90 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 9. — P. acanthus infections developing in T. haemas- 

 toma exposed to freshly hatched miracidia in 4-inch 

 iowls for 24 hours 



DISCUSSION AND CONCLUSION 



Heavy natural Parorchis acanthus infections 

 destroy the gonad of Thais haem/isfoma thereby 

 reducing its reproductive potential. This method 

 was thought to offer a possible means of control- 

 ling the size of drill populations; however, in all 

 wild drill populations sampled in the present 

 study, the infection rate was low. Incidence of 

 infection was also low in gulls, definitive hosts of 

 the parasite. Since the two infection rates are 

 closely related, it would appear that an increase in 

 the number of infected gidls in a given locality 

 would increase the drill infection rate and thereby 

 tend to obtain a measure of control over the drill 

 population-size. This is more difficult to attain 

 than is at first apparent. 



Given a good definitive host-species as a source 

 of supply, successful dissemination of the parasite 

 among members of a wild drill population de- 

 pends, in part, on the numters of miracidia re- 

 leased in the vicinity and, in part, on the numbers 

 of miracidia actually infecting drills. The for- 

 mer depends on the intensity and rate of infection 

 in local gulls, the latter on opportunity of the mira- 

 cidia to contact drills and on susceptibility of the 

 drills to infection. 



The generally low intensity attained in experi- 

 mentally induced gull infections, despite admin- 

 istration of large doses of cysts, suggests that very 

 large numbers of gidls would have to be infected 

 and released neai'by in order to pi"ovide a signifi- 

 cant increase in the number of miracidia available 

 to infect drills of a given population. This con- 

 stitutes, in my opinion, a serious obstacle to suc- 

 cessful employment of P. acanthu» in drill conti'ol. 



Some evidence suggests that, in a given locality, 

 the pattern of distribution of naturally infected 

 drills may be correlated with the concentration of 

 gulls on feeding grounds. Thus, drills not living 

 on or near the feeding grounds would be less likely 

 to become infected. This may be a factor con- 

 tributing to the maintenance of the low natural 

 infection rates observed in Gulf coast localities 

 and would have to be dealt with in order to in- 

 crease infection rates and reduce the size of drill 

 populations. 



Some laboratory experiments suggest that 

 susceptibility of uninfected drills to P. acanthus is 

 considerably higher than natural rates would lead 

 one to suspect. If true, this would indicate that 

 an as yet unrealized capacity for higher infection 

 rates probably exists in wild drill populations. 

 This capacity cannot be realized in a given drill 

 population until larger numbers of miracidia are 

 available in its vicinity. 



The present study has fomid no effective means 

 of spreading the parasite in increased numbers. 

 With the available avian hosts and techniques, 

 there is no evidence that infection rates can be sig- 

 nificantly increased in wild drill populations. 

 Therefore, the conclusion is inescapable that, using 

 these methods, P. acanthii,s cannot l)e employed as 

 an effective biological control of the drill, Thais 

 haemastoma. 



