The effects of beta-ecdysone and ponasterone a on the soft tick ornithodoros porcinus porcinus walton,1962
Mango, Christine K A
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The lack of basic information in the field of tick endocrinology led to this investigation on the effects of an insect moulting hormone, beta-ecdysone, and a phytoecdysone, ponasterone A, on adult and nymphal O. p. porcinus ticks. Ticks were bred at 2SoC with a relative humidity of 75 to SO% on four laboratory hosts : rabbit, chicken, rat and guinea pig. In vitro breeding techniques were also used to study bovine and porcine defibrinated blood as possible substrates for the mass rearing of ticks. The suitability of a host or substrate was assessed by the feeding performance of the ticks, their reproductive efficiency, the percent hatch of eggs and subsequent moulting success and rate of adult maturation. Observations indicated that rabbit- and chicken-fed ticks laid more eggs per unit weight of bloodmeal and t~e resulting eggs had better hatching rates than rat- and guinea pig-fed ticks. The moulting success and rates of development and adult maturation were also better among rabbit- and chicken-fed ticks than rat- or guinea pig-fed ticks. Porcine blood wa3 an excellent substrate for the -, '. promotion of high levels of eggs output while bovine blood gave poor results. In some respects, rabbit blood was a superior substrate in comparision to porcin~. blood but rabbit blood haemolyses easily and it is not suitable because of resulting tick deaths. Porcine blood was greatly superior to bovine blood in most parameters of tick feeding and development. Topical application of beta-ecdysone (1 to 5~g/tick) to adult O. p. porcinus gave higher mortality among males than females, while both males and females supermoulted. Topical application of beta-ecdysone and ponasterone A to 2nd through 5th nymphal instar ticks, the same day or 4 days post-feeding, produced higher mortality among treated nymphs but the premoulting period was also shortened among nymphs treated 4 days post-feeding. There were differences in the rates of moult acceleration among the surviving nymphs with different ecdysteroid doses and in some cases the dose responses were linear. Ticks fed on b Lcodme aI wi th added ecdys teroids at 1 to 8f..lg/molf blood showed low (5.2%) and high (41.3%)mo rtali. ty responses among female and male ticks, respectively. Among ponasterone A treated ticks, a high frequency of mortality ranging from 3.2 to 90%, was observed among both male and female ticks. Supermoulting among beta-ecdysone fed ticks occurred at higher (74%) and lower (19.3%) rates among females and males,respectively. Ponasterone A fed ticks also showed higher (55.8%) rates among females than among males (21.6%). Some betaecdysone fed females underwent a second supermoult on the ecdysone from the first bloodmeal and the dose response curve for this response was linear. Among nymphal ins tars, feeding on ecdysteroids caused moult acceleration,the rate of which depended on the nymphal stage. Older (and larger) nymphal stages experienced better moult acceleration than the younger (and smaller) ones. Age did not affect supermoulting potential ~n o. p. porcinus. Both A. persicus and O. tholozani showeda very slight supermoulting response when fed on bloodmeal with added e cdys te ro i ds . Moulting hormone equi,v'a.lent titre changes (determined by MUsca bioassay) in the haemolymph o.f normally-fed 5th instar nymphs occurred in a pattern with 2 'peaks; the lower 1st peak appeared on day 4 and the higher 2nd peak "appeared on day 9, jus t before moulting. Ecdysone-fed 5th ins tar nymphs showed an early titre peak on day one followed by a normal pattern with the 2 basic peaks. Normally-fed and mated females showed a low titre peak on the 7th day post-feeding which coincided with the onset of vitellogenesis. In ecdysone-fed unmated females, the 2 basic peaks of the nymphal moulting pattern were observed; an early lower moulting peak appeared on day 5 and a 2nd higher peak appeared on day la, preceding supermoulting. In the ecdysone- fed mated females, the 2 basic titre peaks of the nymphal moulting pattern·also occurred. Comparisons between the equivalent moulting hormone titre changes in normally-fed 5th ins tar nymphs and ecdysone-fed fern~les were essentially the same, both in terms of timing and in the magni tude of the changes. 