Significance of medium chainn-alkanes as accompanying compounds in hemipteran defensive secretions: An investigation based on the defensive secretion ofCoridius janus

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Abstract
A mixture of trans -2-hexenal; n -tridecane (60:40, w/w), the natural combination present in the defensive secretion of Coridius janus (Hemiptera; Pentatomidae) was shown by comparison with similar aldehyde mixtures having longer and shorter chain n -alkanes, to be the optimal combination as a fumigant and a repellent against three test insect species, Anoplolepis longipes, Sitotroga cerealella , and Culex quinquefasciatus . Toxic values obtained for the three insects, respectively: 1/LC_50, 1.72, 4.54, and 6.66 ppm while repellencies were 63%, 50%, and 69%. This study revealed that among t -2-hexenal: n -alkane combinations those with medium carbon chains, viz C-11 and C-12, also possessed high toxicities and repellencies comparable to that of the natural combination while those with shorter and longer n -alkanes possessed lower activity. Toxicities and repellencies of n -alkane series were only moderate to low showing highest toxic values for n -tridecane at 1/LC_50, 0.39, 2.32, and 2.32 ppm and repellencies at 31%, 30%, and 32% for the three test insects, respectively. This series, nevertheless, showed similar variation, medium length chains, C-11, C-12, and C-13 showing comparatively higher activity than other alkanes of shorter and longer chains. This study also revealed that the fumigant property of both alkane and aldehyde are of equal importance while repellency is heavily dependent on the aldehyde.