266 Journal of Nematology, Volume 13, No. 3, july 1981 8. Klein, M. G., W. R. Nickle. P. R. Benedict, logical control of insects. Boca Raton, Florida: and D. M. Dunbar. 1976. Psammomermis sp. C.R.C. Press. (Nematoda: Mermithidae): A new nematode para- 14. Poinar, G. O., Jr., and G. G. Gyrisco. 1962. site of the Japanese beetle, Popillia japonica Studies on the bionomics of Hexamermis arvalis (Coleoptera: Scarabaeidae). Proc. Helm. Soc. Wash. Poinar & Gyrisco, a mermitbid parasite of the 43:235-236. alfalfa weevil, Hypera postica (Gyllenhal). J. Insect 9. Nickle, W. R. 1974. Nematode infections. Pp. Path. 4:469-483. 327-376 in G. E. Cantwe]l, ed. Insect diseases. New 15. I'oinar, G, 0., Jr., and H. E, Welch. 1968. A York: Marcel Dekker. new nematode, Filipjevimermis leipsandra sp. n. 10. Nickle, W. R. 1978. On the biology and life (Mermithidae), parasitic in Chyrsomelid larvae history of some terrestrial mermithids parasitic on (Coleoptera). J. Invert. Pathol. 12:259-262. agricultural pest insects. J. Nematol. 10:295 (Abstr.). 16. Polozhentsev, P. A. 1952. New Mermithidae 11. Nickle, W. R., and P. Grijpma, 1974. Studies of sandy soil of pine forests. Trudy Helmintb. Lab. on the shootborer, Hypsipyla grandella (Zeller) 6:376-~82. (Lep.. Pyralidae) XXV. Hexamermis albicans (Sie- 17. Wheeler, W. M. 1928. Mermis parasitism and bold) (Nematoda: Mermithidae) a parasite of the intercastes among ants. J. Exp. Zool. 50:165-237. larva. Turrialba 24:222-226. 18. Welch, H. E. 1963. Nematode infections. Pp. 12. Poinar, G. O., Jr. 1975. Entomogenous 363-392 in E. A, Steinhaus, ed. Insect pathology. nematodes. Leiden: E. J. Brill. New York: Academic Press. 13. Poinar, G. O., Jr. 1979. Nematodes for hip- Mermithid Nematodes: Physiological Relationships with their Insect Hosts 1 Roger Gordon ~ Abstract: This paper assesses our state of knowledge of physiological processes involved in the relationships between insects and their mermithid nematode parasites. Three major components of the host-parasite relationship(s) are reviewed: effects of mermithids on host physiology, effects of host physiology on mermithids, and the physiology of the nematodes themselves. Mermithids induce an array of changes in host physiology, and the effects on host metabolism and endocri- nology are discussed at some length. Few studies have been done to ascertain the effects of the host on the parasites from a physiological standpoint. Whereas host immunity mechanisms against mermithids have been described at the ultrastructural level, the physiological basis of such responses is not known. Mermithids are atypical nematodes, both structurally and physio- logically. In the absence of a functional gut, nutrients are absorbed across the outer cuticle and stored in a trophosome. The transcuticular mode of feeding, storage within the trophosome, and metabolism of storage products are discussed. The usefulness of physiological information toward expediting in vitro culture of these nematodes is discussed, and problems that need to be ad- dressed are defined. Key words: cuticle, fat body, Filipjevimermis leipsandra, Gastromermis boophthorae, hemolytnph, immunity, Mermis nigrescens, mermithid nematode, Neomesomermis flumenalis, Romanomermis culicivorax, trophosome. Mermithid nematodes (Enoplida: Mer- sterile. The potential of mermithids for mithidae) have considerable potential for insect biocontrol, however, has yet to be biocontrol of insect pests. The insect host realized. Only one species of mermithid, the invariably dies when the juvenile nematode mosquito parasite Romanomerrnis culici- completes its parasitic development and vorax, has been mass cultivated on the scale exits from the host's hemocoel (the para- needed to permit field trials and for this an site's microenvironment). When adult fe- in vivo procedure was used (29). Several male insects are infected, they are rendered authorities (28,46) have advocated the es- tablishment of in vitro cultivation pro- Received for publication 9 February 1981. cedures because they are potentially 1Symposium paper presented at the annual meeting of the Society of Nematologists, New Orleans, Louisiana, Au- cheaper, more efficient, and easier to main- gust 1980. tain than in vivo methods. However, at- 2Department of Biology, Memorial University of New- foundland, St. John's, Newfoundland, Canada AIB 3X9. tempts to culture mermithids in vitro (8) Continuing financial support from the Natural Sciences /lave met with limited success. and Engineering Research Council of Canada (Grant No. A6679) is gratefully acknowledged. The paucity of information about the Mermithids--Physiological Relationships: Gordon 267 physiological interrelationships between in- tological examinations of chironomids: sects and mernaithids has