|I. Trägårdh (1923)
[ Forest entomological studies II. 1. The Spruce resin fly. Cheilosia morio Zett. ] Swe.
Medd. Stat. Skogsförsöksanst. 20: 401-424
When one is looking for the Giant Phloem Borer (=Sirex?) by systematically examining the resin flows on spruce trees (granstammer), one sees that these flows harbour a rather rich insect fauna. Often the larva of the 'vecklarejärilar' (?) or 'mott' (?), but more often are found the empty puparia of diptera in the resin. I had observed and collected these last on other occasions, but never had succeeded in rearing the adult, for which reason the species name remained unknown.
However, in September 1920 Huntmaster Edw. Lundberg of Kloten directed my attention to the appearance of a fly larva in the resin flow on spruce trees, sending in material of the same. Since then the larva has been repeatedly observed under conditions which will be described below. Every attempt to rear through the adult has however been unsuccessful. In the spring of 1921 on a study tour of Germany and Austria I visited the old Academy of Forestry in Tharandt near Dresden, and there Dr. W.Baer mentioned that he had observed a similar larva, and even had succeeded in rearing it and that it had been identified by a dipterologist A. Kuntze as Cheilosia morio Zett. Material collected in Sweden was then sent down to Dr. Baer, who kindly made an accurate comparison of the larvae and failed to find any differences between them. One can therefore assume that the name of the species is the one mentioned above. For the forester it does not matter which Cheilosia it is, and therefore I will not delay publication of this information about the fly until the latin name is known absolutely for certain since this could take a long time. I propose the swedish name 'Spruce resin fly' for this species.
Fly larvae have already been found in resin flows and wounds of coniferous trees, apart from the many species found in bored passages in bark (of which some are predators and others are detritivores - Trägårdh, 1.). Baer refers to the larva Cheilosia morio in a footnote in his essay "[Observations and studies on Dioryctria splendidiella H.S. and abietella S.V.]"., but has not described the find or the larva in any detail. Even from North America there is information available about fly larvae belonging to the same genus Cheilosia with a very similar life style (Burke, 1905), of which we will discuss further in the following.
As it is a question of the larva of a syrphid or hoverfly, it is natural to think about the lives of other insects that are found in resin flows. A closer study of the literature about the lifestyle of hoverfly larvae shows however that even if the majority are predators living for the most part on aphids or scale insects, there are many exceptions from this rule. C.L. Metcalf has in his work 'Syrphidae of Maine' divided hoverflies into several groups according to their lifestyle and habits. He distinguishes the following groups:-
1. Aphidophagous type: with completely flat abdomen, the body pointed in front and rounded at the back, with a short posterior respiratory process (prp) bearing the spiracles. Between the spiracles there are a few bristles.
2. The boring type, with an almost cylindrical body, narrowing less anteriorly than type 1. Prp short with a depressed spiracular plate, and fingerlike branched feathery spiracular hairs. The type for this group is Merodon equestris.
3. The short-tailed type living in mud and unclean places. Cylindrical with a rounded front part and both tracheal trunks clearly visible in the prp, which carries three pairs of lappets as in type 2. E.g. Syritta pipiens.
4. The long-tailed type, living in unclean places. Prp long, flexible, telescopic, extensible to at least half of the length of the body. E.g. the mudfly Eristalis.
Notice that Metcalf in his description of these groups has not taken Burke's work into account, as he himself admits. Also it must be mentioned that F. Eckstein (1920) described a syrphid larva which, according to Prof. Escherich, was found in the passages made by larvae of Pine Weevils [= Hylobius abietis]. He believes it to be related to Neoascia, whose larva is depicted by Lundbeck (p. 374). Lundbeck's book 'Diptera Danica' contains a lot of information about the lifestyles of syrphid larvae, from which it appears that larvae of similar species of Cheilosia live in the roots of plants, e.g. the species Carduus and Cirsium. On the other hand he does not mention any Cheilosia species among those said to live in the sap flows of trees.
