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1953
The Lepidopterists News
57
THE SIBLING SPECIES OF BUDWORM MOTHS IN CANADA
The entire April, 1953, issue of The Canadian Entomologist is devoted to papers on the Spruce and Jack Pine Budworms. These six contributions constitute together a superb exposition of the biological details of the differences and similarities of these extremely closely related species. In the first paper Dr. Freeman describes and names the Jack Pine species as Choristoneura pinus sp.n. and discusses the existence of several other possible "cryptic species" of Choristoneura feeding on conifers in Canada and the northern and western U. S. A. Eighteen specimens of C. pinus and its sibling, C. fumiferana, are beautifully figured in color. There are identification differences of ground-color, wing-pattern, size, and $ genitalia; for each there is probably some overlap. Miss Cox analyzes genitalic and larval differences in terms of frequency distribution in the third article.
In the second paper Miss MacKay shows, with fine figures, the larval differences between the two species. These seem to be more absolutely characteristic than the adult differences.
Mr. Campbell shows color differences in the pupa (with color photographs) and in the egg-clusters. C. pinus lays an average of 37 eggs per cluster; the average for C. fumiferana is 19.
The most remarkable discoveries by this Canadian group are analyzed by Dr. Smith. The distinctness of the two Choristoneura is maintained, in the many areas where the ranges meet, by inherited reproductive isolation. C. fumiferana larvae feed on Balsam Fir, Black and White Spruce, and Larch; C. pinus feeds almost solely on Jack Pine. The eclosion periods (28 June -13 July and 11 July - 28 July) of the two species hardly overlap, and C. fumiferana copulates slightly earlier in the day, so that temporal isolation is nearly complete. Mate-choice tests in cages show that each species strongly prefers to mate with its own species. No notable sterility appeared in the hybrid eggs, but eggs laid by hybrid 9 9 showed as much as 25% sterility, due presumably to chromosomal differences. Dr. Smith makes an apparently unwarranted statement that hybrid inviability does not operate "in preventing the flow of genes between these two species of Choristoneura." Even with full fertility and fecundity, if the foodplant suitability is as exclusive as is stated, the hybrid larvae would be expected to have some combinations of nutrition genes giving reduced survival. One aspect of this cooperative study of Choristoneura siblings about which one would like particularly to know more is the matter of comparative nutritional physiology. Dr. Smith's footnote 6 suggests that tolerance of either species and their hybrids to Jack Pine has not been tested in the laboratory. It seems possible that 1) F2 and backcross sterility due to the chromosomal differences and 2) nutritional inviability of hybrids are the most important factors causing separation of these two Choristoneura populations as species and that these arose during former geographic isolation. It may be that mate-choice, oviposition-plant selection, and temporal isolation arose after the populations again became sympatric and were then strongly favored by natural selection. There seems to be no evidence against such a neo-Darwinian interpretation.
58
REMINGTON: Review on Choristoneura Siblings Vol.7, no.2
The final paper, by Dr. Walley, lists the 13 parasitic Hymenoptera known to have C. pinus as a host, 9 of which were also known from C. jumiferana.
These papers place the two Choristoneura in the literature beside Mit-oura hesseli and gryneus, Colias australis and hyale, Strymon caryaevorus and jalacer, Hyalophora angulijera and promethea, and other foodplant-based complexes of sibling species of Lepidoptera which provide important information on the origin of species.
The papers on Choristoneura under review are as follows:
1. Freeman, T. N., "The Spruce Budworm, Choristoneura jumiferana (Clem.) and an allied new species on pine." Can. Ent. 85: 121-127, 22 figs.
2. MacKay, Margaret R., "The larvae of Choristoneura fumijerana (Clem.) and C. pinus Free." Can. Ent. 85: 128-133, 14 figs.
3. Campbell, I. M., "Morphological differences between the pupae and egg clusters of Choristoneura jumiferana (Clem.) and C. pinus Free." Can. Ent. 85: 134-135, 6 figs.
4. Cox, Constance E., "Analysis of frequency distribution of adults and larvae of Choristoneura fumiferana (Clem.) and C. pinus Free." Can. Ent. 85: 136-141, 4 figs.
5. Smith, Stanley G., "Reproductive isolation and the integrity of two sympatric species of Choristoneura." Can. Ent. 85: 141-151, 1 fig.
6. Walley, G. Stuart, "Hymenopterous parasites of Choristoneura pinus Free, in Canada." Can. Ent. 85: 152.
C. L. REMINGTON, Osborn Zoological Lab., Yale University, New Haven 11, Conn., U.S.A.
The Entomologists' Gazette commenced publication in 1950 (Vol. 1). The emphasis is on British Lepidoptera, but other orders of insects are regularly represented in its pages. The Gazette is a quarterly, published in January, April, July, and October. The Editor is the entomological bookseller and publisher, E. W. CLASSEY, and the Assistant Editor is H. S. ROBINSON. The annual subscription rate is one pound. The address is 91 Belfont Lane, Feltham, Middlesex, England.
C. L. R.