Combating unwanted insects such as mosquito’s using biological organisms is known as biological control or sometimes called biocontrol. Mosquito control is everywhere is the news. With the threat of Zika virus and encephalitis on the rise, eradication of these pests are a major concern. So what can you do in your own yard to control mosquitoes?
Many believe that effective mosquito control consists solely of spraying insecticide – that doing so is sufficient to eliminate mosquitoes around one’s residence or office. But while it can kill mosquitoes within one’s immediate vicinity, it often does not do enough to get rid of mosquitoes where they hide and breed. Even if such places are themselves sprayed, such treatments may not last very long. So in the interest of getting rid of these mosquitoes altogether, one should work to get rid of these places where they lurk.
Good biocontrol of back yard mosquitoes involves seeking out the areas where mosquitoes breed and draining the water in them so that these insects cannot return to breed further. Professional pest control technicians check for possible places where water may accumulate and stagnate, such as clogged rain gutters, drains, sewers, flowerpots, and discarded tires. When such places are found, they make sure that water is drained completely, and they either overturn or cover said place so that water cannot accumulate in it again.
Next, deal with areas with thick undergrowth, such as bushes and trees. Many species of mosquitoes roost in these places when they are not seeking blood meals, and they hide from the sun and from predators such as birds, bats, and dragonflies. When pest control professionals cut away the undergrowth, mosquitoes will be forced to head elsewhere or be devoured by such predators.
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BIOLOGICAL AND ALTERNATIVE CONTROL
The use of biological organisms or their byproducts to combat pest insects, such as mosquitoes, is termed biological control, or biocontrol. Biocontrol is defined as the study and utilization of parasites, predators, and pathogens to regulate pest populations. Generally, this definition includes natural and genetically modified organisms and means that the agent must be alive and able to attack the mosquito. The overall premise is simple: Biocontrol agents that attack mosquitoes naturally are grown in the lab and then released into the environment, usually in far greater numbers than they normally occur, and often in habitats that previously were devoid of them, so as to control targeted mosquito species.
Biocontrol is not a magic bullet for mosquito control in the 21st century. It should be considered a set of tools that a mosquito control program can use when it is economically feasible. When combined with conventional chemicals and physical control procedures, biocontrol agents can provide short and, occasionally, long-term control. Biocontrol, as a conventional control method, should aim at the weakest link of the life cycle of the mosquito. In most cases, this is the larval life stage.
One advantage of biocontrol agents is host-specificity. This factor affords minimal disturbance to non-target species and to the environment. Ironically, it is this specificity and the sometimes large start-up costs that deter commercialization and application of biocontrol agents, in addition to the generally narrow market for industry, increased outlays of capital, and the training required for personnel for mosquito control programs. However, in the future, societal changes such as environmental conservation are likely to increase interest in the use of biocontrol agents. Thus, increased knowledge of alternative control strategies such as biocontrol is needed.
A detailed discussion of the philosophical concept of biocontrol or of the many environmental and biotic factors that influence its effectiveness is beyond the scope of this white paper. Scientific and popular literature on these subjects is extensive and readily available. (Also see 7.6 References and General Readings.) The objective of this chapter is to characterize the biocontrol agents and alternative control strategies tested or used by Florida mosquito control programs.
7.2 FLORIDA DEVELOPMENT AND USE OF BIOLOGICAL CONTROL AGENTS
Mosquito fish are currently the most extensively used biocontrol agent. These fish, which feed on mosquito larvae, can be placed in a variety of permanent and semi-permanent water habitats. Differences of opinion exist on the utility and actual control benefits derived from Gambusia implementation in an Integrated Pest Management (IPM) program with results reported from excellent control to no control at all. Recently, concerns over placing Gambusia in habitats where other fish species assemblages are threatened have arisen. Care must be taken in placement of this cosmopolitan species in areas where endemic fish species are sensitive to further environmental perturbation. Additionally, targeting endemic fish species in these areas of concern deserves greater attention. An example of this is Rivulus fish species. The potential of Rivulus as mosquito predators is currently being evaluated in saltwater habitats, especially in Brevard County.
In some aquatic habitats, fish function as an excellent mosquito biocontrol mechanism. These typically are permanent habitats where Culex and Anopheles are the primary mosquito residents and where the mosquito densities are not excessive. However, in habitats such as salt marshes fish are unable to control the sudden explosion of larvae produced by rainfall or rising tides. Here, the mosquito population numerically exceeds what the fish can consume during the brief immature mosquito developmental period. In salt marshes, fish must rely on things other than mosquito larvae for their nutritional needs most of the time, simply because there may be long delays between hatches of larvae. Mosquito larvae present an abundant food source, but only for a few days during their rapid development.
In specific containers, Toxorhynchites may consume a large number of prey mosquito larvae, such as Aedes aegypti and Ae. albopictus. However, this predator does not disperse well enough to impact the vast number of natural and artificial containers used by these mosquitoes. Additionally their life-cycle is 2-3 times that of their prey making it impossible for them to keep up with the other more rapidly developing mosquitoes (Service 1983).
Species of predacious mosquitoes in the genus Toxorhynchites have been studied in a variety of urban areas for control of container-inhabiting mosquitoes, such as the Asian tiger mosquito (Ae. albopictus). Toxorhynchites mosquitoes also affect mosquito populations that develop in the tree-hole environment; however, their introduction into urban container habitats has proven unsuccessful (Jones & Schreiber 1993; Schreiber & Jones 1993).
Another group of biocontrol agents with promise for mosquito control is the predacious copepods. In Florida, season-long control was achieved in trials in tire habitats in Florida (Schreiber, in preparation). Copepods are easy to rear and to deliver to target sites in the field (Hallmon et al. 1993), and they perform well when used with insecticides (Tietze et al. 1993).
