Author Institution: Ohio Agricultural Experiment Station, Wooste

Miller, William E.

The Ohio Journal of Science

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ECOLOGICAL NOTES ON THE PINE MIDGES RETINODIPLOSISRESINICOLA (OSTEN SACKEN) AND R. INOPIS(OSTEN SACKEN) IN SOUTHERN OHIO(DIPTERA, ITONIDIDAE)WILLIAM E. MILLER1Ohio Agricultural Experiment Station, WoosterThe midges Retinodiplosis resinicola (Osten Sacken) and R. inopis (OstenSacken) develop in masses of resin on pine twigs (fig. 1) in aggregations of severalindividuals to a mass. Occasionally their feeding becomes of economic importance(Barnes, 1951). The two species were studied in southern Ohio mainly in theShawnee State Forest during three growing seasons from 1951 through 1953.Midge adults in this study were identified by R. H. Foote of the Insect Identi-fication and Parasite Introduction Section, U. S. Department of Agriculture.Parasites were identified by specialists of the same section as follows: Pteromalidaeand Eupelmidae, B. D. Burks; Platygasteridae, C. F. W. Muesebeck.INCIDENCERetinodiplosis spp. adults were reared or their larvae were seen developingat several widely scattered localities in southern Ohio. These insects probablyoccur throughout the ranges of their host pines in Ohio. Authentically identifiedspecimens of R. resinicola and R. inopis were reared from pitch pine, Pinus rigidaMiller, and specimens of R. inopis were reared from Virginia pine, Pinus virginianaMiller. These determinations were made with male specimens because femalesof the two species cannot be reliably differentiated (Barnes, 1951). Samplespecimens sent to the identifier proved to be mainly females. Both midgesoccurred in the same pine stands, but R. inopis was less abundant than R. resinicola.At a time when developing maggots were abundant, aggregations were examinedfor size and age of that part of the host supporting them. Aggregations werefound on pitch pine twigs, branches, and trunks up to 2 inches in diameter and upto 5 years old. Many more twigs than branches and more branches than trunkswere infested. Pitch pine trees from 2 to approximately 12 feet in height wereseen supporting dense maggot populations.INJURYExactly what the maggots eat is still not clear. Barnes (1951) has reviewedthe available information on this subject. Infestation causes swellings and dis-tortions on young twigs (fig. 1, 2, 3) and probably a reduction in the vigor of thetree. Healed feeding places on older stems form resin pockets in the wood ("bird's-eye pine"). Twigs and even trees are said to be killed in severe infestations(Barnes, 1951), but no infested twigs in the Shawnee Forest died.LIFE HISTORIESTracing the seasonal histories of the midges was difficult because of a markeddecline in their abundance following emergence of the spring brood in May. Theobservations in table 1 suggest that approximately 4 broods a year are produced.xNow with the Forest Service, U. S. Department of Agriculture; Lower Peninsula ForestResearch Center, Michigan State University, East Lansing, Michigan. This work was donewhile the author was employed as a research assistant at the Ohio Agricultural ExperimentStation on a pine tip and twig moth problem under the direction of R. B. Neiswander of theentomology staff.THE OHIO JOURNAL OF SCIENCE 58(4): 231, July, 1958.282 WILLIAM E. MILLER Vol. 58The maximum number of broods per year may be greater; there is probably muchoverlapping of generations after May because of the long emergence period of thespring brood. The late summer and early spring observations (table 1) suggestthat the winter is spent in the larval stage. Moreover, typical Retinodiplosislarval aggregations on pitch pine were seen during the winter in the vicinity ofWashington, D. C. Other observers have also given the larval stage as thewintering form (Barnes, 1951).EXPLANATION OF FIGURES 1-41. Resin masses of Retinodiplosis sp. probably inopis on pitch pine twigs. Needles wereremoved for the photograph.2. Pitch pine twigs one year after deformation by Retinodiplosis sp. probably inopis. Needleswere removed for the photograph.3. Pitch pine twigs two years after deformation by Retinodiplosis sp. probably inopis.4. Resinous cocoons of