TIME TO KILL PHOSPHINE RESISTANT RHYZOPERTHA DOMINICA USING A CONCENTRATION PROFILE TYPICAL OF FIELD EXPOSURE

A considerable number of studies have been carried out on the response of stored product pests to phosphine (PH3). These usually involve constant concentrations for defined exposure periods. These studies show that the concentration by time product required to obtain a defined level of kill varies considerably with concentration (or exposure time). This kind of study is useful in setting dosage regimes for methods of application where a more or less constant concentration is maintained. However, in commercial PH3 treatments (using PH3 generated in situ from metal phosphide) the concentration builds up to a maximum over 1 to 3 days and then decays over the remaining exposure period. Many factors affect the concentration profile including the nature of the phosphide preparation, leakage, sorption, temperature and humidity. This makes defining the required dosage regime a complex process. The laboratory exposures reported here mimic various field concentrations. The results are used to judge the adequacy of particular regimes by observing the level of kill at various times through the exposure period. The response to the concentration by time product (Ct) at each time end point is then compared to that predicted with a constant concentration exposures. The relationship between (Ct) obtained in these two manners is discussed.