THE EFFECT OF WATER AVAILABILITY, RELATIVE HUMIDITY AND PHYSICAL PROPERTIES OF SILICA DUST ON ITS EFFICACY AS A CONTROL AGENT FOR BLATTELLA GERMANICA (DICTYOPTERA: BLATTILLIDAE) (L.)

Abstract:
Silica dusts with different physical properties were tested against the German cockroach Blattella germanica. The dusts varied in their effects on cockroach mortality according to their physical properties. Oil adsorption, pore volume and surface area were the most important characteristics while particle size showed no significant effect. The rank order of decreasing toxicity was Dri-die, Gasil 23D, Gasil 114, Gasil EBN, Gasil HP25, a silica gel S4 and Gasil 200DF. When silica was applied as a suspension in water the exposure time was found to have a greater influence on efficacy than the concentration used. A 72 hour exposure at 3gllitre gave 100% mortality whereas exposure for 1 hr on l00g / litre caused only 33% mortality. Exposure of male and female cockroaches to silica dust Gasil 23D reduced the time the cockroaches were able to survive without water from 5 and 12 days respectively to 3 and 4 days, and to only 1 and 2 days in the presence of the silica dust, Dri-die. The KD50 and LD50 for a range of silica dusts were determined at different humidities. Humidities of up to 60% did not significantly affect the knock-down or mortality of cockroaches for the unfluorinated silica Gasil 23D and fluorinated silica Dri-die, but when humidity was increased to more than 70% it reduced efficacy of the dusts, particularly Gasil 23D.