CORRELATION BETWEEN HEMOLYMPH ECDYSTEROID TITER, SALIVARY-GLAND DEGENERATION AND OVARIAN DEVELOPMENT IN THE IXODID TICK, AMBLYOMMA-HEBRAEUM KOCH

In the female tick, Amblyomma hebraeum Koch (Acari: Ixodidae), the salivary glands degenerate and vitellogenesis begins, provided the tick exceeds a critical weight (approx. 300-350 mg). The following were measured in ticks: (1) at various times following normal engorgement (1000-3000 mg) and (2) at various stages (weights) of the feeding cycle, 7 days after removal from the host: salivary fluid secretory competence and ecdysteroid content (by radioimmunoassay) of haemolymph and ovary extracts. (1) Haemolymph ecdysteroid titre rose from < 10 ng/ml on the day of engorgement, to 300-400 ng/ml by day 6 after engorgement. Ecdysteroid content of the ovary rose in parallel from less than 10 ng/g wet weight to 1000-2000 ng/g by day 6. (2) In ticks weighing up to 300 mg, there was a progressive rise in fluid secretory competence, and haemolymph ecdysteroid titre remained below 50 ng/ml. In ticks weighing 300-400 mg, haemolymph ecdysteroid titre was approx. 90 ng/ml and salivary fluid secretory competence had declined by 50% relative to ticks weighing 200-300 mg. In ticks weighing 400-500 mg, haemolymph ecdysteroid titre was approx. 140 ng/ml and fluid secretory competence had fallen by over 90% compared to ticks weighing 200-300 mg. Haemolymph ecdysteroid titre continued to rise in larger ticks, peaking at about 800 ng/ml. Ecdysteroids were more concentrated in the ovary than in the haemolymph, peaking at about 3500 ng/g in large engorged ticks, 7 days after removal. The data support the following hypothesis: when the tick exceeds the critical weight, ecdysteroids are released (from an unknown endocrine organ) into the haemolymph, where they trigger salivary gland degeneration. The hormone also accumulates in the ovary where it may play a role in tick development and/or hatching.