In r-selected species such as insects, early immature stages often exhibit high mortality due to the vulnerability to predation pressure, thus the acquisition of effective defense systems is important for increasing viability. Food-for-protection mutualism is effective for avoiding natural enemies, but costs of nutritional rewards render immatures difficult to establish the mutualism. Many species of lycaenid butterflies rely on ants to protect from natural enemies in their immature stages. However, in the species showing facultative mutualism, early instars cannot always attract ants because of their insufficient development of nectary organs. In this study, we focused on coexistence of conspecific larvae in a facultative myrmecophilous lycaenid, Arhopala japonica, and tested whether the coexistence produces extra attractiveness to attending ants without extra nectary production. Field surveys revealed that the coexisting condition significantly increased the percentages of ant attendance in first and second instars, although these instars showed significantly lower attendance rates than other instars when they were alone. Laboratory experiments confirmed that a pair of coexisting second instars attract ants similarly to a third instar larva without extra nectary secretions or nectary-organ development. These results suggest that the coexistence of conspecific larvae in early immature stages compensates the insufficient attractiveness to partners.