ARS | Publication request: Filtration effects due to bioassay cage design and screen type

Submitted to: Journal of the American Mosquito Control Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 8, 2010
Publication Date: December 1, 2010
Citation: Fritz, B.K., Hoffmann, W.C., Farooq, M., Walker, T., Bonds, J. 2010. Filtration effects due to bioassay cage design and screen type. Journal of the American Mosquito Control Association. 26:411-421.

Interpretive Summary: The use of bioassay cages provides researchers with a quick, easy and repeatable measure of insect mortality resulting from spray applications from new or novel spray compounds or new application techniques or technologies. There are a number of cage designs being used, each with different shape and screening materials, which affect how the ambient air and spray cloud interact with them. The cages are designed prevent insect escape while allowing applied sprays to penetrate, however some filtration effects are still present restricting both airflow and spray droplets. Replicated studies were conducted in the USDA-ARS Low Speed Wind Tunnel in College Station, TX to determine the effects on airspeed and droplet size inside the cage for different cages that varied in cage size and shape, as well as, the screening material used on the cage. No simple relationships between screen fiber size and porosity and airspeed reduction were observed, but screens with lower porosities, smaller cages types, and cylindrical cages tended to provide greater resistance to airflow through the cage. Spray droplet size inside the cages were reduced anywhere from 0 to over 10 percent for the different cages tested. As a result of this work, field researchers involved in assessing the efficacy of vector control applications will have a better understanding of the airspeeds and spray droplet sizes penetrating the cages and interacting with the cage insects.

Technical Abstract: The use of bioassay cages in the efficacy assessment of specific compounds, application techniques and technologies is a common practice. There are a number of cage designs being used that range across a variety of cage shapes and sizes and mesh types. The objective of this work was to examine a range of cages and mesh types for their filtration effects on airspeed and spray droplet size. Tests were conducted measuring airspeed inside and outside of bioassay cages across in a wind tunnel. Airspeeds of 0.5, 1, 2, and 4 m/sec and cage face orientation (relative to the airstream) or 0, 10, 22.5 and 45° were tested. Generally, higher percent reductions in airspeed were seen at lower airspeeds with overall reductions ranging from 30 to 88%, depending on cage type and tunnel airspeed. No simple relationships between screen fiber size and porosity and airspeed reduction were observed, but cages with screens with lower porosities and smaller cylindrical shaped cages tended to provide greater resistance to airflow through the cage. Overall, spray droplet size inside the cages were reduced anywhere from 0 to over 10 percent. As with the airspeed tests, face orientation relative to the mean air direction did not have a significant effect on droplet size reduction levels.