A review of the literature is given which shows that changes in the susceptibility ofinsects to insecticides occur during development when the poison is applied as a fumigant, as a stomach poison and as a contact poison.The assessment of changes in susceptibility to contact poisons is discussed. The basis of assessment may be the concentration required to kill a given number of individuals, or the amount of poison required to kill unit weight of insect material. The great majority of the earlier work used the former basis of assessment, but where the insect is changing in size and shape during development and hence changing in the proportion of surface area to body weight, this method has limited significance when the poison is applied as a spray of constant deposit evenly over the surface area. A mathematical method is given for transforming the results obtained in terms of concentration to kill a given number of individuals into weights of poison to kill unit weight of insect material. Rearingmethods are outlined which enable batches of various stages of Diataraxia oleracea (L.) (tomato moth), Tenebrio molitor L. (meal worm) and Periplaneta americana (L.) (American cockroach) to be obtained at a known age and stage of development.Experimental results are given for a spraying technique using DDT and the pyrethrins as insecticides on the comparative resistance of the eggs, larvae and pupae of D. oleracea, the larvae, pupae and adults of T. molitor and the nymphs and adults of P. americana.On the basis of the concentration of insecticide required to kill a given percentage of individuals, it is shown that great differences can occur in the resistance of different instars of one species and considerable differences may occur within the instar. If the data for the larval and nymphal instars are considered on the basis of the weight of poison required to kill unit weight of insect material, differences still exist but are much reduced.When toxicity is estimated on the basis of the concentration of poison in a constant spray deposit required to kill a given percentage of individuals the overall variations of resistance during development measured for DDT in terms of median lethal concentrations were: —D. oleracea eggs and larvae fifty times (0·002 per cent.–0·1 per cent. w/v), the pupa proved resistant to 0·5 per cent, w/v; T. molitor larvae, pupae and adults thirty-seven and a half times (0·0008 per cent.–0·03 per cent, w/v.) and P. americana nymphs and adults eight and a half times (0·0035 per cent.–0·03 per cent. w/v). The figures for pyrethrins were: D. oleracea eggs, larvae and pupae thirty-seven times (0·0027 per cent.–0·1 per cent. w/v), T. molitor larvae, pupae and adult forty times (0·005 per cent.–0·2 per cent. w/v), and P. americana nymphs and adults ten times (0·00031 per cent.–0·003 per cent. w/v).The figures show that the range of variation of resistance during development may be very large, over 250 times in the case of DDT and D. oleracea, where the pupa, is resistant. The maximum variation that was found within an instar was 16·6 times where the resistance to pyrethrins of the 1-day old pupa of T. molitor was compared with that of the 4-day old pupa. The data show that the amount of variation in resistance that can occur varies with the test species and with the insecticide. Furthermore that the order of resistance of the developmental stages of any given species will differ with the insecticide and that with any given insecticide the order will vary with the species.It may be inferred from these data that any comparison between insecticides on one stage of development of one instar of one species will not necessarily hold true of any other stage of development of that species or of any other species.Observations were made on the action of the insecticides on the various instars and the symptoms of poisoning are described. It was observed that DDT at the highest concentration used (0·075 per cent. w/v) failed to prevent the development of the embryo inside the egg-shell and death only occurred after the fully developed embryo had eaten the egg-shell which it normally does prior to emergence. Pyrethrins on the other hand if applied at sufficiently high concentrations could prevent embryonic development although at lower concentrations a high percentage of eggs formed fully developed embryos.At the highest concentrations used neither DDT (0·5 per cent, w/v.) nor the pyrethrins (2·5 per cent, w/v.) were able to prevent pupal development, and partial emergence often occurred before death. DDT differed from the pyrethrins in that it was ineffective on the pupae of D. oleracea.Using data given in the literature and from some preliminary experiments on respiration rates, it was possible to deduce some correlation between metabolic rate and susceptibility, and changes in the permeability of the cuticle and chorion and susceptibility, but the evidence is unsatisfactory and the causes of the changes in susceptibility await further detailed investigation.
Evaluation of Reference Genes for Transcriptional Profiling in Two Cockroach Models
