The relationship between ‘critical weight’ and 20-hydroxyecdysone in the female ixodid tick, Amblyomma hebraeum

The feeding cycle of adult female ticks (Acari: Ixodidae) is divided into preparatory, slow and rapid feeding phases. At the transition from slow to rapid feeding, Amblyomma hebraeum (Koch) females reach a ‘critical weight’ (CW; approx. 10× the unfed weight) that is characterized by several behavioural and physiological changes. Five of these changes were used as criteria to establish a more precise estimate of CW than we have to date. The CW as defined by re-attachment to the host was 9× the unfed weight, while for haemolymph ecdysteroid titre, salivary gland degeneration, ovary weight, oocyte length and oocyte vitellin content the CW was 10×, 10×, 12×, 12× and 13×, respectively. CW thus varies depending on the parameter measured. Although previous studies have established the influence of ecdysteroids on salivary gland degeneration and vitellogenesis, here we demonstrate a further effect of ecdysteroids: inhibiting re-attachment to the host.