Toxicity of Fractionated and Degraded Mexican Marigold Floral Extract to Adult Sitophilus zeamais (Coleoptera: Curculionidae)

Abstract

Floral extract of Mexican marigold, Tagetes minuta (L.), was toxic to adult male and female maize weevils, Sitophilus zeamais Motschulsky, under illuminated and dark conditions. The LD50 values ranged from 35 to 47 μg per weevil across sex and photoperiod, whereas LD99 values ranged from 319 to 816 μg per weevil. Fractionation tended to decrease toxicity. Generally, both sexes were similarly susceptible to a given treatment. Although the original extract and certain fractions probably contained photoactive thiophenes, which are toxic to certain larval Lepidoptera, there was no obvious effect of illumination on toxicity for this beetle species. Air exposure for 24 h, in the presence or absence of light, greatly reduced the toxicity of the unfractionated extract. Chromatographic profiles indicated that fractionation procedures had a significant effect on sample composition and on the amount of nonvolatile material. Air exposure for 24 h had a much more dramatic effect, with concomitant 365-nm UVA or incandescent light causing even greater degradation. Degradation under illumination is the end result of the process that causes photoactivation in susceptible species. Apparently, S. zeamais was not adversely affected by the amount of photoactivation that occurred as the extracts or fractions were degrading under our test conditions. These results are discussed in terms of maximizing the control potential of labile material in subsistence agriculture, particularly when purification is required to remove undesired compounds.