Lastly, the writer many years ago found a mine of a Cheilosia larva in the leaf of Sedum telephium; in later years while studying the biology of Pine Weevils I found numerous syrphid larvae under the bark of fir-tree roots - under similar circumstances to the one described hibernating by Eckstein: I have not been able to get adults to emerge from these.
From this information it appears firstly that hoverfly larvae from America hibernate under similar circumstances to those of the spruce-resin fly; secondly, that phytophagy is not unknown amongst the Syrphidae, even if most of them are aphidophagous.
The larva is white or yellow-white in colour, with a somewhat flattened ventral side and arched back, declining from the 4th abdominal segment anteriorly and the 3rd posteriorly. It is rounded anteriorly, while the posterior is drawn out into a long telescopic siphon clearly consisting of three equally long sections.
The cuticle is covered with fine bristle-like pegs. The segments are divided into number of cross-folds by numerous dorsal cross furrows which go down the sides, but are almost obliterated on the ventral side. On the ventral surface of each of the first 8 segments sits a pair of small wart-like swellings which serve as pseudopodia. The rear pair of these are a little drawn out and drop-shaped.
The prp is almost half the length of the body if completely extended (pulled out); it consists of three parts, with the first and second sections cone-like, and the third almost cylindrical. Subdorsal near the end of the first section sits a pair of drop-shaped lappets, and subventral just past the middle of the second section is another pair the same shape but somewhat (coarser?). Such lappets are found in many syrphid larvae with clearly extended prp's. They are even found on the larva described by Eckstein. But although that one is clearly deformed because of preservation in too strong alcohol, and the description must therefore be treated with caution, the differences are still so clear and distinct that it cannot be the same larva. This is corroborated by Eckstein's observation that his larva completely lacks mouth-hooks.
Moreover we lack here the opportunity of closer examination of the exterior morphology of the larva; I will merely describe two organs whose construction bear close relation to the lifestyle of the larva.
Above all there are the mouth-hooks, which as a rule ought to show clearly their adaptations to the diet. In aphidophagous species the jaws look sharply bent and pointed, as far as one can judge from Metcalf's figures (Pl.32 fig. 47 & Pl.33 fig 11).
In Cheilosia morio on the other hand (Fig. 3) they are slightly bent and rather blunt, carrying a row of blunt teeth in a ventral groove. As only a small number of syrphid larvae are so accurately described or illustrated that one knows the construction of their mouthparts, it is impossible to know if this characteristic type of mouth-hook is found in other species with the same life-style.
The shape of the mouth-hooks suggests that their function is as an aid to locomotion, or otherwise they could serve as a sort of grating tool.
Of great interest too is how the prp is constructed. As Fig. 4 shows, the end of the prp is flattened and somewhat depressed in the middle. On either side the longitudinal tracheal trunks divide into three branches, each ending in a button-like structure whose topside nevertheless has no opening: the opening of the spiracles is to be found under the outer freely projecting edge of the structure, and consequently is in a much more sheltered position (than usual).
In the middle of the prp one sees a lighter area with thinner cuticle produced by a dwindling down of two oval 'ytor'(? yt = surface, exterior) and surrounding two oval areas with radial folds.
These so-called ring-fields (the circular plate of Metcalf) are unusually well developed and large in Cheilosia morio. On the edge of the spiracular plate are four pairs of curious bifid, then repeated-in-the-form-of-a-fan forked bristles which are normally bent forwards.
If we compare the construction of the posterior spiracles of Cheilosia larva with those of other syrphid larvae, some curious features will be seen.
Firstly the spiracular slits have a hidden location, while on other larvae they are lying on the surface and are drawn out into long slits. Moreover, the spiracular plate (ring-fields) and the feather-like hairs are of a considerable size. We see here a series of adaptations to the lifestyle of the larva. The spiracular slits are sheltered by virtue of their position as an adaptation to living in the resin. The elastic ring-fields (different 'ring-fields' from the spiracular plate?) without doubt make it possible for the middle section to be drawn in and out. On contact with the air inspiration occurs; then when the siphon is drawn through the resin, an air bubble occurs, which (empties?) when the eight feather hairs are bent backwards by movement. It looks very much like the flotation device in the tip of the prp that is possessed by many aquatic insect larvae.
