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Focus on Residual Sprays, PT. 2

Text & Photos by: The Editor.
Manuel F. LLuberas MS, IDHA

Residual insecticides applications continue to play a pivotal role in vector-borne disease control programs around the globe (see Focus on Residual Sprays, Hudson News, Volume 6, No.1). In that issue, we focused on factors affected by the handling and operation of the Hudson X-Pert® sprayer -or any compression sprayer- and their effect on the spray output and coverage. In this issue we will focus our attention on the impact an operator has on the outcome and effectiveness of the vector control program.

We mentioned in the first section dedicated to the subject that maintaining the internal pressure of the compression sprayer used at the recommended operational range of between 55 to 35 psi is the first step towards achieving adequate insecticide deposition. Anyone applying residual insecticides 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 - please see related images in last issue. However, as we mentioned in our last issue, even some of the most experienced technicians are surprised when provided with palpable evidence of this.

A couple of key factors that are frequently overlooked by residual spray technicians and not always sufficiently emphasized during training are operator technique and spray timing. These factors are solely operator-dependent and are directly influenced by the technician's training and his/her attention to detail. The first factor is spray rhythm or speed. This can be defined as the time required to deliver a uniform deposition over a particular surface. In most cases, adequate deposition can be obtained by spraying the material at about 2.5 seconds per linear meter of surface -provided that a 8002E nozzle is maintained at the required distance from the surface (45cm) and all the requirement mentioned in Part I are met. However, delivering good deposition and pattern over a surface does not stop here. The second operator-dependent factor is spray technique. A spray application executed with good technique is one where the operator carefully maintains the nozzle at 45cm from the target surface and a 5cm overlap is maintained between the down stroke and up stroke while an adequate spray rhythm is kept. Applications conducted by poorly trained technicians or by those rare few who do not pay close attention to their technique may produce one or more of the following: unevenly sprayed surfaces, insufficient material deposited, unevenly spray surface with insufficient material deposited, or too much material deposited.

Unevenly sprayed surfaces can be defined as those that end up looking more like hour-glasses than rectangles (see accompanying image). Placing two of these figures side by side it should be painfully obvious, even to the untrained eye, that there are large gaps where little or no material has been applied where the vectors may rest. This defeats the purpose of spraying and results in wasted time and material.

In areas where the spray pattern is adequate but where the application is too fast, say two seconds per linear meter, the surface may have uniform and consistent sides but the sur-face ends up with a sub-lethal amount of material. This has essentially the same result as leaving gaps in the pattern, and has little or no effect on the vector. In fact, these surfaces may have a more deleterious effect on the vector management program than any other as the vector exposed to a sub-lethal dose of material can develop tolerance to this material and eventually acquire and pass on to their offspring resistance to it. Surfaces with an uneven spray surface and insufficient material deposited may also contribute to the development of insecticide resistance.

Surfaces with an adequate spray pattern but where the spray timing is too slow produce a series of potential problems. The first problem is one of possible repellency effect. Some insecticides labeled for residual applications are known to have repellent effect if applied at higher than recommended dosages. The second problem is related to cost. The excess material applied represents a waste of time and money as it confers no further protection and may actually repel the vector.

It should be evident to the reader that spray technique and timing are key factors in any spray program. The sprayer's nozzle must be maintained at the prescribed distance from the surface to be treated and spray timing must be closely observed. The only way to ensure that spray missions are carried out effectively and overcome these factors and those mentioned in Part I of this series is to provide exhaustive training and practice to spray teams prior to the beginning of any spray season. Moreover, annual cascade training for spray team supervisors should also be completed.

The editor would like to thank Mr. Anton Gericke of Avima in South Africa (with the sprayer) and John Maseko of EcoMark in Zimbabwe for conducting the field demonstration depicted in this photograph.