,Oobius agrili, a solitary, parthenogenic egg parasitoid, and Spathius agrili, a gregarious larval ectoparasitoid. These have been introduced and released into the United States of America as a possible biological control of the emerald ash borer. Initial results have shown promise with Tetrastichus planipennisi and it is now being released along with Beauveria bassiana, a fungal pathogen with known insecticidal properties.[19][20][21]

Plants

The legume vine Mucuna pruriens is used in the countries of Benin and Vietnam as a biological control for problematic Imperata cylindrica grass. Mucuna pruriens is said not to be invasive outside its cultivated area.[22] Desmodium uncinatum can be used in push-pull farming to stop the parasitic plant, Striga.[23]

Indirect control

Pests may be controlled by biological control agents that do not prey directly upon them. For example, the Australian bush fly, Musca vetustissima, is a major nuisance pest in Australia, but native decomposers found in Australia are not adapted to feeding on cow dung, which is where bush flies breed. Therefore, the Australian Dung Beetle Project (1965–1985,) led by Dr. George Bornemissza of the Commonwealth Scientific and Industrial Research Organisation, released forty-nine species of dung beetle,[24] with the aim of reducing the amount of dung and therefore also breeding sites of the fly.[25]

Effects of biological control

Effects on native biodiversity

The cane toad, Bufo marinus

Biological control can potentially have positive and negative effects on biodiversity.[3] The most common problems with biological control occur via predation, parasitism, pathogenicity, competition, or other attacks on non-target species.[26] Often a biological control agent is imported into an area to reduce the competitive advantage of an exotic species that has previously invaded or been introduced there, the aim being to thereby protect the existing native species and ecology. However the introduced control does not always target only the intended species; it can also target native species.[27] In Hawaii during the 1940s parasitic wasps were introduced to control a lepidopteran pest and the wasps are still found there today. This may have a negative impact on the native ecosystem, however, host range and impacts need to be studied before declaring their impact on the environment.[28]

Over the past 15 years with the rise in biological control interest there has become a greater focus on the non-target impacts that could occur.[3] In the past many biological control releases were not thoroughly examined and agents of biological control were released without any consideration. When introducing a biological control agent to a new area, a primary concern is its host-specificity. Generalist feeders (control agents that are not restricted to preying on a single species or a small range of species) often make poor biological control agents, and may become invasive species themselves. For this reason potential biological control agents should be subject to extensive testing and quarantine before release into any new environment. If a species is introduced and attacks a native species, the biodiversity in that area can change dramatically. When one native species is removed from an area, it may have filled an essential ecological niche. When this niche is absent it may directly affect the entire ecosystem.

Vertebrate animals tend to be generalist feeders, and seldom make good biological control agents; many of the classic cases of "biocontrol gone awry" involve vertebrates. For example, the cane toad, Bufo marinus, was intentionally introduced to Australia to control the introduced French's Cane Beetle and the Greyback Cane Beetle,[29] pests of sugar cane. 102 toads were obtained from Hawaii and bred in captivity to increase their numbers until they were released into the sugar cane fields of the tropic north in 1935. It was later discovered that the toads could not jump very high and so they could not eat the cane beetles which stayed up on the upper stalks of the cane plants. However the toad thrived by feeding on other insects and it soon spread very rapidly; it took over native amphibian habitat and brought foreign disease to native toads and frogs, dramatically reducing their populations. Also when it is threatened or handled, the cane toad releases poison from parotid glands on its shoulders; native Australian species such as goannas, tiger snakes, dingos and northern quolls that attempted to eat the toad were harmed or killed.[30] This example shows how small mis-introduced organisms can alter the native biodiversity in large ecosystems. If native species are reduced or eradicated, a domino effect can take place until a new equilibrium is reached.

Other examples of biological control agents that subsequently crossed over to native species are:

Living organisms, through the process of evolution, may achieve increased resistance to biological, chemical, and physical methods of control over time. In the event the target pest population is not completely exterminated or is still capable of reproduction (were the pest control means a form of sterilization), the surviving population could acquire a tolerance to the applied pressures - this can result in an evolutionary arms race with the control method. Successful biological control reduces the population density of the target species over several years, thus providing the potential for native species to re-establish. In addition, regeneration and reestablishment programs can aid the recovery of native species. To develop or find a biological control that exerts control only on the targeted species is a very lengthy process of research and experiments.