E. Lundberg writes about his discovery in a letter thus:
“I made my observations on a walk in a neighbouring spruce forest in a recently thinned stand, and many of the tree trunks were (furnished with?) strong resin flows, which resembled the resin flows one usually sees everywhere in spruce forests. I did however perform a closer examination of one, and it turned out that though the surface of the resin did not have any holes, (I could see) holes within the resin, and by chance I thought that then the resin moved, and upon closer examination I found a larva swimming in the resin. I examined a number of others, and found larvae in the majority of cases. They could well have been present even in cases where I did not find them, since the larvae are very difficult to separate from the sluggishly flowing resin in which they swim. In no attack did I find more than one larva.”
In the material of resin lumps sent in by Lundberg were also larvae of Dioryctria, as a result of which I expressed the opinion that the syrphids lived off these. Further examination by Lundberg and my own repeated observations above all with mature ('long-standing') trunks has produced the conclusion that these syrphid larvae live alone in the resin flows and undoubtedly feed from them. Sometimes they can of course be associated with the Giant Phloem Borer (?Sirex sawfly) or with Dioryctria larvae, but in these cases it must be accidental.
It would be of great interest to know whether the ovipositing female has managed to injure the bark of the spruce tree herself, or if she is content to lay in already existing resin flows. As no female material is accessible to me to examine her ovipositor - which should give an indication in this matter - it is impossible to say anything with certainty.
In the meantime the writer as well as Lundberg have found the larvae very commonly in resin flows after thin circular scratches/scoring (presumably a management operation of foresters). Lundberg writes about it in response to my request that affected trees should be marked before I came to check them out. “Marking of suspect trees for possible study in the spring turned out to be unnecessary since such trees are found in unlimited numbers. Almost every big lump of resin contains a larva or a puparial case or 1-2 puparia. On the spots where thin rings are marked with scratches from last summer, larvae are to be found in practically every wound, from which it would seem that the larvae are not the primary cause of the wound or the resin flow, but no doubt it irritates the tree to produce a violent resin flow.”
Even with old stands where an area occurs where thin-ring scratches were common, inside their resin flows Cheilosia larvae were found as a rule. Possibly wounds or resin flow have in other cases been the occasion for entry of the larva (as in the case when caused by the Giant Phloem Borer (?Sirex), but then vacated by it).
It looks therefore as if the ovipositing female chooses preferentially places where the bark has been damaged. This agrees with what Burke writes from America when describing Cheilosia alaskensis Hunt [sic], of which he writes that he has not found any proof that the larva can make a hole in the bark by itself. Burke usually found the larva in holes, which he assumed were made by the bark-boring (beetle).
On the other hand the differences count, since in the american Cheilosia larva the prp is as long as the body, and it can therefore without difficulty keep the tip in contact with the air at the same time as working at the wound. In Cheilosia morio this would not be possible, so one must assume that the larva is less stationary, and embarks on wanderings out to the surface of the resin lump. Fig. 5 shows how such a resin flow looks when it is not connected with scorings, but it must be noted that the resin dried several together, and the photo shows a dry specimen. Fig. 6 shows the dorsal part of the same specimen. We see here a passage a couple of centimetres long in the sap-wood surface, filled with resin.
It would be interesting to know for how long such a wound is inhabited by the Cheilosia larva. No conclusive observations exist on this point. The rather normal frequency of empty puparia observed by Lundberg embedded in resin flows where larvae are also found indicates that one wound is inhabited by succeeding generations of larvae over a couple of years at least. Likewise we know little of the future destiny (of the tree), e.g. if the spruce-resin fly can through its activity prepare the ground for further attack by other insects.
This problem therefore deserves further study quite apart from the fact that its harmful effect should be brought to the foresters attention, because it is something easily mistaken for evidence of the Giant Phloem Borer (?Sirex).
translated by Margrethe Sylvest Jensen Gilbert