"It is concluded that a virus epizootic may be responsible for marked fluctuations in populations and that, as a result of this, one can expect to record years of heavy migration of certain species when the population is at a maximum and years of light migration when the population is at a minimum. Crow (1957) has pointed out that the cycles of population is a response to virus infection followed by a host strain resistance. It would appear, from these observational data, that a resistant strain may occur in a well-defined geographical area and that, if the species is a migrant, this will result in the reappearance of the species where it had been absent for a period of time.
It would be of considerable interest to those of us involved in migration studies to have comparative data on the fluctuations in numbers of those species that are not well-defined migrants or are not remigrants such as species that enter areas formerly unoccupied by them. An examination of reared specimens of other migrants during periods of low population density may indicate an epizootic similar to that here described and perhaps follow a cycle similar to that of the monarch butterfly both in time and duration. There is also the possibility that the distance travelled during migration varies with population density, with greater distances during maximum and lesser during minimum density.
The data obtained for the monarch butterfly, although admittedly not of an exact nautre to allow for statistical analysis, is presented so that others interested in following the movements of migrant insects may wish to carry out a similar study."
Note: If any one wants this original pamphlet of F.A. Urquhart's study, I will mail it to him/her. It was fun reading. In an era with no computers, internet, GPS systems, digital cameras and cell phones, it must have been difficult to amass field observations that spread from Canada to Mexico. The video (below) taken with a digital camcorder of Monarch Butterflies swarming in Iowa could have been sent to Dr. Urquhart's office at the University of Toronto at the speed of light!
A neat video! Dr. Urquhart mentioned in the above study, that "...the monarch buterfly reached a peak of abundance in 1950 and 1951 which period was marked by countless thousands of specimens that clustered in hug masses on various species of trees during the annual fall migration. This period of peak abundance was followed by a marked reduction in numbrs in 1952, and by 1953 no roosting colonies could be found and only seven specimens were collected during our summer field studies. By 1954 a few small roosting clusters were located and 58 specimens were collected. By 1956 the population had once again reached a peak of abundance. This cycle occupied a period of six years."
Urquhart also mentioned, "We had noticed, throughout the periods of scarcity, that a disease, marked by a black, foul-smelling liquid in the bodies of dead specimens, seemed to be responsible for the decline. Our laboratory colonies were almost completely destroyed- as much as 98.2% mortality- which hindered our research on other phases of the ecology of this species. During the last population decline we were able to ascertain that the pathogen involved was a polyhedrosis virus (Urquhart, 1966). This virus was later identified as a cyctoplasmic virus (Howard et al, 1968). That a virus is responsible for cyclical fluctuations in other species has been previously reported on, as for example, Balch and Bird (1944); Steinhaus (1948); Crow (1957); Bird and Burke (1962)."
Interesting, from a "backyard" Monarch enthusiast's point of view, with the Monarch population way down, I'm wondering about the primary cause of the decline. In my area, roadside cuttings and sprayings have been all but absent. Large highway interchange areas have prospered with milkweed and nectaring wildflowers. But, there were few eggs laid on the plentiful supply of milkweeds! Is the decline's primary cause due to OE and other parasites, or is it due to man-made destruction of Monarch habitat? Of the chrysalis' I had this season, a large percentage had the "black, foul-smelling liquid" as Dr. Urquhart referred to. Of the last 33 chrysalis' I had this season, 12 were infected and never opened. Will we have a dramatic increase in the Monarch population in the cyclical time frame Dr. Urguhart observed years ago?