Effects on invasive species

The invasive species Alternanthera philoxeroides (alligator weed) was controlled in Florida (U.S.) by the introduction of Agasicles hygrophila (alligator weed flea beetle)

Biological control programs aim to reduce or eliminate populations of ecologically and agriculturally harmful invasive species. Examples where this has been achieved include:

Grower education

A potential obstacle to the adoption of biological pest control measures is growers sticking to the familiar use of pesticides. It has been claimed that many of the pests that are controlled today using pesticides, actually became pests because pesticide use reduced or eliminated natural predators.[36] A method of increasing grower adoption of biocontrol involves is letting growers learn by doing, for example showing them simple field experiments, having observations of live predation of pests, or collections of parasitised pests. In the Philippines, early season sprays against leaf folder caterpillars were common practice, but growers were asked to follow a 'rule of thumb' of not spraying against leaf folders for the first 30 days after transplanting; participation in this resulted in a reduction of insecticide use by 1/3 and a change in grower perception of insecticide use.[37]

See also

References

  1. ↑ Flint, Maria Louise & Dreistadt, Steve H. (1998). Clark, Jack K., ed. Natural Enemies Handbook: The Illustrated Guide to Biological Pest Control. University of California Press. ISBN 978-0-520-21801-7.
  2. ↑ "Biological control: Approaches and Applications". University of Minnesota. Retrieved 3 September 2012.
  3. 1 2 3 Follett PA, Duan JJ (2000) 'Nontarget effects of biological control.' (Kluwer Academic Publishers).
  4. ↑ "the only biogenic amines whose physiological significance is presumably restricted to invertebrates, pharmacologists have focused their attention on the corresponding receptors, which are still believed to represent promising targets for new insecticides" TYRAMINE AND OCTOPAMINE: Ruling Behavior and Metabolism T Roeder - Annual Review of Entomology, 2005 - Annual Reviews
  5. ↑ Gurr, Geoff M. (22 February 2016). "Multi-country evidence that crop diversification promotes ecological intensification of agriculture". Nature Plants. doi:10.1038/nplants.2016.14.
  6. 1 2 Pauline Pears (2005), HDRA encyclopedia of organic gardening, Dorling Kindersley, ISBN 978-1405308915
  7. ↑ Kaya, Harry K. et al. (1993). "An Overview of Insect-Parasitic and Entomopathogenic Nematodes". In Bedding, R.A. Nematodes and the Biological Control of Insect Pests. CSIRO Publishing. ISBN 978-0-643-10591-1.
  8. ↑ Spider mites and their natural enemies
  9. ↑ White flies and their natural enemies
  10. ↑ Hoddle M.S., Grandgirard J., Petit J., Roderick G.K., Davies N. (2006). "Glassy-winged sharpshooter Ko'ed - First round - in French Polynesia". Biocontrol News and Information 27 (3): 47N–62N.
  11. 1 2 Smith SM (1996) Biological control with Trichogramma: advances, successes, and potential of their use. In 'Annual Review of Entomology' pp. 375–406.
  12. ↑ Wajnberg, E., Hassan, S.A. (1994). Biological Control with Egg Parasitoids. UK: CABI Publishing.
  13. ↑ L.A. Swan. 1964. Beneficial Insects. 1st ed. page 249.
  14. ↑ I.M. Hall & P.H. Dunn, Entomophthorous Fungi Parasitic on the Spotted Alfalfa Aphid, Hilgardia, Sept 1957.
  15. ↑ Voigt K., A.V. Marano, & F.H. Gleason. 2013. Ecological & Economical Importance of Parasitic Zoosporic True Fungi. in: The Mycota: A Comprehensive Treatise on Fungi as Experimental Systems for Basic & Applied Research Vol. 11 Agricultural Applications. 2nd edition Eds: K. Esser & F. Kempken. Springer: New York pg. 243–270.
