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year. In such a case, where the life-cycle is confined to the Spruce, there are two generations (a winged and a wingless) in one year. In this whole life-cycle only females are found, so that the reproduction is a parthenogenesis or "virgin birth.”
But simultaneously with the above simpler life-history there may be a more complex life-cycle spread over two years, and consisting of five generations. Generation i. of this cycle passes, as before, the winter on the Spruce, and lays its eggs in spring. Generation ii. hatches out from these, lives at first in the cone-like galls, and ultimately becomes winged. Some of these winged summer forms migrate from the Spruce to Larch (and sometimes also to Pine), and deposit their eggs on the Larch foliage. This form is then known as C. laricis. Soon Generation iii, hatches out from these last-laid eggs, small yellow larvæ appearing which suck the Larch-needles for a short time in
autumn and then crawl to the bark, where they hiberFig. 169.
nate in fissures and under bark-scales. They become adult in the following spring and lay their eggs on the young Larch-needles. From these ova Generation iv. is hatched out in the forın of yellowish woolly aphides (the woolly form of C. laricis), which pierce and suck the middle of the Larch-leaves and make them crick or knee. They afterwards get wings and fly back to the Spruce, where they lay their eggs. Generation v. hatches out from these, and consists of both males and females, —males being thus produced for the first time in the life-cycle. After pairing, the fertilised ova of Generation v. are deposited and hatch out in late summer and autumn as the wingless females forming Generation i. above described, which pass the winter on the Spruce and produce the cone-like galls the following spring. —(MacDougall, op. cit.,
pp. 299-301.) Prevention and Extermination consist in collecting and burning as many of the galls as possible. Useful birds should be encouraged. Spraying with pure paraffin kills the lice after the eggs have hatched out, but does not seem to affect the eggs much (it also kills Pine-sawfly larvæ); but this scorches the foliage of Larch and Spruce, though they recover
during the summer, while Pine - leaves are not Cone - like gall produced by thereby damaged. Spraying in June with 1 lb. of
Chermes abietis on a Spruce soft soap dissolved in every gallon of water kills off twig-natural size.
the insects, but this wash also scorches the young
shoots of Spruce, though it does not seem to affect the Larch or Pine foliage. A milder spray, less likely to damage the foliage, and also fairly satisfactory, may be formed of an emulsion of hard soap } lb., soft water 1 gallon, and paraffin 2 gallons. The soap is dissolved in boiling water and at once added to the warm paraffin, the two being churned till they form a buttery mass. This stock
be diluted with from eight to ten times its volume of water.
1. The Spruce-gall Aphis (Chermes abietis) is common all over Britain, and does not seem to receive the attention it deserves (Fig. 169).
Appearance.—This louse is only about it. of an inch long, and yellowish-green to light-brown, with (when present) white wings and whitish bloom.
Life-history.—It bores into the buds of young Spruce of 10 to 20 years old, and lays about 20 to 30 eggs on the edges of the scales. On hatching out the
1 It is only by collecting and destroying the galls of C. abietis on Spruce, that the Larch aphis (C. laricis) can possibly be exterminated.
young lice suck the sap, and a cone-like gall (false-cone) is formed by the compressed needles. This false-cone (Fig. 169) is at first green, but turns ruddy-brown. After it has opened at the edges of the leaf-bracts to let the fully developed louse emerge in July and August, it becomes dry, hard, and dull-brown. Side-shoots are much more attacked than leaders ; and this is fortunate, because punctured shoots become curved and bent. If badly attacked, the poles soon become sickly and unhealthy.
Prevention and Extermination consist in cutting off the galls, and drying and burning them.
Fig. 170. 2. The Larch Aphis (Chermes laricis), though small, occurs in such large numbers sometimes that it does great damage to plantations (Fig. 170). According to Loudon, it was first observed on the Larch at Raith (Fifeshire) about 1785, and it appeared in the Athole plantations in 1795. It generally attacks unhealthy trees and plantations, and is seldom found in healthy and thriving woods. Its attacks might be far less frequent and destructive if the Larch were grown in mixed woods. The aphis usually appears, and most frequently on trees of 10 to 20 years old, about the beginning of May, and soon seems to be enveloped in a whitish coating like filaments of cotton. The lice are sometimes found in such numbers as to make the trees look white all over. During the whole season they go on producing eggs.
Appearance.-A small, oval, purplish-black or blackish-brown louse, less than of an inch long, Damage done by the Larch-Bug or and covered with fluffy white woolly down, but be
Larch Aphis (Chermes laricis)
-natural size. coming dirty-green when it gets wings. This is
a. The lice at work sucking out merely the sexless form of Chermes abietis. It has
the sap from the leaves, which a long bristle-like sucker, with which the louse
consequently get bent at these penetrates the Larch-needles to feed on their sap.
places. Life-history.—Towards the end of April this sexless form of aphis lays 40 to 50 eggs on the twigs. On hatching out the young lice scatter themselves over the needles, and do not live enclosed in a gall. At first very minute, they grow quickly, and get covered with whitish woolly down from the honey-dew exuded from their pores. When these swarm in large numbers, the trees look as if covered with scattered snow-crystals. From about June till August they get wings and fly about and lay eggs, from which further broods are produced till the autumn. This aphis may attack Larch of all ages, but is most frequently found on poles of 10 to 20 years of age. It is often also troublesome in nurseries.
