Removal of the glandular trichome exudate from leaflets of the wild tomato Lycopersicon hirsutum f. glabratum PI 134417 by swabbing with ethanol resulted in loss of resistance to larvae of the tomato fruitworm (Heliothis zea). An extract containing the exudate and little else was toxic to first-instar fruitworm larvae. Gas chromatographic analysis of extracts containing only the excised tips of the glandular trichomes revealed the presence of 2-tridecanone, a compound toxic to H. zea and other arthropods. The toxin was found to be nearly absent from the leaflet interior, being present in significant quantities only in extracts of leaflet surfaces. H. zea larvae were killed by fumes from the surface extract and from pure 2-tridecanone. The air surrounding PI 134417 leaflets was found to be rich in 2-tridecanone vapors. Fumigant action is a major avenue of their exposure to 2-tridecanone on the foliage of PI 134417. While larvae are quickly immobilized by fumes when placed on resistant leaflets, most recover within 24 hr, casting doubt on 2-tridecanone as the sole defensive compound in PI 134417 against H. zea.
First instarManduca sexta (L.) larvae confined on foliage fromLycopersicon hirsutum f.glabratum (accession PI 134417) plants grown under a long-day regime exhibited greater mortality than larvae on foliage from plants grown under a short-day regime. 2-Tridecanone, a toxin important in the insect resistance of PI 134417, was significantly more abundant in the foliage of plants grown under the long-than the short-day regimes. Light intensity influenced neither 2-tridecanone levels nor the expression of resistance. The density of glandular trichomes, which secrete 2-tridecanone, was influenced by an interaction between day length and light intensity.
Larvae of the cabbage butterfly,Pieris rapae, refuse to feed on the wild mustard,Erysimum cheiranthoides, due to the presence of alcoholextractable deterrents. The active components were extracted inton-BuOH, and this extract was separated into four fractions (I-IV) by reverse-phase HPLC. Fractions III and IV retained the feeding deterrent activity. The activity of fraction III was found to be due to the cardenolide diglycosides 1 and 2, which were previously reported as oviposition deterrents for gravidP. rapae butterflies. Three active compounds were isolated from fraction IV by column chromatography on silica gel followed by reverse-phase HPLC. These compounds were identified as a monoglycoside, digitoxigenin 3-O-β-D-glucoside (4), and two diglycosides, glucodigigulomethyloside (5) and glucodigifucoside (6). An additional cardenolide isolated from fraction II was identified as cheirotoxin (7). All compounds were identified by UV, NMR ((1)H and(13)C), and mass spectrometry, as well as hydrolysis experiments. The feeding deterrent activity of these compounds was compared with that of related commercially available chemicals and other compounds isolated fromE. cheiranthoides.
Toxicity bioassays of the naturally occurring insecticide 2-tridecanone and a group of structural analogs against the tomato fruitworm (Heliothis zea) reveal a significant influence of chain length on toxicity of methyl ketones. This effect may be due to the differential ability of these compounds to penetrate lipid barriers and reach the active site, since a close relationship is seen between toxicity and lipophilicity. Congeners of chain lenths 15 and over are less active than predicted by this relationship, possibly due to steric effects.
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