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Preventive and Exterminative Measures consist in collecting and destroying the galls. Small birds pick the grub from the gall.
Other common species include
The Oak-Cone Gall-Wasp, Cynips (Aphilotrix) fecundatrix, which forms the small, woody, cone-like excrescences near the tips of Oak twigs. These woody-galls, often formed in clusters, are at first greenish and soft, but turn brown as they harden.
The Oak-Rose Gall-Wasp, Cynips (Teras) terminalis, causes the large, rose-coloured, spongy galls at the tips of Oak twigs. They are generally formed at the terminal bud, where the “roses” are sometimes as big as a potato ; and they consist of several larval chambers clustered together.
The Marble Gall-Wasp (Cynips Kollari) often attacks young Oak in nurseries.
The Acorn Gall-Wasp (Andricus glandium) is another common gall.wasp which attacks acorns of the Turkey Oak and the Cork Oak. In S.E. Europe, Turkey Oak-galls are a commercial product.
IV. Two-winged Flies and Midges or Gnats (Diptera).
Various midges cause malformation of shoots, and malformation of buds like that occasioned by gall-wasps. Gall-midges often do considerable damage in Osier-beds, as the nodes or swellings formed make the rods useless for weaving. The damage done to other kinds of trees (Beech and various conifers) is very trifling.
Gnats or Midges with very small bodies and two comparatively large and often iridescent wings, with three to five longitudinal veins, round and broad at apex, but narrow at lower end ; antennæ thread-like or pearl-shaped ; abdomen cylindrical, with eight segments, pointed in female, and often provided with a movable ovipositor. Larvee long legless maggots or "strigs," mostly pale-yellow or reddish, without horny mouth-parts, but with a horny holdfast embedded ventrally. Generation annual.
The most important injurious insects of this order are the Osier Gallmidges belonging to the family of Cecidomyidæ or gall-gnats.
These lay their eggs on Osier-shoots, and when the larvæ hatch out and begin to suck the sap, nodes or swellings are formed which make the withes useless.
1. The Large Osier Gall-Midge (Cecidomyia salicis).
Appearance.—This midge is small, blackish, and long-legged, being only š to? of an inch long; its abdomen is ringed with red, and has whitish hairs.
Life-history.—The eggs are laid on young shoots (mostly on Salix purpurea)
1 The Gall-gnat family is a large and interesting one, numerous in species. The flies of the family are small and extremely delicate; the hairs on body and wings are very easily rubbed off, and this makes determination of the species a matter of great difficulty, and, because of this, examples of the galls (in the gall-making species) are always desired as an aid to identification. The larvæ or maggots, which hatch from the eggs of the female fies, are found in very diverse places according to the species-e.9., under bark, in fungi, in flowers and fruits, and in galls on leaves or stems. Some of the best known and most injurious are not found in marked swellings or galls—e.g., the Hessian-fly maggots, so destructive to the stems of wheat and barley, and the maggot of the wheat midge, injurious to the young grain. The gall-making species are the commonest, and the galls inhabited by the maggot are to be found on many different plants, both herbaceous plants and trees. More than one species may attack the same tree, and it is not common for the same species to lay on plants far apart in relationship.
Of the gall-guats whose galls are found on trees, several are not uncommon on the Willow, the galls being found on the youngest shoots, or in 2-year-old and older twigs, or in the bud at the apex of the twig, or on the Willow leaves, or on Willow fowers (MacDougall, in Trans. Roy. Scot. Arbor, Socy., 1905, p. 210).
in May, and again in July. When the reddish-yellow maggots or strigs begin to suck they cause spindle-shaped swellings which spoil the withes. Pupation takes place within the swelling.
Prevention and Extermination.-From May onwards, twigs infested should be cut off and burned before the maggots come to maturity. A natural enemy of the gall-gnats is a beautiful little 4-winged Chalcis fly. This lays its eggs in the galls, and on hatching out the larvæ feed on the maggots of the gall-gnat.
2. The Small Osier Gall-Midge (Cecidomyia saliciperda).
Appearance. This midge is still smaller, being only to of an inch long, with blackish-brown body, and milk-white wings.
Life-history. During May the eggs are laid in a long string on the bark of young Willow-shoots (chiefly on Salix alba, S. viminalis, and S. acutifolia), and in June the reddish-yellow maggots eat their way inside. From July till the following April they bore irregular vertical galleries, which cause spindle-shaped nodes or swellings that ultimately become scabby from the bark bursting. Pupation takes place within these nodes.
Prevention and Extermination as for C. salicis.