Prevention and Extermination.—By far the best measure is to collect and destroy the galls of the Spruce aphis (C. abietis), the sexual form of the insect. Otherwise spraying with insecticides, such as soft soap and paraffin emulsions, &c., is the only remedy to try and kill off the pest while in its sexless form on the Larch (see footnote on opposite page).
1 Where this aphis is abundant, the formation of mixed woods of Spruce and Larch must provide the most favourable conditions for breeding this pest on these two trees.
3. The Red Spruce Aphis (Chermes coccineus) is smaller than C. abietis, and brownish-red with a white downy spot behind. Its life-history is similar, but its development is quicker, so that three to four generations in one year are common. The false-cones are much smaller than those of C. abietis, and are usually found at the tip of twigs without any shoots above them. Green at first, they afterwards turn red and then brown.—The galls should be collected and destroyed.
Various aphidæ are found on broad-leaved trees, where they do far less harm than on conifers. These include The Elm-gall Aphis (Tetraneura ulmi), causing the formation of small club-like galls about the size of a pea or bean on the upper side of Elm-leaves. The Elm-blister Aphis (Schizoneura lanuginosa) produces large hairy galls on the tips of Elm shoots and foliage. During the summer these blister-galls, at first green and red, but turning brown as they dry and harden, contain a sticky viscous fluid in which the lice live from June till August. The Ash Aphis (Chermes fraxini) produces canker-like spots on the bark of the Ash. The Thorn-Fly (Aphis cratægi) is found on young Thor. hedges, and on Thorn plants in nurseries.
B. Scale or Blight Insects (Coccide).—It is to this family that the felted scale of the Beech belongs, and other scales common on many kinds of woodland trees, the Scale being a small insect found clinging to the bark.
The females are minute imperfectly formed insects, and never have wings. They attach themselves to bark or leaves, and cover themselves with a felty scale, beneath which they feed on the sap, lay eggs, and die. The adult males are usually winged, but live only a short time.
The Beech, when old and weakly, or after severe attacks of the Beech weevil (Orchestes fagi) or heavy mast years, is apt to be attacked by the felted Beech - Scale (Cryptococcus fagi)—formerly erroneously called the Beech Aphis (Chermes fagi)—but the attacks seldom take place on trees in vigorous growth. When badly attacked, the bark loosens and the tree dies. Where the bark has been punctured small round galls form, which swell and burst, while the gall-tissue in the cortex becomes brown and dies. The canker-fungus (Nectria ditissima) very often attacks Beech-trees at the same time as this insect.
Trees badly attacked usually die within a The Beech-Scale is now a serious pest, and is increasing. The adult female is lemon-yellow, and about sth of an inch long. The male has not as yet been discovered. The best preventive and exterminative measures are brushing with paraffin emulsion, or winter-washing with caustic alkali wash. See Board of Agriculture Leaflet No. 70 (Winter- Washing of Pruit-Trees).
Young Ash - trees after heavy thinning, or when growing on light gravelly soil, are exposed to attacks of the Ash Coccus (Apterococcus frarini), while both Ash and Willow suffer from attacks of the felted White-Scale Coccus (Chionaspis) salicis. In this latter case, where soil or situation is at fault, the trees affected are never likely to thrive; but where they appear to have been injured by too severe thinning, the case is different. Such trees may be strengthened by scraping off the outer bark with the scale, or by applying a caustic alkali solution—or a strong emulsion made of soft soap (dissolved in hot water) and paraffin, or of pint paraffin to 1 gallon coaltar—with a hard brush to destroy the eggs.
The scale is easily distinguished by the appearance of the insect upon the surface of the bark. It presents itself as very numerous small white felty spots, and if a stone be taken and drawn roughly along the bark, it will be tinged red with the blood of the insects. If scraping and washing have not been effective, the young insects may be sprayed with paraffin emulsion in May, when they are seeking their feeding-places.
A black variety of Scale is produced on the bark of branches and twigs of young 5. to 15-year-old Spruce by Coccus racemosus, and on Silver Fir by Coccus corticalis.
VI. Straight-winged Insects (Orthoptera).
The insects of this order have (like the Hemiptera) only an imperfect metamorphosis. The wingless larva closely resembles the complete insect ; while the pupa only differs from the latter
Fig. 171. in having rudimentary wings, not strong enough to fly with. During all these three stages of development the insect crawls about and continues to feed. For the forester, the most important family in this order is the Crickets (Gryllidae).
