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Focus on Residual Sprays, PT. 1

 

By: Manuel F. LLuberas MS, IDHA

Residual sprays continue to play a pivotal role in the control of vector-borne diseases around the globe. They are the mainstay of malaria vector control programs in most of Africa, many countries in Asia and a good portion of the rest of the Americas. In addition, residual applications of insecticides are also employed in the control of Chagas disease vectors throughout Central and South America. Unfortunately, though this method is widely used, vector control programs frequently lack well-trained field staff that can ensure that the insecticides used are applied properly and that adequate deposition and spray patterns are obtained.

Maintaining the internal pressure of compression sprayers at the recommended operational range of between 55 to 35 psi is the first step towards achieving adequate insecticide deposition. An experienced vector control specialist knows that the internal pressure of the compression sprayer will drop as the liquid volume of the sprayer is reduced during its application on a surface.

Fig. 1. Output volume at various pressures.
However, even some of the most experienced specialists are surprised when provided with palpable evidence of this, see Figure 1. As the internal pressure of the sprayer drops from the recommended maximum operational level of 55psi (measuring cylinder #1) to 40psi (measuring cylinder #2) and then to 25psi (measuring cylinder #3) there is a relatively linear and uniform reduction in the output volume. However, the reduction in output volume ceases to be linear in nature when the internal pressure of the sprayer is lowered another 15psi from 25 to 10psi (see measuring cylinder #4).

The reduction in output volume resulting from a drop in the internal pressure of a compression sprayer brought about as its internal volume is depleted brings up another factor frequently overlooked: a measurable variation in the spray pattern (see Figure 2). As the sprayer pressure drops by 15psi from the highest operational pressure of 55psi (two upper band), there is a slight reduction in the effective coverage. This reduction in effective coverage continues as the pressure continues to decrease in a relatively uniform fashion and then becomes essentially useless when the sprayer's internal pressure drops below 25psi.

The last major factor influencing the effectiveness and efficacy of an indoor residual spray program is the type and condition of the nozzle used. Two types of nozzles are recommended, depending on the residual insecticide used: 8002E or 8001E. The latter one is used when insecticides other than pyrethroids are sprayed whereas the former is normally installed when residual formulations of pyrethroids are applied.

Maintaining proper nozzle orientation -see Figure 3., letter "a"- is the first step in ensuring adequate coverage and deposition. Orientation must be tested in a small corner of the surface to be sprayed to ensure that the spray is oriented correctly. Though unnoticeable to the untrained eye, the deposition pattern between an 8002 and 8002E nozzle is significantly different -see letters "b" and "d" on Figure 3. The edges of the material sprayed using an 8002 nozzle (letter "d") are diffuse as compared with those produced by the 8002E (letter "b"). Finally, the material sprayed using an 8001 nozzle is noticeably less than that sprayed using an 8002 nozzle -letter "c" and "b," respectively.

One of the factors most overlooked by many vector control programs conducting indoor residual spraying is nozzle age. A nozzle that has outlived its useful life will produce uneven deposition patterns with gaps where a sub-lethal dose of insecticide is deposited -see letter "e" on Figure 3). This produces an uneven insecticide deposition pattern and provides gaps where vectors can come to rest without coming in contact with a lethal dose of the material sprayed.

The editor would like to thank Mr. Anton Gericke of Avima in South Africa for conducting the field demonstration depicted in the series of photographs in this note.