Submitted to: Mycological Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 5, 1995
Publication Date: N/A
Interpretive Summary: Geographic origins of introduced insects and their natural enemies are often poorly known. Two closely related introduced weevil pests, the alfalfa weevil and the clover leaf weevil, were probably introduced in the 19th century and by now have spread throughout much of North America. The fungal pathogen Zoophthora phytonomi infects only these and other weevils. The fungus was discovered infecting the clover leaf weevil in the 19th century but not until 1973, in Ontario, was it discovered infecting alfalfa weevils. After the 1973 discovery, fungus-infected alfalfa weevils were reported from locations progressively further south, suggesting a rapid spread of a new fungus strain. The objective of this study was to determine whether there is more than one fungal strain present in these beetles in North America, thus indicating a very high degree of host specificity for this fungus. DNA fingerprinting techniques were used to distinguish among fungal strains. Results indicated one group of fungal strains is specific for the clover leaf weevil while another group of strains infects both weevil species. This latter group is genetically related to fungal strains of European origin, suggesting its more recent introduction into North America.
In North America, the fungal pathogen Zoophthora phytonomi has been known to cause significant levels of infection in introduced clover leaf weevil (CLW) populations since 1885. This pathogen was never noted in introduced populations of alfalfa weevil (AW) sympatric CLW, until 1973 when it was found in AW in Ontario. From 1973 through 1981, Z. phytonomi was progressively found further south from Ontario. Whether these reports of Z. phytonomi infecting AW actually demonstrate spread by a novel genotype has previously been proposed and disputed. RAPD techniques were used to compare isolates of Z. phytonomi from both hosts in North America, and from AW in Israel. Analyses demonstrate that two main genotypes of Z. phytonomi occur in North America; one genotype including only CLW isolates with a second more homogeneous and principally including isolates from AW. The genotype principally including isolates from AW was more closely related to oisolates from AW in Israel than to the other North Amercian group, but als included one isolate from CLW. Results do not disprove a hypothesis of a more recent introduction to North America of a novel genotype of Z. phytonomi abundantly infecting AW. Based on affinity with Israeli genotypes, this latter strain may have origninated in the Eurasian areas where AW is endemic. The degree of host specificity of these two North American genotypes of Z. phytonomi will require further investigation.