Use of predators biological control Passive protection

Public Health Significance of Urban Pests Mosquitoes 367 366

11.8.4. Use of predators biological control

Worldwide, research on about 40 biocontrol agents has received the support of a WHO research programme; these agents include bacteria, fungi, protozoa, nematodes, viruses, fish, insects, snails and plants Schrieber Jones, 2000. In some cases, the introduction of fish was successfully used as a biological control agent. The top minnow Gambusia affinis has been introduced into ditches, rice fields and marshes of southern Europe; its persistence depends on the water surface not freezing. An average of six fish per 10m 2 seems to be sufficient for control. While the effectiveness of this predator remains limi- ted in natural environments, because these fish seem to feed on mosquito larvae only in the absence of other food sources, it remains very effective in artificial sites, such as orna- mental ponds. For such ponds, some people might prefer more decorative fish, such as cyprinids such as goldfish, the guppy Poecilia reticulata or tilapia Oreochromis mos- sambicus , which are also efficient predators of mosquito larvae. Other predators have been tested in the United States Rose, 2001. Some have yet to become available, some have production and storage problems and some are still just can- didates. The EPA has registered the entomopathogenic fungus Lagenidium giganteum for mosquito control, but products have not become readily available. For technical rea- sons, the pathogenic protozoon Brachiola Nosema algerae has also been unavailable. Entomoparasitic nematodes, such as Romanomermis culicivorax and Romanomermis iyen- gari , are effective and do not require EPA registration; however, they are not easily pro- duced and have storage viability limitations. Also, the predacious copepod Mesocyclops longisetus preys on mosquito larvae and is a candidate for larval control in water contai- ners under semitropical conditions.

11.8.5. Passive protection

Passive methods of protection may not necessarily eliminate the pest problem, but they can limit its impact. Protection can be achieved by avoiding vector-infested areas, by using physical barriers, such as clothing, screens and nets, by using space repellents, or by applying repellents to skin or clothing, or both Barnard, 2000. throid; WHO, 2002 on the walls of resi- dences, so that either unfed or recently blood-fed mosquitoes will die when they rest on the sprayed walls. T his type of control has a long-lasting effect up to two months or more, but will be effective only against endophilic species such as some biotypes of the northern house mosquito and some Anopheles spp.. It is important that specially trained and equipped pro- fessionals do the spraying. In some cases, the treatment of adults can be done outdoors – that is, against exo- philic mosquitoes – by using ULV appli- cations produced by cold emulsion or thermal diesel suspension foggers, mounted on a vehicle Fig. 11.5 or aircraft. This has no long-lasting effect and must be repeated daily during either periods of high risk of disease outbreak or periods of severe nuisance biting, originating from areas inaccessible to larviciding. The active ingredients available Tables 11.3 and 11.4 are organophosphates such as fenitrothion and malathion or pyrethroids such as deltamethrin and permethrin, depending on compliance with local regulations. Spraying operations are usually carried out early in the morning – before mosquitoes become active around 05:00 in summer and people leave their dwellings – and target the resting places of adult mosquitoes such as hedges and groves close to human habitations WHO, 1996; Chavasse Yap, 1997; WHO, 2003. Aerial spraying can also be undertaken in the evening and at night, espe- cially if it targets Culex spp.; the disadvantage of this is that it is likely that more people will be out of doors and thus exposed to insecticides, and it may also be more difficult or expensive or both to get spraying personnel. Table 11.4. Active insecticide ingredients for residual indoor adulticide treatments AI Dosage AI m 2 Residual effect Mode of action Organophosphates Fenitrothion 1–2 g 3 months or more Contact and ingestion Malathion 1–2 g 2–3 months Contact Pyrethroids Cypermethrin 0.5 g 4 months or more Contact Deltamethrin 0.05 g 2–3 months Contact Permethrin 0.5 g 2–3 months Contact Source: WHO 1996. Table 11.3. Active insecticide ingredients available for outdoor adulticide treatments AI Dosage AI ha cold fog Dosage AI ha thermal fog Organophosphates Fenitrothion 250–300 g 270–300 g Fenthion 112 g — Malathion 112–693 g 500–600 g Pyrethroids Bioresmethrin 5–10 g 20–30 g Deltamethrin 0.5–1.0 g — Permethrin 5–10 g — Source: WHO 1996. Fig. 11.5. Adulticiding against Asian tiger mosquitoes in an urban environment Source: Photo by F. SchaffnerEID Méditerranée Public Health Significance of Urban Pests Mosquitoes 369 368 • cytoplasmic incompatibility natural incompatibility of some allopatric populations of the same species; and • chromosomal translocation sterility inherited by crossing normal individuals with heterozygotes having a translocation. Except for some trials that used irradiated males, the theoretical prediction of the long- term impact of these methods on suppressing nuisance mosquitoes was not conclusive in field applications. Among the various possible reasons for this failure are: • an incomplete mixing of the populations released with the natural populations or beha- vioural differences in reproduction between these two populations; • the density threshold that allows regulation of the population after the introduction of the genetic modification was not reached; • the absence of compensation, by an increase in the number of released genetically modi- fied individuals when the natural population grows or the lack of competitiveness of the released sterile males; and • immigration of fertile females towards the trial area from nearby areas. At present, research is concentrating on the production of transgenic mosquitoes. These are genetically modified to: • prevent pathogen development in and transmission by the insects Blair, Adelman Olson, 2000; Christophides, 2005; • foster insecticide susceptibility or prevent insecticide resistance Carlson et al., 1995; Collins James, 1996; Hemingway, 1999; or • reduce mosquito populations Benedict Robinson, 2003; Pates Curtis 2005. Apart from ethical concerns about the release of uncontrollable transgenic organisms, it is doubtful whether the modified mosquitoes are sufficiently fit to replace the natural populations Spielman, 1994; Catteruccia, Godfray Crisanti, 2003.

11.9. Economic burden of mosquitoes