Toxic and behavioural effects of different modified diatomaceous earths on the German cockroach, Blattella germanica (L.) (Orthoptera: Blattellidae) under simulated field conditions

Defined populations of German cockroaches were observed after exposure to deposits (25 g/m²) of various modified and un-modified diatomaceous earth (DE) formulations by using a computer-aided device measuring motility, circadian rhythm, and mortality under defined environmental and climatic field-simulating conditions. In a humid climate (85% r.h.) with water and food offered ad libitum, complete population eradication could be achieved within 10 days with three out of seven DE formulations tested. Results revealed two basic factors affecting mortality: (a) formulations containing freshwater diatoms from Spain were significantly more effective than those originating from marine diatoms purchased from Fur Island, Denmark (P<0.0001), and (b) the higher the degree of coating with a silica-aerogel to increase oil-carrying capacity, the greater the efficacy (P<0.0001 to 0.07). Exposure to DEs resulted in complete disruption of the species-specific circadian rhythm of German cockroach populations with six out of seven DE formulations investigated. A remaining rudimentary circadian rhythm was found after exposure to one formulation. It is concluded that strongly hydrophobic formulations of DE can be used for effective cockroach control even in tropical environments, especially as part of a least-toxic Integrated Pest Management control strategy. Nevertheless, it remains unclear to what extent newly developed DEs, coated or formulated with different chemical compounds resulting in modified surface characteristics, could potentially produce adverse health effects to humans.