hampered progress "Very little or no food is abstracted from of in vitro culture of mermithids. Mer- the haemolymph of the host, for in the mithids are atypical nematodes. They lack a parasitized forms there is always an increase, functional gut and absorb nutrients from instead of a decrease, in the lipoid content the host's hemolymph through their outer of the blood." Rempel's contention that the cuticle. The parasitic juveniles store these hemolymph does not constitute the primary nutrients in a storage organ (trophosome) source of nutrients for the developing mer- that almost completely fills the pseu- mithid has since been shown to be incorrect. docoelom. Storage reserves are utilized by Whether or not blood lipid levels really the free-living stages which feed only min- were elevated cannot be ascertained from imally or not at all. Such adaptations sug- his study, but such art increase would not gest that mermithids are deeply committed discount the hemolymph as a nutrient to parasitism and rely heavily upon their source. host to provide nutritional requirements Among mermithids, as among nema- and, in all probability, stimuli for growth todes in general, there is considerable uni- and development. For in vitro mermithid formity in morphology and general pattern culture to be effective, these nutrients and of life cycle. It would seem reasonable to stimuli have to be identified and incorpo- suggest that information obtained concern- rated into the culture media. ing the physiology of one species would, in Severe overt effects of parasitism indicate principle, be applicable to the family as a the degree to which the physiology of the whole. The same conclusions may not be host is affected. Mermithids cause either drawn, however, when considering the phys- gross degeneration or suppress the develop- iology of hosts that are so distantly related ment of a number of host tissues (9) which as orthoptera and diptera. In the ensuing in the normal insect fulfill vital metabolic discussion, therefore, effects of mermithid roles. Recent studies by Wfilker (49,50) infection on the two types of host are con- have significantly advanced our understand- sidered separately. ing of the cytological changes that occur in M. nigrescens in locusts: Jutsum and the imaginal discs and gonads of chiron- Goldsworthy (22) reported that relatively omids (Diptera) as a result of mermithid low infections (ca. three worms per host) of parasitism. Mermithids induce a greater M. nigrescens caused pronounced reduc- variety of developmental alterations of their tions in the levels of proteins within the hosts than do other entomogenous nema- blood and fat body of adult male Locusta todes. They prevent molting and, in some migratoria. instances, induce intersexuality (19,49), The physiological condition of the host gynandromorphism (19), or formation of influences the degree to which hemolymph intercastes (26). and tissue metabolite levels are affected by In 1927 Cobb et al. (4), intrigued by the infection. Electropherograms of hemolymph discovery that the sex of mermithids is de- taken from adult female Schistocerca termined by factors inherent in the level of gregaria 3 wk after infection with heavy parasitism, called for physiological studies doses of M. nigrescens (40-50 worms per on the host-parasite relationships. However, host) showed that almost all of the protein little attention was paid to such physiolog- fractions were depleted through parasitism ical considerations until recently. Two para- (12). These locusts had been infected soon sites have constituted the primary focus for after their imaginal molt, and the parasite physiological studies: Mermis nigrescens, prevented vitellogenesis. But in a study parasitic in grasshoppers and locusts, and where infection was delayed until vitello- R. culicivorax, parasitic in larval mos- genesis commenced, the mermithid did not quitoes. affect overall hemolymph protein levels (9). MERMITHIDS The initiation of infection was delayed ON HOST PHYSIOLOGY in these experiments, so the parasitized host commenced vitellogenesis, but the yolk was In 1940 Rempel (36) surmised from his- subsequently resorbed into the hemolymph. 268 Journal of Nematology, Volume 13, No. 3, July 1981 Such a resorption of vitellogenic proteins cause the fat body tissue would be deprived likely compensated [or the reduction in of amino acid precursors. Though the mech- hemolymph (protein) levels that would anisms involved are matters for speculation, otherwise have occurred. The mermithid there would appear to be sufficient evidence was found to cause a significant depletion to support the proposal that M. nigrescens of hemolymph proteins in fifth-instar S. acquires amino acids by inhibiting protein gregaria (infected as fourth