3. Cecidomyia heterobia (Fig. 167) in 1903 and 1904 caused much damage to Osier-beds at Castlecomer (S.E. Ireland) by attacking the best varieties of Salix triandra (Black Norfolks, Black Mules, and Spaniards), and forming galls on the terminal shoots (see Trans. Roy. Scot. Arbor. Socy., vol. xviii., 1905, p. 211).
4. The Beech Gall-Midge, Cecidomyia (Hormomyia) fagi, produces the long, hard, conical green and red galls often to be seen on the upper side of Beech leaves.
Appearance. This midge is only about of an inch long, and has a blackishbrown body with flesh-coloured abdomen, and brownish wings with grey hairs.
Life-history. The midges fly in April, when the female pierces holes in the tender cuticle on the upper side of the young Beech leaves, and lays an egg in each hole. Above the puncture the green cone containing the maggot forms, and gradually hardens and changes from green to deep red in colour. About October it drops to the ground, and early in the following spring pupation takes place within the gall lying on the ground, the midge emerging in April to pair and reproduce itself.
Prevention and Extermination consist in collecting and burning leaves infested.
The species found on conifers include the Pine Gall-midge (Cecidomyia brachyntera), the Spruce Gall-midges (C. picece and C. abietiperda), the Larch Gall-midge (C. Kellneri, Fig. 168), and the Yew Gall-midge (C. ta.ci); but none of these does any serious damage.
V. Half-winged Insects (Hemiptera).
The metamorphosis of Hemiptera (as also of Orthoptera) being incomplete, the larvæ which hatch out of the eggs are already more or less like the perfect insect. They moult their skin several times, the last moult freeing the wings of the perfect insect. Any definite larval or pupal stage is wanting Two families of this order are of special interest to the forester—Plant-lice (Aphide) and Scale-insects (Coccide), which are both very often destructive.
A. Plant-Lice (Aphide) form a very destructive family. The Aphide emit honey-dew, for which some kinds are regularly milked by ants. This they extract from the sap of the leaves, and afterwards eject it in a purified state. Plant-lice possess an extraordinary power of multiplication; hence they easily become very destructive to any kind of plant they attack.
Aphides with membranous wings, often wanting in the female ; antennæ long and thread-like or silky; head usually with a well-developed proboscis ; legs generally long and thin. Generation many times within the year, and very prolific.
The genus of importance in woodlands is Chermes, various specific forms of which infest conifer plantations.
The genus Chermes includes the plant-lice which attack Spruce, Larch, Pine, and Silver Fir. By means of their proboscis they prick the leaf-buds which then form galls, or they pierce and bend the needles, or they suck the sap from
Fig. 168. the bark. Whichever means of attack is employed, the tree gradually sickens and
may die off.
The Chermes infesting different conifers are not really different species, but only specific forms in the generation of one species (analogous to similar changes in the generation of the fungus Melampsora on Willow or Birch, and Chrysomyxa on Larch and Pine). Thus, from Spruce-galls a generation of lice may issue, which attack Larch, Pine, or Silver Fir, and look as if they were entirely different species.
There are three kinds of galls made on the Spruce by members of the genus Chermes-viz., C. abietis, C. strobilobius, and C. coccineus. In the case
a of the first two the plant-lice from the galls migrate to the Larch, and sometimes to the Pine, where the next stages in the life - history are passed. C. coccineus is also thought to have a migrant generation, which moves from the Spruce to the Silver Fir and becomes the woolly aphis C. picea.
Much remains to be learned about the difficult life-history of these insects, but the following details are well established as to the common C. abietis :Small, wingless, grey-green aphides, which have Galls produced on a twig by the Larch passed the winter on the Spruce, and which may be Gall-Wasp (Cecidomyia Kellneri) found under cover of cotton-like threads, undergo
natural size. several metamorphoses in early spring and in warm a. Infected buds, rendered abortive. weather prick Spruce buds and lay their eggs. The bases of the leaves swell and a gall is formed (Fig. 169) like a small fir-cone, with the tops of the needles projecting all over the gall. The gall is hollow, and small yellowish lice hatch out there and wander into its various chambers before these become closed by growth. Development takes place inside, and in August the galls, at first green and now brown, open at the edges and winged aphides with black heads and yellowish bodies emerge and lay eggs on the needles of the Spruce under cover of downy threads. From these eggs wingless forms hatch out, which form the galls on the Spruce in the following
1 Interesting details concerning the genus Chermes and the results of spraying experiments made in East Lothian will be found in an article by MacDougall in Trans. High. and Agri. Socy. Scot., vol. xii., 1900, pp. 298-303, from which the following details are taken. See also Board of Agriculture Leaflets Nos. 34 (Woolly Aphis) and 104 (Aphides or Plant-Lice). VOL. II.