Retinodiplosis inopis on buds and needles of pitch pine.The resin masses inhabited by R. resinicola frequently were larger, containedmore individuals, and occurred on larger stems than the masses of R. inopis. R.resinicola masses were usually opalescent while R. inopis masses were often trans-parent. In 24 aggregations examined in the spring, the number of nearly or fullygrown Retinodiplosis spp. larvae and pupae per aggregation varied from 2 to 29,with the mode being 5.The larvae of R. resinicola pupated in silken cells spun within their resin masses.These cells protected the pupae from the sticky resin. Pupae were oriented withNo. 4 ECOLOGICAL NOTES ON PINE MIDGES 233their heads outward from the mass. Just before emergence of adults, the pupaeworked themselves about half way out of their cells by manipulating their abdo-mens. Following adult emergence, the empty pupal skins protruded from resinmasses.In contrast to R. resinicola, the larvae of R. inopis crawled away from theirresin masses and pupated in resinous cells elsewhere on the twig (fig. 4). Barnes(1951) mentioned the different pupating habits of the two species as diagnosticcharacters.R. resinicola and R. inopis adults of the spring brood emerged concurrentlyin the insectary in May. Spring insectary emergence began in 1952 on May 4,and in 1953 on May 3. The flies issued over periods ranging from 24 to 38 days.Adults were short lived. Sixteen R. resinicola individuals which had emergedduring the previous 24 hours were put in a battery jar. The following day, halfof the midges in the jar were dead, and two days later, all but three were dead.There had also been placed in the jar a freshly cut pine twig and three microscopeslides each containing a droplet of fresh resin. During the first 24 hours, theTABLE 1Seasonal history observations on Retinodiplosis spp. insouthern OhioDate ObservationApril 2, 1953 Full grown larvae.April 16 Partially grown larvae.April 30 Full grown larvae.May 7 PupaeMay 14 Pupae and full grown larvae.May 28 Pupae and empty pupal cases.June 19 Full grown larvae.July 1, 1952 Pupae and empty pupal cases.July 9, 1953 Full grown larvae.July 30 Do.September 3 Do.confined midges laid several hundred eggs. The midges were attracted to theresin for oviposition, a fact well brought out earlier by Eckel (1903). Some ofthe eggs were laid in the resin on the slides, and many on the cut ends of the twigwhere resin had exuded; but most were laid on the walls of the jar where almostimperceptible traces of resin had been deposited accidentally in handling.PARASITIZATIONEmergence records from randomly collected aggregations of both species ofRetinodiplosis during the springs of 1952 and 1953 showed that 15 percent of themidges were parasitized. Listed in descending order of their abundance, theparasites (all hymenopterous) were Systasis diplosidis Eckel, Habrocytus sp.,Norbanus sp. (Pteromalidae); Platygaster diplosidis (Ashmead) (Platygasteridae);and Eupelmus sp. (Eupelmidae). Definite associations of parasites with hostspecies were noted for only two of the parasites: Systasis diplosidis on Retinodiplo-sis resinicola and Platygaster diplosidis on Retinodiplosis inopis. Eckel (1903)found Systasis diplosidis, a, Eupelmus sp., and Platygaster diplosidis parasitizingRetinodiplosis resinicola in Massachusetts.Systasis diplosidis accounted for about three-fourths of all parasitization.This parasite appeared in the insectary all three rearing years. It was solitaryin its development and apparently did not affect all individuals in a host aggrega-234 WILLIAM E. MILLER Vol. 58tion. Besides emerging in spring, it is known to have developed on at least onelater host generation. Adults began issuing in spring during the last one-third toone-fourth of the host emergence period. Of 30 parasite specimens obtained fromseveral maggot aggregations, 19 were males and 11 females.Habrocytus sp. accounted for most of the remaining parasitization. Thisparasite emerged concurrently with Systasis diplosidis. Of 7 Habrocytus sp.individuals picked at random for identification, 3 were males and 4 females.Two aggregations of Platygaster diplosidis issued in the insectary on May 11,1953, through several round holes gnawed by the