Toxicity studies of analogs of 2-tridecanone, a naturally occurring toxicant from a wild tomato.

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.

Chemical constituents of an unacceptable crucifer,Erysimum cheiranthoides, deter feeding byPieris rapae.

The wild cruciferErysimum cheiranthoides was found to contain extractable constituents that deterred feeding by larvae of the crucifer specialistPieris rapae when applied to cabbage leaf disks in both choice and nochoice bioassays. High-performance liquid chromatography was used to separate the extract into several fractions, two of which retained the feeding deterrent activity of the extract. UV-absorption spectra of the fractions suggested that one contained cardenolides similar or identical to those reported to deter oviposition byP. rapae onE. cheiranthoides. The other active fraction evidently contains a compound that deters larval feeding but not adult oviposition. The results suggest that the chemical defense ofE. cheiranthoides depends on two types of compounds acting on separate developmental stages of the insect.

Cardenolides fromErysimum cheiranthoides: Feeding deterrents toPieris rapae larvae.

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-ß-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.

Effect of day length and light intensity on 2-tridecanone levels and resistance inLycopersicon hirsutum f.glabratum toManduca sexta.

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.

Integrative Cloning, Expression, and Stability of the cryIA(c) Gene from Bacillus thuringiensis subsp. kurstaki in a Recombinant Strain of Clavibacter xyli subsp. cynodontis.

A bacterial endophyte was engineered for insecticidal activity against the European corn borer. The cryIA(c) gene from Bacillus thuringiensis subsp. kurstaki was introduced into the chromosome of Clavibacter xyli subsp. cynodontis by using an integrative plasmid vector. The integration vectors pCG740 and pCG741 included the replicon pGEM5Zf(+), which is maintained in Escherichia coli but not in C. xyli subsp. cynodontis; tetM as a marker for selection in C. xyli subsp. cynodontis; and a chromosomal fragment of C. xyli subsp. cynodontis to allow for homologous recombination between the vector and the bacterial chromosome. Insertion of vector DNA into the chromosome was demonstrated by DNA hybridization. Recombinant strains MDR1.583 and MDR1.586 containing the cryIA(c) gene were shown to produce the 133,000-kDa protoxin and several smaller immunoreactive proteins. Both strains were equally toxic to insect larvae in bioassays. Significant insecticidal activity was demonstrated in planta. The cryIA(c) gene and the tetM gene introduced into strain MDR1.586 were shown to be deleted from some cells, thereby giving rise to a noninsecticidal segregant population. In DNA hybridization experiments and insect bioassays, these segregants were indistinguishable from the wild-type strain. Overall, these results demonstrate the plausibility of genetically engineered bacterial endophytes for insect control.