A new member of the unresolved Aristotelia corallina Walsingham species complex (Gelechiidae) from a Costa Rican dry forest (Parque Nacional Palo Verde) is reported. Its larval behavior and possible relationships to the rest of the complex are described and analyzed. Field and laboratory observations revealed that these caterpillars consumed the extrafloral nectar, Beltian bodies, and young foliage of two acacia species (Vachellia collinsii and Vachellia farnesiana; Fabaceae: Mimosoideae). The caterpillars were observed building individual silk webs surrounding a single stem node's thorns and leaf rachises. If open domatia are included in shelters, the caterpillars reside in the domatia between feeding bouts and during pupation. These caterpillars are the only non-myrmecophilous caterpillars known to regularly exploit a host plant's ant rewards. DNA barcoding of the Costa Rican specimens showed that they are significantly different from the United States members of A. corallina (12–13% difference in cox1 sequence). An examination of host plant herbarium material for caterpillar shelters demonstrated likely differences in host plant use, with webs consistent with A. corallina only found on specimens of United States Chamaecrista nictitans (Fabaceae: Caesalpiniodeae), but not Vachellia farnesiana. By contrast, webs were found on V. collinsii, V. cornigera, and V. farnesiana from Central America (including Mexico), but not on Central American C. nictitans. This ecological evidence suggests that the Costa Rican populations of A. corallina may be more closely related to the Mexican populations than to the US populations.