adults in their Retinodiplosisinopis cells. These aggregations numbered 9 and 10 adults. Before parasiteemergence, the host cells looked black because of the dark color of the parasitesinside them.Two females of Norbanus sp. emerged on May 31, 1953, and one female ofEupelmus sp. on May 25, 1953.ANNUAL POPULATION FLUCTUATIONFor three years it was consistently observed that aggregations of developingRetinodiplosis resinicola midges were abundant in the spring, but scarce duringthe remainder of the growing season. This annual fluctuation correlates withavailability of potential development sites, a factor which seems to be determinedby the host growing schedule.In southern Ohio, pitch pine undergoes its most rapid growth in early spring.At this time, twigs swell and many tiny lesions frequently form in the bark of youngtrees. Small amounts of resin exude from these lesions. Eckel (1903) snowedthat minute exudates are ideal for the establishment of young R. resinicola maggots.That the midges are attracted to such exudates for oviposition seems well estab-lished by Eckel's work and the insectary observations just described. Thuspotential development sites are abundant early in the spring when the over-wintering brood emerges and begins ovipositing. Consequently, the populationlevel of the following generation soars. By the time members of this large broodare ready to oviposit, however, conditions have changed. Host growth is slowerand earlier lesions have healed or the exudates have hardened. As a result, fewerpotential development sites are available and population levels drop. The samefluctuation can be inferred from Eckel's paper on R. resinicola in Massachusetts.The extent to which this phenomenon occurs in R. inopis is not known.An experiment was done to test the hypothesis that this annual fluctuation inthe Shawnee Forest is due mainly to availability of potential development places.On July 9, 1953, a date when maggots were hard to find, roadside pitch pinesaplings at two midge focal points were blazed liberally with a pocket knife.Three weeks later resin exudates from these blazes were examined, and manysmall aggregations of maggots were found. Not all of the exudates containedinsects, but the results clearly supported the hypothesis.A spatial rather than temporal rhythm in abundance of a western species ofRetinodiplosis was correlated by Austin et al. (1945) with presence or absence ofnatural resin exudation from shoots of different ponderosa pine trees in the samestand. Trees whose shoots were viscid with resin exudation were heavily infestedwhereas those whose shoots were not viscid were lightly infested.During the summers, when R. resinicola population levels in the ShawneeForest were low, the midges were found developing mostly in resin exudatescaused by other insects or by unknown agencies. Resin masses of previous midgegenerations were never found harboring new aggregations. Two insect speciesproviding habitations in which midges were observed developing were the pitchpine tip moth, Rhyacionia rigidana (Fernald), and the Virginia pitch-nodulemoth, Petrova wenzeli (Kearfott) ( = P. virginiana (Busck)). Also, ComstockNo. 4 ECOLOGICAL NOTES ON PINE MIDGES 235(1880) noted that the maggots occurred in exudates associated with the pitchtwig moth, P. comstockiana (Fernald).LITERATURE CITEDAustin, L., J. S. Yuill, and K. G. Brecheen. 1945. Use of shoot characters in selecting ponderosapines resistant to resin midge. Ecology 26: 288-296.Barnes, H. F. 1951. Gall Midges of Trees. Vol. 5 of Gall Midges of Economic Importance.London: Crosby Lockwood and Son, Ltd. 270 pp.Comstock, J. H. 1880. Report of the entomologist. Pp. 185-348 in Ann. Rept. (U. S.)Commissioner Agric, 1879: 1-621.Eckel, L. S. 1903. The resin-gnat Diplosis and three of its parasites. Entom. News 14:279-284.

Ecological Notes on the Pine Midges Retinodiplosis Resinicola (Osten Sacken) and R. Inopis (Osten Sacken) in Southern Ohio (Diptera, Itonididae)

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Ecological Notes on the Pine Midges Retinodiplosis Resinicola (Osten Sacken) and R. Inopis (Osten Sacken) in Southern Ohio (Diptera, Itonididae)

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