Zapping mosquitoes with biopesticides 

Mosquito control can be an expensive and hazardous process relying on highly toxic and persistent insecticides. Recent experiences in Central America show that biological control is effective, safer and cheaper than chemical control.

Malaria is a debilitating and often fatal disease. The insecticides to control carriers of this and other tropical illnesses were first used in Central America in the late 1950s. Pesticides used included dieldrin and DDT, sometimes in weekly applications. Insecticides sprayed routinely in and around buildings and breeding sites to control adult mosquitoes also exposed spray operators and residents. The rapid development of resistance to particular products by the mosquitoes has been a continual problem, leading to the introduction of organophosphates (OPs) in the 1960s, then carbamates, followed by pyrethroids in the 1980s, and back to OPs. By the 1990s rising control costs due to resistance, in conjunction with increased awareness of the negative environmental effects, prompted the health authorities in the region to undertake a major rethink of their control strategies. Supported by the European Union, a programme to control the focal points of malaria epidemics by biological means began in 1996(1), spearheaded by Nicaragua’s Ministry of Health, who decided to stop their routine applications of the OP fenthion in the same year.

Learning to use biopesticides
The main aim was to see whether large-scale applications of commercially available biopesticides for mosquitoes could actually reduce the number of disease cases in malarial hotspots, such as the poor, lakeshore neighbourhoods of Nicaragua’s capital Managua. The main product used was a liquid formulation of spores of the bacterium Bacillus sphaericus, produced in Cuba under the trade name Griselesf. This bacterium, specific to mosquito species which feed on blood, infects mosquito larvae in their aquatic habitat, producing high mortality 72-96 hours after application to water bodies. Griselesf and other B. sphaericus products offer the advantage that the bacteria can survive and reproduce for up to 11 months, thus reducing the need for frequent applications. Other larvicides based on the mosquito-specific israelensis variety of the bacterium Bacillus thuringiensis, (Bt) were also tested for additional control. Bt mosquito biopesticides can achieve high larval mortality within 48 hours of application but only persist for 15-60 days and cannot reproduce, thus they are best for emergency use or in short-term breeding sites.
    Just as important as the trialling of the biopesticides was the intensive training of Health Ministry technical and manual staff in biological control techniques and evaluation. Staff learn’t how to map breeding sites and sample mosquito larvae using simple scoops to calculate larval densities and growth stages and to identify when densities reached action thresholds for control measures. Before applying biopesticides, staff needed to cut back vegetation and improve drainage around the breeding sites, to increase the effectiveness of the spraying, application was by pressurised knapsack sprayers or motor-driven mistblowers. 
    After spraying, larval densities were monitored by weekly sampling and adult numbers in houses were assessed by staff recording the number of mosquito bites they received per hour on exposed legs during the early evening. Medical staff could relate larval sampling data pre- and post-spraying to changes in adult population levels and malaria cases recorded in each neighbourhood.
    Spray operators received practical training and an illustrated manual covering topics of vector and parasite biology, malaria transmission, natural enemies of mosquitoes and environmental impact of chemical application, as well as biopesticide safe use, persistence and impact.

Cost savings
Results by 1998 in Nicaragua showed that three out of five Bt israelensis applications led to a 90-100% reduction in mosquito populations for three to five weeks. Lakeshore assessments over three years revealed that the vector population could be adequately controlled with one to two applications of Griselesf and three to five applications of Bt israelensis per year. Malaria cases in Managua dropped from over 19,000 in 1996 to 1,575 in 2000. 
    In Guatemala B. sphaericus products gave positive results and in Honduras they proved effective in reducing mosquito populations, while Bt israelensis was also used for controlling the Aedes aegypti mosquito which transmits dengue fever. 
    Direct control costs were compared with conventional chemical control using fenthion (Table 1). Guatemala’s Ministry of Health estimated enormous savings of 92-97% for an area of 12,500m2 over six months by using B. sphaericus 2362 instead of Fenthion 50% powder. The savings were due to lower costs for the biopesticide and the fact that only one to three applications were needed, compared with 22.5 applications of fenthion.

Safer and cleaner
It is hard to estimate precisely the reduced negative effects on human health and environment, but the programme acknowledges that the indirect costs of reliance on toxic, broad-spectrum insecticides must have been substantial. These would include costs incurred by the repeated development of mosquito resistance. Once widespread insecticide application was stopped, natural predators of mosquitoes were able to re-establish in breeding sites, as well as other beneficial insects and wildlife. Moreover, as a result of switching to biological control, Managua’s public health spray operators no longer suffer the acute poisoning symptoms which used to affect many of them when applying fenthion. (SW)

Reference
1. Sustainable strategies for malaria control: a guide for public health personnel, Movimondo and Nicaraguan Ministry of Health, Managua, Nicaragua, 2001 (in Spanish).

[This article first appeared in Pesticides News No. 54, December 2001, page 9]