The Action of some Organic Compounds when used as Stomach Poisons for Caterpillars

Abstract

This work was commenced in 1919 and continued in 1920 and 1923. The writer's thanks are due to Prof. H. Maxwell Lefroy, who provided facilities for the carrying out of the work at Westbrook House, Heston, and showed great interest in its progress. Publication of the results has been delayed in the hope that further experiment could be made in the near future.It is known that some organic compounds, e.g., naphthalene, are of use as stomach poisons for caterpillars, but little work has been done except with mineral compounds, of which Lefroy and Finlow tested a large number. The effect of some organic substances on house-flies is described in a paper by Jackson and Lefroy, who found the dinitrocresylates and salicylates to be first-class poisons.The present tests were made in an endeavour to find some efficient substitutes for the arsenates which would be innocuous or much less toxic to man and other animals, and to indicate the directions for further research. Any available organic compound either metallic or non-metallic was tried, in addition to some inorganic substances. One per cent, mixtures were first used, and where a fairly good killing effect was obtained, other strengths were tried in order to give some indication of the optimum. It appears to be advisable in any future tests that at least three different concentrations be used in every case.The caterpillars used were those of Pieris rapae,L-, and Spilosoma lubricipeda, Esp., with leaves of cabbage and lupin respectively for food, the Spilosoma being indicated in the tables by the letters BE. Larvae about half-grown were generally used, but towards the end of the season sufficient of these were not available, and pupation interfered to some extent with the tests. The larvae that died as a result of wilt disease, parasites or cannibalism, as well as any which pupated, are not included in the figures except those of the controls.Compounds were applied as emulsions where possible, made with a standard Liquid soap composed of olein 37*5 cc, 90 per cent, alcohol 19-0 cc, ammonia (0-880) 9-0 cc. Some of them, mainly acids, would not emulsify and for these a one per cent, starch paste was found to be a satisfactory carrier. Technique.The compound (0-25 gram) was finely ground in a mortar, well mixed with (a) 1-0 cc. of the liquid soap, 24 cc. of distilled water being gradually added so as to form a good one per cent, emulsion, or (b) 25 cc. of the starch paste. Each was well agitated before use.The leaves were first immersed in 90 per cent, alcohol to break the surface film, and allowed to become nearly dry. The mixture was then applied by means of a small brush, care being taken to cover the whole of the upper and lower leaf surfaces uniformly. This was found to be the only satisfactory method, as with dipping or spraying the distribution of the mixtures was unreliable. The treated leaves were allowed to dry, then placed in a covered jar with the caterpillars, generally five. Although this number is not large, it was thought to be sufficient to indicate the re...