5.3.5. General discussion
More research has been published on in vitro brood feeding test. Descriptions of laboratory methods have been provided over almost half a century (Weaver, 1955; Rembold and Lackner, 1981; Wittmann and Engels, 1981; Vandenberg and Shimanuki, 1987; Davis et al., 1988; Czoppelt, 1990; Engels, 1990; Peng et al., 1992; Malone et al., 2002; Brodsgaard et al., 2003). These methods generally provide LD50 or LC50 for the treated larval stage. In 1981, Wittmann and Engels suggested to use the in vitro brood feeding test as a routine method for screening insecticides and classifying chemicals according to their toxicity to larvae. Considering both the laboratory toxicity of a product to larvae and exposure data of brood to this product in natural conditions, the in vitro larval feeding test seems an appropriate starting point of the brood risk assessment, in other terms a tier 1 study. However, objections have been raised against the in vitro method and its regulatory use, in particular doubts on the standardisation of the protocol, criticisms on the frequent high mortality and the presence of intercasts in the control samples. The difference of food quality and mode of dispensing between natural (Haydack, 1968) and artificial conditions described by authors may account for these weaknesses. See a detailed review of in vitro larval rearing in Crailsheim et al., 2013.