Through field surveys and laboratory rearing observations, we systematically investigated the larval morphological characteristics, biological traits, and occurrence patterns of two pest species in the genus Achaea—A. janata (Linnaeus) and A. serva (Fabricius)—in Guangdong Province. The larval morphological features and key diagnostic characteristics of both species were described in detail. Field studies revealed that these two pests complete 6-8 generations per year in Guangdong, with larvae exhibiting distinct defensive behaviors and causing severe damage to various forest trees and economic crops under high population densities. Additionally, new host plant records are reported for the first time: Excoecaria cochinchinensis, Excoecaria agallocha, and Phyllanthus tenellus for A. janata, and Sinosideroxylon wightianum and Sinosideroxylon pedunculatum for A. serva. The findings provide important insights for the identification and management of Achaea pests in mangrove and subtropical evergreen broad-leaved secondary forest ecosystems.