Crickets with cylindrical bodies and strong fore - legs adapted for burrowing ; hind - wings folding lengthways and extending beyond the wing-cases, but often absent or aborted ; by rubbing the wing - cases together a chirping sound is produced ; some kinds with, and some without, a long ovipositor; antennæ long and bristly; head free and thick. Generation annual.
The Mole-Cricket (Gryllus gryllotalpa, syn. Gryllotalpa vulgaris).
The mole-cricket is for the most part carnivorous, and it is thus sometimes useful in destroying noxious insects, such as cockchafer-grubs and other larvæ.
The Mole-Cricket (Gryllotalpa vulgaris)But in forming its underground runs
natural size. it is also destructive of seedlings in
a. Imago when full-grown (natural size). nurseries, for in making its runs (of about 8. C. Larva at different stages of early
development. the breadth of a finger) it bites through all roots in its way, and makes seedlings and transplants wither and die. Damage of this sort is usually worst in Pine and Spruce seed-beds.
Appearance.—This insect is easily recognisable by its powerful hand-shaped fore-legs with strong claws, formed like a mole's for digging. The wingless larva resembles the fully developed insect, and there is no distinct pupal stage, gradual changes taking place at each time of moulting the skin. The pseudo-pupa (nymph) only differs from the perfect insect by having rudimentary instead of as yet completely developed wings; it crawls about and feeds like both larva and imago (Fig. 171). The cricket is 14 to 14 inch long, ruddy or dark-brown above, and lighter below; the short wing-cases, veined with black, do not cover the whole of the hind-wings ; abdomen with two long tail-processes ; fore-legs strong, and armed with claws for burrowing.
Life-history.—Pairing takes place in May and June, when both the male and female chirp underground. The female lays up to about 150 to 200 pale-yellow eggs, about the size of hem pseed, inside a nest-hole hollowed out of a clump of earth as big as a fist, cemented with slimy saliva, and formed about 3 to 4 inches deep in the ground.
The larvæ hatch out in July, in about two to three weeks. At first whitish, but turning brownish, and then darker later on, they soon wander throughout the soil and feed. In October or November they hibernate under ground (after three moults), and in all moult their skin four times before the “nymph” or masked form of pupa finally develops into the perfect insect in the following May or June. The female cricket keeps careful watch over the entrance-hole of the nest containing her eggs, but at the sanie time also actually devours a large number of her progeny.
Prevention and Extermination.—The eggs may be taken from the nests in June and July, if these be found by tracking the converging runs that go deeper into the ground as they get near the nest. The runs are marked by air-holes in the soil, about the size of holes made with the point of a walking-stick, and by the withering and dying seedlings ; yet it is not easy to find the nests, as they often lie about 10 inches deep.
Laying out ordinary flower-pots (with the bottom-holes stopped with cork) about 6 feet apart, and stretching lath-like pieces of wood from pot to pot, sometimes traps crickets wandering after nightfall during the pairing-time. Being forced to go along the edge of the lath they tumble into the pots, where they can be collected and destroyed.
When the mole-crickets are chirping and calling to each other while pairing in May and June, they can often be caught by cautiously approaching the place whence the chirping comes, and then suddenly dislodging the insect by lifting a clod with a hoe.
As the freshly-formed runs are easily distinguishable after rain, a spoonful of petroleum, oil, or tar may be poured into them, and then water added until the runs are full. Whenever the cricket is touched by the oil it at once ascends for fresh air, and then can easily be caught and killed.
Mites (Acarina), a low order of the Arachnida, constituting a sub-class of the Arthropoda, and including scorpions, spiders, mites, &c., include at least one species which often infests the bark and leaves of many trees, the so-called Red Spider, Spinning Mite, or “Fire-blast Insect" (Tetranychus telarius), found under the bark and upon the leaves of Elm, Poplar, Lime, Spruce, and Scots Pine. It causes the leaves of trees attacked to turn brown (“fire-blast”), and generally to fall off altogether; so that in this way many trees are weakened and their bark injured by these pests. They abound chiefly in dry warm weather, and are seldom to be found during a wet season.
This Mite spends the winter in crevices of wood (living or dead) and under stones, then emerges about middle of May, and attacks leaves of woodland and orchard trees and shrubs, sucking the sap, and clogging the pores with a finely-spun web. Within this web it lays its eggs, which soon hatch out and multiply quickly. The young are 6-legged at first, but afterwards have 8 legs. In hot dry years the larvæ collect like red dust on the lower side of the leaves ; but heavy rain kills them in large numbers.
Prevention and Extermination. - Washing with a strong emulsion of soft soap and paraffin, or spraying with a lye formed of boiling 1 lb. of flour of sulphur and 2 lb. fresh lime in 4 gallons water, sprayed on from time to time till the pests are destroyed. 21 lb. of sulphide of potassium (liver of sulphur) dissolved in 100 gallons of water is a good spray on hops. See also Board of Agriculture Leaflet No. 41 (Red Spiders).
Two other genera of Red Spiders (Bryobia, Tenuipalpus) are destructive to fruittrees and berry-bushes.