Our adult oviposition preference tests further reveal the narrow
host range of the two leaf-rollers. In these tests, both insects
strongly preferred to oviposition R. ellipticus over other plant species,
suggesting high specificity to R. ellipticus. Though E. ustulana
also laid eggs on R. coreanus var. tomentosus, R. irenaeus and
R. mesogaeus, the numbers of eggs were nearly 20 times lower than
those on R. ellipticus. Similarly, the numbers of eggs of E. tetragonana
on R. ellipticus were 10–40 times higher than those on other
plants. Because the larvae and pupae live inside the leaf rolls, adult
oviposition is the key step for host use (Thompson and Pellmyr,
1991). Since our tests were conducted in cages, space limitation
might restrict adult host selection. Therefore, adults may use even
less plant species for oviposition under natural conditions. Our
field surveys support this view, as we found only R. ellipticus was
attacked by E. tetragonana and E. ustulana and no leaf-rolls by these
two insects were found on other species.