J. G. Rodriguez

Strawberry foliage headspace vapor components at periods of susceptibility and resistance toTetranychus urticae Koch

Headspace components from strawberry foliage have been isolated by nitrogen entrainment and Tenax trapping. Traps were eluted with hexane, and components were analyzed by gas chromatography-mass spectrometry. Fifteen compounds were identified by comparison with authentic standards,trans-2-hexenal, 1-hexanol,trans-2-hexen-1-ol,cis-3-hexen-1-ol, hexyl acetate,cis-3-hexenyl acetate, 6-methyl-5-hepten-2-ol, 1-octanol, 1-octen-3-ol, linalool, α-terpineol, methyl salicylate, ethyl saiicylate, benzyl alcohol, and 2-phenylethanol. The relative amounts of these components were compared at flowering and after fruit harvest when plants were more resistant to the two-spotted spider mite,Tetranychus urticae Koch. The predominant components,cis-3-hexen-1-ol and its acetate, did not change markedly between the sampling periods, but methyl salicylate increased approximately 10-fold after fruit harvest. Methyl salicylate at low concentrations under bioassay conditions did not affect mite behavior. The biosynthetic relationship of this compound to other phenols which have been implicated in plant resistance is discussed.

Strawberry resistance toTetranychus urticae Koch: Effects of flower, fruit, and foliage removal—comparisons of air- vs. nitrogen-entrained volatile compounds

An increase in resistance to the two-spotted spider mite (TSSM),Tetranychus urticae Koch, is observed in field-grown strawberry plants during the period from flowering to postharvest. This seasonal phenomenon was investigated to determine the influence of the metabolic sink, that is, fruiting in the plant. Removal of flowers and fruit and partial removal of foliage did not alter the pattern of resistance of the strawberry plant to TSSM. Bioassays were conducted in concert with chemical analyses. Headspace chemicals emitted from foliage samples were entrained in air and trapped on Tenax, identified, and compared with those entrained in nitrogen and trapped. Terpenes were among the major compounds entrained in air, whereas alcohols were obtained with nitrogen. The air-entrained headspace compounds did not appear to correlate quantitatively with the development of mite resistance in the control plants or those subjected to metabolic sink (flower and fruit) removal. Evidence was obtained for the presence of heretofore unreported strawberry foliage headspace components, namely, (Z)-3-hexenyl 2-meth-ylbutyrate, (Z)-3-hexenyl tiglate, (E)-β-ocimene, (Z)-β-ocimene, α-farnesene, and germacrene D.

Behavior ofTetranychus urticae toward essential oil mixtures from strawberry foliage

A standard essential oil mixture (SEOM) was formulated containing volatile compounds in the relative proportions found in the essential oil of the foliage of “Citation,” a strawberry cultivar relatively resistant toTetranychus urticae. Other mixtures contained varied levels, relative to the SEOM, oftrans-2-hexen-1-ol, nonanal, α-terpineol, and methyl salicylate. The behavior ofT. urticae females in response to these mixtures at several concentrations in propylene glycol was studied in choice tube (preference) tests. Feeding effects were measured by incorporating the mixtures and individual components into sucrose with32P. In the choice tube tests, mites were generally attracted by mixtures at concentrations of 0.1% or below unless the level of methyl salicylate was below 0.5 × that in the SEOM, or the level of nonanal was above that of the SEOM. When mixtures were incorporated into food, methyl-salicylate-stimulated feeding and nonanal levels were inversely related to the amount of food ingested.