  16. ↑ Li, Z.; Dong, Q.; Albright, T.P.; Guo, Q. (2011). "Natural and human dimensions of a quasi-natural wild species: the case of kudzu". Biological Invasions 13: 2167–2179. doi:10.1007/s10530-011-0042-7.
  17. ↑ Beard, Karen H., and Eric M. O'Neill. "Infection of an invasive frog Eleutherodactylus coqui by the chytrid fungus Batrachochytrium dendrobatidis in Hawaii." Biological Conservation 126.4 (2005): 591-595.
  18. ↑ Sparrow FK. 1960. Aquatic Phycomyetes. The University of Michigan Press:Ann Arbor. 2nd edition
  19. ↑ Gould, Juli; Bauer, Leah. "Biological Control of Emerald Ash Borer (Agrilus planipennis)" (PDF). Animal and Plant Health Inspection Service (APHIS) website (United States Department of Agriculture). Retrieved 28 April 2011 External link in |work= (help)
  20. ↑ Bauer, L.S.; Liu, H-P; Miller, D.; Gould, J. (2008). "Developing a classical biological control program for Agrilus planipennis (Coleoptera: Buprestidae), an invasive ash pest in North America" (PDF). Newsletter of the Michigan Entomological Society 53 (3&4): 38–39. Retrieved 29 April 2011.
  21. ↑ "Biocontrol: Fungus and Wasps Released to Control Emerald Ash Borer". Science News. ScienceDaily. 26 April 2011. Retrieved 27 April 2011.
  22. ↑ "Factsheet - Mucuna pruriens". www.tropicalforages.info. Retrieved 2008-05-21.
  23. ↑ Khan, Z.; Midega, C. A. O.; Amudavi, D. M.; Hassanali, A.; Pickett, J. A. (2008). "On-farm evaluation of the 'push–pull' technology for the control of stemborers and striga weed on maize in western Kenya". Field Crops Research 106 (3): 224–233. doi:10.1016/j.fcr.2007.12.002.
  24. ↑ Bornemissza, G. F. (1976). "The Australian dung beetle project 1965–1975". Australian Meat Research Committee Review 30: 1–30.
  25. ↑ http://www.csiropedia.csiro.au/display/CSIROpedia/Dung+beetle+program
  26. ↑ Council NR (1996) 'Ecologically Based Pest Management:New Solutions for a New Century.' (The National Academies Press)
  27. ↑ Society for Conservation Biology (2002), "Biocontrol backfires again," http://www.scienceblog.com/community/older/2002/C/20025043.html, accessed July 31, 2009.
  28. ↑ Wright, MG; Hoffmann, MP; Kuhar, TP; Gardner, J; Pitcher, SA (2005). "Evaluating risks of biological control introductions: A probabilistic risk-assessment approach". Biological Control 35: 338–347. doi:10.1016/j.biocontrol.2005.02.002.
  29. ↑ "Cane Toad". Exotic Animals - Major Pests. Northern Territory Government, Australia. Retrieved 14 March 2011.
  30. ↑ http://fdrproject.org.au/pages/toads.htm
  31. ↑ National Research Council (U.S.). Board on Agriculture and Natural Resources (June 2000). Incorporating science, economics, and sociology in developing sanitary and phytosanitary standards in international trade: proceedings of a conference. National Academies Press. p. 97. ISBN 978-0-309-07090-4. Retrieved 12 August 2011.
  32. ↑ "Alternanthera philoxeroides information from NPGS/GRIN". www.ars-grin.gov. Retrieved 2008-08-09.
  33. ↑ Cofrancesco (2007)
  34. ↑ http://www.environment.gov.au/biodiversity/invasive/weeds/publications/guidelines/wons/pubs/s-molesta.pdf
  35. ↑ http://www.daff.qld.gov.au/documents/Biosecurity_EnvironmentalPests/IPA-Prickly-Pear-Control-PP29.pdf
  36. ↑ Thacker, J.R.M. 2002. An Introduction to Arthropod Pest Control. Cambridge University Press.
  37. ↑ Heong, K.L.; Escalada, M. M. (1998). "Changing rice farmers' pest management practices through participation in a small-scale experiment". Int. J. Pest Management 44: 191–197. doi:10.1080/096708798228095.

Further reading

General
Effects on native biodiversity
Effects on invasive species
Economic effects

External links

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