Attention is called to the need of methods for measuring the quantitative effect of introduced entomophagous insects on the survival of their host populations. One of the principal obstacles has been the difficulty of excluding the introduced enemies from test plots. An expt. is described in which this was accomplished by the use of sleeves of fine-meshed cloth, , in such a way that the effect of the presence and absence of parasites on the survival of their host populations could be measured.