The two-spotted spider mite, Tetranychus urticae Koch, occurs in two colour forms in greenhouses in the Netherlands: a red form on tomato and a green form on cucumber. The evolutionary status of these strains was analysed by studying genetic differentiation, host plant preference, and mate choice. Males of the tomato strain preferred the female (30 h-old) teleiochrysales from the same strain to those of the cucumber strain, independent of the host plants (tomato, cucumber, bean) on which the teleiochrysales were placed. In contrast, males of the cucumber strain were not selective. In a Y-tube olfactometer, females of the cucumber strain were not responsive to host plant volatiles alone. However, in two-choice disc experiments, where females were exposed to both volatile and contact cues, they settled on cucumber leaves in preference to tomato leaves. Females of the tomato strain preferred the odour of tomato leaves and settled on tomato leaves in preference to cucumber leaves. These experimental results provide the first evidence for (1) host-plant independent mate selection in male spider mites and (2) olfactory discrimination between host plants in female spider mites.

Electrophoretic analysis showed much genetic differentiation at the phosphoglucose isomerase locus. The cucumber strain showed large variation with 5 alleles, whereas the tomato strain was fixed for the most common allele of the cucumber strain.

The results suggest that the two strains represent host races. We hypothesize that the tomato strain has originated from the cucumber strain because (1) tomato represents a more hostile host plant to spider mites (due to toxic compounds and glandular hairs) and (2) the tomato strain is genetically impoverished suggesting that it passed through one or more bottlenecks.