Biochemistry of the exocrine secretion from gypsy moth caterpillars (Lepidoptera: Lymantriidae)

Gypsy moth, Lymantria dispar (L.), caterpillars have unpaired dorsal abdominal glands on the 6th and the 7th segments, and pairs of smaller glands on the Ist to 4th abdominal segments. Normally, material from these glands becomes sticky and is regularly dispersed onto setae by the caterpillars, but if larvae are held at saturated humidity for 3-4 d, droplets accumulate on the glands and remain fluid. The secretion is an aqueous mixture of low molecular weight molecules including the biogenic amine, gamma-aminobutyric acid, short-chain hydroxy acids (e.g., alpha-hydroxyisobutyric acid), and Krebs cycle acids (e.g., isocitric acid), plus higher molecular weight compounds (equal to or greater than 30,000 MW). 2-Isobutyl-3-methoxypyrazine, considered to be a warning odor in many aposematic insects, also occurs in the secretion in minute amounts (equal to or less than 1 picogram per larva), yet is mainly responsible for the exudate odor which is detectable from individual caterpillars. Natural secretion from L. dispar larvae was a feeding deterrent to foraging fire ants, Solenopsis geminata (F.), in a laboratory bioassay. All Lymantriidae have dorsal abdominal glands; therefore, it is likely that secretion from these glands contributes to the irritating and allergenic properties associated with setae from tussock moth caterpillars.