OBSERVATIONS OF THE RHYTHM OF DETACHMENT OF ENGORGED FEMALE BOOPHILUS-ANNULATUS (ACARI, IXODIDAE) FROM CATTLE

A bimodal, diurnal rhythm of detachment was observed for engorged fema le Boophilus annulatus fed on Hereford heifers confined in individual stalls in an open-sided barn and exposed to ambient climate and light. In 3 experiments during which hosts were fed each morning and evening , a morning period of increased detachment activity occurred during a 6-h period centered around the 2-h collecting interval in which sunris e occurred. A 2nd peak of activity occurred during the 2-h interval th at began 6 - 8 h later. Seventy-four to 84% of the total detachment oc curred during these two 6-h intervals and most of the detachment (44-6 2%) occurred during the afternoon period. These experiments were done in July, September, and January when daylengths were 13 h, 27 min; 12 h, 22 min; and 10 h, 37 min, respectively. In a 4th experiment in whic h groups of hosts were fed at different times of the day, maximal perc entages of 41.5 and 38.3 of the total detachment from morning-fed or b oth morning and evening-fed heifers, respectively, occurred during the 2-h interval in which the sun rose instead of during the evening. Dur ing the evening peak period, the number of engorged females that detac hed from these 2 groups of heifers was approximate to 1/3-1/4 the numb er that detached during the peak period in the morning. In contrast, 4 3.6% of the engorged females in the evening-fed group detached during an 8-h morning period of increased detachment activity that began in t he interval between 0200 and 0400 hours. About the same percentage of the ticks (38.5) detached during a 6-h period that began at 1400 hours . The less precisely delimited pattern of detachment of engorged femal es from cattle fed only in the evening, as contrasted with the rhythm observed for ticks that detached from heifers fed in the morning or bo th morning and evening, indicates that the nutritional or physiologica l state of the host may influence the rhythm exogenously. Whether the morning or evening peak of detachment activity is dominant is a questi on that remains unresolved.