Strawberry resistance to Tetranychus 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.