Applications of the mermithid nematode, Romanomermis culicivorax Ross and Smith, 1976, for mosquito control in Manitoba and taxonomic investigations in the genus Romanomermis Coman, 1961
Successful invasion by the mermithid Romanomeris culivorax declined linearly from 93.6 to 1.5% in Culex tasalis and from 73.1 to 1.6% in Aedes dorsalis larvae exposed in the laboratory at 18, 16, 14, 12 and 10°C for 48 hours. Larvae of C. tarsalis were more susceptible than those of A. dorsalis at 18 and 16°C, but this relationship was reversed at 12°C. Larval mortality during the 48 hr exposure period was due primarily to nematode infection. Photoperiod had no apparent effect on infection. Low temperature was the primary factor limiting infection by R. culicivorax in mosquito larvae which develop in snow-melt pools. About 20% of A. canadensis and A. pionips larvae were infected at 5-10°C in pools treated with 50,000 preparasites per m². Little or no infection occurred in A. dorsalis, A. spencerii or A. communis larvae at temperatures below 10°C. R. culicivorax survived the winter in a natural mosquito habitat in Manitoba. Mosquito larvae were prematurely killed by high levels of multiple infection. Eight to 20% of A. vexans larvae which hatched after an early spring rain were infected when a pool was treated with 50,000 preparasites per m². Melanized mematodes were discovered in A. communis, A. canadensis and A. vexans larvae. No infection of Mochlonyx velutinus and Dixa sp. larvae or Paraleptophlebia (?) sp. nymphs was observed. Romanomeris culicivorax cannot be recommended at this time for the control of spring Aedes mosquitoes in Manitoba because of the low levels of infection, the number of applications and the high number of preparasites required to obtain infection. Mermithid parasites were obtained from stock cultures at 27 ± 0.5°C 3 weeks after post parasitic juveniles of R. culicivorax were placed in moist sand. Peak preparasite production occurred between 8 and 14 weeks, and then declined rapidly. After 32 weeks, only a small number of preparasites could be obtained from samples. When a 10 wk old culture was kept at 15°C, the number of preparasites obtained from samples diminshed progressively over a 7 wk period. Pre-treatment levels of preparasites were obtained 6 week after the culture was returned to 27°C. Parasitism by R. culicivorax caused symptoms characteristic of starvation in A. vexans larvae both in the laboratory and in the field. Infected larvae were reduced in size throughout their development and moulting was delayed through successive instars. Food supply of the host affected development time of the nematodes in laboratory experiments. It was apparent that A. vexans larvae suffered significant premature mortality as a result of mermithid infection. The percentage of larvae infected in artificial pools varied considerably over time and between pools treated at the same level. Excluding samples taken after the onset of pupation of uninfected larvae, mean per cent infection ranged from 7.2-18.8, 12.4-24.8, 9.2-23.2 and 38.0-49.0 when preparasites were applied at 10.000, 20,000, 50,000 and 100,000 per m² respectively to artificial pools. Nematodes were occasionally melanized by the host. It was concluded that control of floodwater mosquitoes using R. culicvorax on a large scale in Manitoba is not practical at this time. A brief taxonomic review of the Romanomris species parasitizing mosquitoes is presented, along with the description of Romanomermis communensis sp. n. from Goose Creek, Manitoba. The type locality for R. communensis was the same as for Hydromermis churchillensis Welch, 1960, however, the latter species was not found in either 1974, 1975 or 1976. Consideration is given to the present status of H. churchillensis. Although R. communensis and R. culicivorax Ross and Smith, 1976 were successfully cross-mated in the laboratory, they are considered here as separate species. The description of R. hermaphrodita Ross and Smith, 1976 is supplemented by specimens collected from the type locality and host.