3 The biology of the supermoulted ticks revealed that while the supermoulted ticks grew larger with subsequent ecdysone-meals, their conversion of normal bloodmeal into eggs was less effective compared to that of normal females. Their total oxygen consumption was also comparable to that of normal females during the period of post-bloodmeal digestion and oocyte maturation. Anatomical and histological comparison~revealed significant differences between the adjusted mean size indices (in terms of unfed weight) of normal and ecdysteroid treated females. Thus supermoulted females weigh less than would be expected for their size. These supermoulted ticks also had enlarged salivary glands and. the salivary glands of twice supermoulted females were proportionally larger than those of normal or once supermoulted females. Cuticular patterns of normal and supermoulted adult females also showed differences; the number of mammillae per optic field became fewer as they g rew larger and became more prominent wi th each subsequent moul t . Supermoulted females also showed accumulation of salt crystals around their mouth parts which may imply that faulty water balance mechanisms contribute to their greater mortality in comparison -,.: . to normal females. From the foregoing observations it was concluded that : 1. For ~n vivo mass rearing, rabbits and chickens are better laboratory hosts than are rats and guinea pigs. The female ticks fed on rabbits and chickens can be expected to have higher levels of egg production. Nymphal ticks fed on rabbit and chicken hosts also show low levels of mortality coupled wi th rapid moulting and adul t development. 2. Defibrinated porcine blood is an excellent substrate for the promotion of good in vitro feeding and high levels of egg production. Nymphs fed in vitro on this substrate had good develQP~ental rates with early adult maturation. These positive effects of defibrinated pig blood make the in iri tvo breeding technique the method of choice for colony rear1ng and maintenance. 3. Adult male O. p. porc1-nus show significantly higher mortality responses to topically applied bet a+e cdys one (at any particular dose level) than do adult females. Both sexes show low supermoulting responses to topical, ecdysteroids with males responding at a slightly higher rate. 4. Topical application of ecdysteroids to nymphs will produce high mortality responses with the sensitivity of nymphs increasing significantly from day I to day 4 post-feeding. Since mortality was always less than 80%, other ecdysteroids should be tested at a range of practical dose levels. As a potentially successful growth regulator, any ecdysteroid must cause more than 90% mortality before it could be tested for potential in the field. 5. The moulting acceleration which was observed among the surviving nymphs is undesirable in terms of tick control. 6. At lower doses(lto4~g/ml), ingestion of ecdysteroids will cause supermoulting and low mortality among beta-ecdys~ne and po nas terone A fed ticks. Higher do seslf to l~g/ml) cause mortality of up to 90% among both males and females. Ecdy- -",:. sene-fed ticks may sllpermoult as many as 3 times. Ornithodoros p. porcinus'adults do not lose their supermou lti.ng potential with Lnc reasi ng age and they can also respond to many other ecdysteroids. 7. Ingestion of beta-ecdysone or ponasterone A causes moderate to high mortality among nymphal instars. Because of the absencq of a generally linear dose-response curve it is not now feasible to test either of these ecdysteroids as a systemic growth regulator for ticks. Ingestion of these ecdys teroi.d s al so causes nymphal moul t acce lera tion among the survivors. 8. <There are 2 basic pe~~s of moulting hormone activity 1n the haemolymph of fed nymphs and supermoulting adults. The two peaks appear to be related to the moulting process. In normally-fed and mated females a peak of moulting hormone 5 activity appears on day 7 and this seems to be related to egg maturation. Therefore ticks show titre changes in association with both moulting and reproductive cycles and may have endocrine patterns and mechanisms similar to those found in insects. 9. Supermoulted female O. p. porci.nus take larger bloodmeals and lay more eggs than normal females. On a comparative basis, however, their conversion of bloodmeal into eggs is less efficient and they have shorter adult lives. The shortening of adult life seems to be related to physiological defects 1n their water balance mechanisms. Accordingly, the total egg production of supermoulted females might not be greater than that of the longer-lived and more efficient normal females. 10. There may be some potential for the development of an analogue to the natural tick ecdysone(s) which could be used for tick control by systemic administration through livestock hosts. Any such analogue would have to persist at' relatively high levels in the host circulation and should not contribute to th ec.s upe rmou Lti.ng of adults or moult acceleration of nymphs.