Salivary gland degeneration and ovary development in the Rocky Mountain wood tick, Dermacentor andersoni Stiles (Acari: Ixodidae). II. Determination of the ‘critical weight’

The feeding cycle of female ixodid ticks is divided into preparatory, slow, and rapid feeding phases. When a female Amblyomma hebraeum is removed from the host after achieving a minimum size of about 10–13× the unfed weight, various physiological and behavioural changes occur: (a) haemolymph ecdysteroid concentration rises dramatically, (b) the tick does not reattach to the host when given the opportunity, (c) its salivary glands degenerate within about 4 days (if mated) or 8 days (if virgin), and (d) oocyte maturation and oviposition occur (Kaufman and Lomas, 1996; Invert. Repr. Devel. 30: 191–198). None of these changes occur if the tick is removed from the host at smaller sizes. This transition, which occurs when the tick enters the rapid phase of engorgement, has been named the ‘critical weight’. To date, the critical weight has been determined for A. hebraeum only. The present study established that, in both mated and virgin D. andersoni, the critical weight is similar to that of A. hebraeum. Although a small percentage of virgin A. hebraeum do exceed the critical weight, achieving perhaps 20× the unfed weight, virgin D. andersoni regularly fed well beyond their CW (>50× the unfed weight) and occasionally engorged completely (100× the unfed weight), although they did not detach spontaneously from the host within 21 days of attachment.