The Biological Relationship between Septobasidium retiforme (B. & C.) Pat. and Aspidiotus Osborni New. and Ckll. By John ?f. Couch,1 University of North Carolina, U.S.A. With Plates 15-19 and 60 Text-figures. CONTENTS. PAGE I. INTRODUCTION ......... 384 II. HISTORICAL ......... 384 III. THE PROBLEM 388 IV. LIFE-HISTORY OF SEPTOBASIDIUM RETIFORME . 389 V. LIFE-HISTORY OF ASPIDIOTUS OSBORNI . 395 VI. OBSERVATIONS ON THE RELATIONSHIP BETWEEN THE FUNGUS AND SCALE INSECTS AT DIFFERENT TIMES OF THE YEAR 1. Dormant season ....... 402 2. Growing season ........ 405 3. Transition from growing to dormant season . 425 VII. ADVANTAGES OF THE FUNGUS-INSECT ASSOCIATION TO THE FUNGUS ......... 427 VIII. ADVANTAGES OF THE ASSOCIATION TO THE INSECTS . 429 IX. NATURE OF ASSOCIATION BETWEEN FUNGUS AND SCALE INSECTS 432 X. SUMMARY 433 1 The scale insects have been identified by Dr. Harold Morrison, Chief of Taxonomic Investigations, Bureau of Entomology, U.S.D.A., Washing- ton, D.C. Dr. Morrison has also helped me with the literature on scale insects. In the preparation of the plates and figures I have received help from several sources. Miss Alma Holland inked most of the Text-figures, and part of Plate 19. Mrs. E. R. Couch inked a number of the Text-figures and Plate 16. Dr. J. M. Valentine retouched Plates 17 and 18. It is also a pleasure to thank the editor, Professor E. S. Goodrich, for his kind advice in the arrangement of the figures and plates and for retouching several of them. Paper presented at the International Botanical Congress, Cambridge, England, August 19, 1930. 384 JOHN N. COUCH I. INTRODUCTION. ONE of the most unusual and interesting genera of fungi is Septobasidium. Allied to the mushrooms through the Thelephoraceae on the one hand, by the flattened resupinate habit and exposed fruiting surface, and to the rusts on the other hand by the similarity of their reproductive cells, the genus occupies a very anomalous position taxonomically. Though interesting from this standpoint, the genus is of far greater interest in its biology. The seventy odd species, which have so far been described, grow, as a rule, in flattened patches over the bark and leaves, and sometimes the roots of living plants. In some species the fungus growth is so extensive as to clothe the leeward side of the trunk and the under side of many of the branches of trees fifteen metres high (Petch, 1911, 1921, and Couch, 1929). The majority of species, however, grow in smaller patches. In spite of such extensive growth, observers have so far reported, in the great majority of species studied, that the fungus does not penetrate into the living tissues of the plant. If the upper regions of the fungus are scraped away with a knife, numerous scale insects are exposed. Borne of the scale insects are imbedded in the tissue of the fungus; others adhere to the wood. Some of the insects are dead, their remains appearing only as empty shells; others are alive. Usually the insects, whether dead or alive, are very abundant. No one can observe such an association of fungus and insects on a living tree without propounding the question as to how the fungus is nourished. Several explanations have been offered. II. HISTORICAL. By some observers the fungus has been regarded as a parasite, causing peculiar wounds (Galloway, 1890) in the wood in some species, and witches' brooms in other species (Rick, 1906, 1907; see also Sawada, 1911; Jaap, 1916; Gaiimann, 1927). Others have considered the genus as epiphytic (Patouillard, 1892). In 1907, von Hohnel announced his discovery of scale insects beneath the stroma of Septobasidium abnorme (P. Henn.) v. H. et L. Von Hohnel found, furthermore, that the SEPTOBASIDIUM AND ASPIDIOTUS 385 fungus did not penetrate at all into the tissue of the leaf. He states, therefore, that if the same condition can be found in other species, there can be no doubt that a real biological relationship exists between the species of Septobasidium and the scale insects. 'It may be parasitism or saprophytism, or it may be that a complicated symbiotic relationship exists' [between the scale insects and Septobasidium]. Since this very remarkable discovery, by far the greater majority of species which have been described have been found associated with colonies of scale insects; and the species described before 1907, upon reinvestigation, have also been found to be associated with scale insects. Petch (1921), who has studied entomogenous fungi in Ceylon for a number of years, states that all species of Septobasidium so far found in Ceylon are associated with scale insects. Indeed, there is only one species of Septo- basidium which is known not to be associated with scale insects. This species was collected in Jamaica by W. E. Maxon (Couch, 1929), and grows on the fertile side of fern sporophylls, parasitizing the young sporangia. Since von Hohnel's discovery in 