Here is an example that explains plant morphology from the perspective of defense against herbivorous arthropods. Adult female spider mites, which are the dispersal stage, have a strong tendency to walk upwards, and it is difficult to stop this behavior. How do wild plants that protect themselves from pests overcome these challenges? For plants, the inflorescence—its reproductive organ—is most important, followed by the upper leaves that receive ample sunlight, while the older lower leaves are of the least value. As a plant defense strategy, spider mites should be stopped on lower leaves and not allowed to climb up to the inflorescence. The distinguishing feature between the closely related Asteraceae plants Erigeron philadelphicus and E. annuus is that the leaves of the former encircle the stem. Therefore, both plants can be used for natural treatment plots (E. philadelphicus) and control plots (E. annuus) to verify the effect of stem-encircling leaves in suppressing the upward movement of spider mites. When adult female Tetranychus kanzawai were released onto the lower stems of both plants and observed, they settled on one of the upper leaves within 10 minutes. Each spider mites and plant stems were used only once. As a result, over 90% of females (n = 19) on E. philadelphicus were trapped on the underside of the leaf directly above the release point, whereas over half of females (n = 29) on E. annuus moved to the underside of second or third leaves above the release point. Since the leaves of E. philadelphicus that encircle the stem guide females climbing the stem to the underside of the leaves, females seeking shelter from rain or UV radiation appear to settle on the underside of lower leaves rather than returning to the stem via the upper leaf surfaces. In other words, the leaves encircling the stem can be regarded as a sophisticated defense mechanism against spider mites invading upward. Regarding growth potential, as measured by plant height and number of leaves, E. annuus (n = 37) outperformed E. philadelphicus (n = 35); however, regarding defensive capacity, as measured by spider mite fecundity, E. philadelphicus (n = 31) outperformed E. annuus (n = 26), suggesting a trade-off between growth and defense in both plants. That is, the perennial E. philadelphicus, which is more easily detected by herbivorous pests, seemed to be more heavily defended than the annual E. annuus.