Research on other possible predators of mosquitoes, such as fish, dragonflies and frogs is ongoing. Currently, however, there is no small- or large-scale development program for the application of these organisms for biocontrol purposes. Two other potential biocontrol agents which have a great deal of commercial and application interest are the bacteria Bacillus sphaericus, which is effective in waters with high organic content, such as sewage treatment areas and dairy lagoons, and the fungus Lagenidium giganteum, which is effective in still water environments.
Birds and bats are often promoted as potential biocontrol agents of adult mosquitoes. While both predators eat adult mosquitoes, they do not do so in sufficient amounts to impact the mosquito populations. Mosquitoes provide such a small amount of nutrition that birds or bats expel more energy pursuing and eating mosquitoes than they derive from them. They are not a primary food source for these predators. Additionally, with mosquito flight behavior being crepuscular they are not active during the feeding periods of most birds. While bats are active during the correct time period, they simply can not impact the massive numbers of adult mosquitoes available.
As the development of mosquito control technology moves forward, new biocontrol agents will be discovered. Ultimately, biological control hopefully will become an integral part of Florida mosquito control programs.
7.3 ALTERNATIVE CONTROL TECHNIQUES
The term alternative control techniques in essence is a catch all for removal-trapping techniques, repellents, mechanical traps, bug zappers, biotechnological developments, and socio-cultural changes. The following is a thumbnail sketch of some of these alternative control techniques, their merits, and their disadvantages.
7.3.1 Removal Trapping Techniques
The premise underlying removal trapping is that a trap attracts, captures, and removes a significant portion of the biting mosquito population. Subsequent mosquito populations are smaller and, therefore, require fewer chemical applications for control. Removal trapping is currently being evaluated in Collier County in cooperation with the USDA at the Center for Medical, Agricultural, and Veterinary Entomology (CMAVE) in Gainesville and the Collier County Mosquito Control District, and by the University of Florida at the Florida Medical Entomology Laboratory (FMEL), in other parts of the state. In these studies, answers are being sought to such questions as:
• How many and what type of trap should be used?
• What is the impact of removal trapping on the target mosquito population?
• Is the utility of removal trapping limited to geographically isolated sites, such as islands, as opposed to the mainland?
• What influence does the flight range of the target species have on the effectiveness of removal trapping?
The use of synthetic or natural chemicals applied to the human skin for prevention of mosquito bites has a long history that includes some unconventional approaches (e.g., cover oneself with mud or eat lots of garlic cloves) as well as some proven techniques, such as the use of DEET. Research to develop new and improved repellents continues and requires further support. Most repellents on the market have been tested and provide some protection from mosquito bites. However, some so-called natural mosquito repellents (i.e., the mosquito plant Citrosa) do not repel mosquitoes (Cilek & Schreiber 1994). In general, claims regarding natural mosquito repellents should be viewed with skepticism until they have been verified by scientific studies.
7.3.3 Mechanical Traps and Bug Zappers
The use of electronic devices to attract and kill flying mosquitoes or to repel them by sound is not supported on the basis of scientific research. None of the currently marketed products functions as advertised. A course of legal action targeted at the marketing of these devices should be developed and implemented.
Autocidal control can be achieved via the release of sterile males or by manipulation or alteration of the genome of the target population. The theory of sterile-male release comes from the successful sterile-male eradication program for the screw worm fly in North America. Implementation of sterile-male technology for controlling Florida mosquitoes has a number of logistical and economic obstacles. Much basic research would have to be conducted before sterile- male technology could be used for control of any given species in Florida.
Sterile-male release works well in situations where target populations are low and dispersal is limited, such as screw worm flies and Mediterranean fruit flies. With mosquitoes, such as Ae. taeniorhynchus and Ae. vexans, the number of fertile males emerging from a typical brood is so large that it would be impractical to mass rear sufficient sterile males to compete with them. Berryman et al. (1973) indicated that ratios of sterile:fertile males of 40:1 can fail to reduce populations. Consider the number of sterile males that would have to be synchronously raised to compete with the single emergence from a 500 acre salt marsh, where billions of adult Ae. taeniorhynchus emerge in a 24 hour period.
Genetic manipulation of mosquito populations requires much additional research if operational methods are to be developed. Some possible examples are the introduction of a lethal gene into the mosquito population and the incorporation of the autogency characteristics in female mosquitoes so that eggs could be laid without the requirement for a bloodmeal. Genetic biotechnology may also allow us to incorporate the genes that code for the B. thuringiensis toxin into algae, which mosquito larvae ingest.
7.4 SOCIO-CULTURAL CHANGES
This category of alternative controls includes human avoidance of mosquitoes. Two lifestyle changes in Florida that have greatly reduced mosquito contact with human beings are air conditioning and television viewing. Today people seldom sit on a porch at dusk or at night during the summer. Additional cultural changes that may be forthcoming include the wearing of protective clothing and the curtailment of outdoor activities when the potential for mosquito attack is greatest. Mosquito control can implement these changes and can alter the public perception of mosquito problems through educational programs in schools, through the media, and by cooperation with government agencies at the local, state, and federal level. However with our continuing urbanization, residents seem to be becoming even less tolerant of insect pests.
While it may be desirable to increase the use of biocontrol and alternative control methods into Florida mosquito control programs, many unanswered questions prevent immediate implementation. The only biocontrol agent or alternative technology currently included in mosquito control programs is the mosquito fish Gambusia spp. Other technologies may be included in the future, but these may be limited to fewer programs with one or two specific mosquito problems.
Changes in the state laws which govern the sale and advertisement of mosquito control devices or repellent devices of doubtful efficacy are strongly recommended.
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