(7) F, G. Tromba, Biological Control of Helminthic 
Diseases, Vet, Med, 60, 69 (1965), 
(8) D. T. Clark and C, K, Smith, Gnotobiotics in 
Animal Parasite Research, Devlpmt, in Indus, 
Microbiol, 4, 187 (1963), 
J. P. Doll and CC,’ K, Franker, Experimental 
Histomoniasis in Gnotobiotic Turkeys, I, In- 
fection and Histopathology of the Bacteria- 
Free Host, Jour, Parasitol: 49, 411 (1963), 
C, K, Franker and J, P,. Doll, Experimental 
Histomoniasis in Gnotobiotic Turkeys, II, Ef- 
fects of Some Cecal Bacteria on Pathogenesis, 
Jour, Parasitol, 50, 636 (1964), 
Personal observations, unpublished, 
P, W. Brian, The Ecological Significance of 
Antibiotic Production, Microbial Ecol, 168 
(1957), 
I, W. Parnell and H, McL, Gordon, Predaceous 
Fungi: A Possible Method of Biological Control 
of Parasitic Nematodes, Jour, Heliminthol, 37, 
339 (1963), 
D, Pramer, Nematode-Trapping Fungi, Science 
144, 382 (1964), 
T. B, Stewart andH, J, Godwin, Cuticular Lesions 
of Ascaris suum Caused by Pseudomonas sp, 
Jour, Parasitol, 49, 231 (1963), 
J. Robinson, Pilobolus spp, and the Translation 
of the Infective Larvae of Dictyocaulus vivi- 
parus from Faeces of Pastures, Nature 199, 
353 (1962), 
W. E. Bizell and H, Ciordia, The Dissemination 
of Infective Larvae of Trichostrongylid Para- 
sites of Ruminants from Feces to Pasture by 
the Fungus Pilobolus spp, Jour, Parasitol, [In 
press, ] Fag 
F, F, Ferguson and J, R, Palmer, Biological 
Notes on Marisa cornuarietis, a Predator of 
Australorbis glabratus, the Snail Intermediate 
Host of Schistosomiasis in Puerto Rico, Amer, 
Jour, Trop, Med, and Hyg, 7, 640 (1958), 
J. Oliver-Gonzalez and F,F, Ferguson, Probable 
Biological Control of Schistosomiasis mansoni 
in a Puerto Rican Watershed, Amer, Jour, 
Trop, Med, and Hyg, 8, 56 (1959), 
(9) 
(10) 
(11) 
(12) 
(13) 
(14) 
(15) 

(16) 

(17) 
(18) 
(19) 
(20) C, O, Berg. New Approach to Control of Flukes, 
Farm Res, 25, 8 (1959), 
M, M, Farr, Survival of the Protozoan Parasite 
Histomonas meleagridis, in the Feces of Inf- 
fected Birds, Cornell Vet, 46, 178 (1956), 
Further Observations on Survival of 
the Protozoan Parasite, Histomonas mele- 
agridis, and Eggs of Poultry Nematodes in 
Feces of Infected Birds, Cornell Vet, 51, 
3 (1961), 
Personal observations, unpublished, 
E, E,. Lund, War on Blackhead Still to Be Won, 
Turkey World 33, 12 (1958), 
E, E, Lund, E, E, Wehr, and D, J. Ellis, Role of 
Earthworms in Transmission of Heterakis and 
Histomonas to Turkeys and Chickens, Jour, 
Parasitol, 49, (sup.) 50 (1963), 
E, E, Lund, Earthworm Transmission of His- 
tomoniasis in Poultry, Presented at Ist Inter- 
natl, Cong, Parasitol, Rome, 1964, 
H. M, DeVolt and C, R, Davis, Blackhead (In- 
fectious Enterohepatitis) in Turkeys, with Notes 
on Other Intestinal Protozoa, Univ, Md, Agr. 
Expt, Sta, Bul, 392, 516 (1936), 
J. F. Frank, A Note on the Experimental Trans- 
mission of Enterohepatitis of Turkeys by Ar- 
thropods, Canad, Jour, Compar, Med, 17, 230 
(1953), 
Personal observations, unpublished, 
E, E, Tyzzer, Studies on Histomoniasis, or 
*'Blackhead'"' Infection, in the Chicken and the 
Turkey, Amer, Acad, Arts and Sci, Proc, 69, 
189 (1934), 
E. -E. Wehr and L, G, Olivier, Limitations of 
Phenothiazine in the Control of Cecal Worms 
and Blackhead Disease of Turkeys, Poultry Sci, 
25, 199 (1946), 
E, E, Lund, Blackhead of Turkeys and Chickens, 
How To Control It, U.S, Dept. Agr, Leaflet 
404 (1956), 
Properly constructed stone yards have remained 
sufficiently free of earthworms for several 
years, so no clinical histomoniasis appeared in 
turkeys raised thereon, 
(21) 
(22) 
(23) 
(24) 
(25) 
(26) 
(27) 
(28) 
(29) 
(30) 
(31) 
(32) 
(33) 
PEST CONTROL BY GENETIC MEANS 
Martin G, Weiss, Assistant to Deputy Administrator, Agricultural Research Service, 
U.S, Department of Agriculture, Washington, D.C, 
Previously in this symposium we considered 
pest control by biological means. Pest control 
by genetic means is in actuality also aform of 
biological control. In this form of control the 
characteristics of the host that condition re- 
sistance to a pest are heritable. The virulence 
116 
of a parasitic organism or its ability to attack 
a host, however, also is under genetic control. 
Pest control by genetic means therefore must 
involve determination of the interaction between 
the genetic systems of the host and the parasite, 
commonly called host-parasite interactions. 