1907, attention has been directed to the relationship between the fungus and scale insects, rather than to the relationship between the fungus and the tree (or other host plant). One group of investigators claim that the fungus parasitizes the bodies of the scale insects, completely overgrowing and finally destroying them (Petch, 1911; Burt, 1916; Coker, 1920; Snell, 1922; Boedijn and Steinmann, 1930); the other claims that, as far as is known at the present time, Septobasidium lives saprophytically or epiphytically on the excretions of scale insects (Gaumann, 1926). I shall review only the evidence for parasitism, since Gaumann does not present any evidence in support of the view stated in his text-book.1 Petch (1911), in a brief note on the biology of the genus 1 It should be borne in mind, however, that Gaumann (1922), in his extended research on S. borgoriense Pat., was not attempting to discover the fungus-insect relationship, but was rather trying to determine if the fungus would grow on a large number of trees and shrubs without insects .(see p. 428 below). 386 JOHN N. COUCH Septobasidium, states: 'Prom an examination of a long series of specimens, it has been determined that these Fungi are parasitic on colonies of scale insects, which they overgrow and completely destroy. These Fungi live, not on the secre- tions of the insects, as in the case of M e 1 i o 1 a, but upon the insects themselves.' Again, in 1921, p. 34, Petch makes essenti- ally the same statement.1 Burt's (1916) observations point to an association of Septo- basidium with scale insects, and he seems inclined to follow Petch in believing that the fungus completely overgrows and destroys the insects. Burt states that Mr. Seagle from North Carolina wrote that the old fructifications of S. pseudo- pedicellatum disappeared from his apple trees in late spring and early summer, and new fructifications grew, which became large by early winter. I have found, contrary to Mr. Seagle's observations, that S. pseudopedicellatum Burt is perennial on several varieties of oak, ash, black gum; pear, and other trees. Indeed, I have had several individual colonies of S. pseudopedicellatum Burt marked for three years. Coker (1920) reporting on S. pseudopedicellatum Burt and S. retiforme (B. and C.) Pat. states, ' This species attacks colonies of scale insects on living bark;' and, farther on, he says, ' the plant is parasitic on the insects only and does not injure the tree.' Snell (1922), describing S. pinicola Snell on Pin us s t r o b u s, says that the fungus lives as a pure epiphyte on the tree, but is associated with scale insects, parasitizing and destroying them, as cursory observations reveal. About the nature of this association, Snell says, ' The insects are overgrown 1 Petch (1921) states, however, that there are certain exceptions to the rule, that some species after destroying the scale-insects attack the plant. Gaumann (1927) in a brief note 'On the parasitism of S. bogoriense Pat.' states that he found this fungus growing on hybrid roses imported from Europe in such luxuriance as almost to exterminate the roses. He makes no statement about the fungus-insect relationship but states that the fungus is parasitic on the rose. This is particularly interesting in view of the fact that in an earlier extended research of Gaumann's (1922) he reports that S. bogoriense grew very poorly on trees and shrubs without scale insects. SEPTOBASIDIUM AND ASPIDIOTUS 387 and intergrown with mycelium of the fungus, which can be determined to be within their bodies. The hyphae in the young- est insects were hyaline and on the older ones were dark like that of the context.' Snell presents a photograph (fig. 2, PI. 12) to show this condition. An examination of material sent me from Pennsylvania by Professor Overholts has shown, however, an entirely different condition from that described by Snell. In spite of the fact that this material was examined two weeks after it was collected, and hence was almost completely dried out, it showed beneath the fungus stroma: (1) non-parasitized insects; (2) non-parasitized adults containing young; (3) para- sitized insects. The latter contained numerous haustoria of the fungus in the shape of much gnarled coils, which resemble, in a striking manner, a glomerulus of the kidney. These haustoria within the parasitized insects' bodies are unmistakable, and it is evident from Snell's statements and photograph that he did not recognize the true haustoria of S. pinicola. In 1930, Boedijn and Steinmann, working in Java, report as follows about the fungus-insect relationship: ' As a result of our studies of several species, and of a great amount of material, we have found that these fungi are parasitic on scale insects, living not on their secretions, as is the case with sooty moulds, but on the insects themselves, which they overgrow and finally destroy.