Mechanisms of Location of Amblyomma variegatum (Fabricius) and other Livestock Ticks by the Parasitoid Ixodiphagus hookeri Howard

Abstract

The control of Amblyomma variegatum and other economically important ticks, like Rhipicephalus appendicu/atus, has relied mainly on the use of acaricides worldwide. However, due to the drawbacks of chemical control, future tick control will have to move more in the direction of integrated approaches emphasizing more regular use of environmentally-friendly non-chemical methods, such as tick parasitoids. lxodiphagus hookeri, one of the seven species of tick parasitoids known, occurs naturally in Kenya and is specific to A. variegatum ticks . This study investigated the host location cues used by the parasitoids, visual evaluation and recognition of hosts, acceptance of A. variegatum and R. appendiculatus nymphs by the parasitoids for oviposition as well as the suitability of the two tick species for the development of parasitoid immatures. Observations were also made on the host seeking and dispersal behaviour of the parasitoids both in the laboratory and the field. Experiments carried out in a Y-tube olfactometer showed that /. hookeri females were not attracted to grass odours (Cynodon dactylon, Eragrostis superba & Digitaria seriata). Cattle waste odours (urine and dung) and odours from tick-free dewlaps and heels were attractive to the parasitoids. Odours from tick-free cattle ears and scrota were not attractive. Off-host A. variegatum nymphs did not attract the parasitoids, while feeding nymphs were attractive. Experiments done in a T-tube olfactometer showed that hexane washes and faeces of A. variegatum nymphs were attractive to the parasitoids. Gas chromatographic analyses of A. variegatum and R. appendiculatus washes and faecal extracts showed that there were differences in the chemical composition of volatiles emitted by the two tick species. Petri-dish/vial bioassays revealed that the parasitoids recognized and evaluated the sizes, feeding status and species of hosts visually before contact. Movement and colour were not important in visual host recognition. Glass vial experiments showed that the parasitoids oviposited more in host nymphs than in non-host nymphs. The conditioning of R. appendiculatus nymphs with A. variegatum nymphal odours and integument prolonged the time these nymphs were attacked by the parasitoids, however, oviposition was rare. Scanning electron microscopy revealed differences in the appearance of the integument surfaces of the two tick species. All R. appendiculatus and unfed A. variegatum nymphs attacked were not suitable for the development of /. hookeri immatures while fed A. variegatum nymphs attacked yielded progeny. /. hookeri females spent less time ovipositing in parasitized than in unparasitized hosts. Dermal gland secretions from mechanically disturbed fed A. variegatum nymphs had a short-lived repellent effect on the parasitoids. In the field, parasitoids searched longer on cattle heels for hosts compared to other parts of the legs. A. variegatum nymphs collected from cattle heels were more parasitized than nymphs collected from the other body regions. In the field and wind tunnel, the parasitoids dispersed through crawling, jumping and flight. Jumping was more common while flight was rare. Release distances did not significantly influence the numbers of parasitoids reaching the odour sources.