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So far as insects injurious to woodlands and nurseries are concerned, by far the most important of these orders are the beetles (Coleoptera) and the moths (Lepidoptera). In beetles the structure and the different segments of the mature insect are already externally distinguishable in the pupa, but in moths they are still masked or indistinct.
Extent of Damage.—Conifers are usually much more often attacked by insects than broad-leaved trees; and the attacks are generally far more destructive, owing to the want of recuperative power in consequence of the very much smaller reserves of starchy and nitrogenous substances as compared with deciduous trees. Pine and Spruce suffer far more than any other trees. Oak, Beech, Poplar, and Willow are subject to the attacks of more insects than Ash, Elm, Maple, Sycamore, Birch, or Alder. When Oaks have been almost entirely defoliated in spring by caterpillars of the leaf-roller oth, they can utilise their nutrient reserves and flush into leaf again in July ; but when caterpillars have devoured about four-fifths of the foliage of Scots Pine and Spruce, the recovery of the Pine is very doubtful, and the Spruce is almost certain to die. In Britain, the Larch often suffers severely from mining-moths and leaf-lice.
Damage to seedlings and young crops is always more serious to the individual plants than when older poles and trees are attacked. And attacks made in spring are more harmful than those in summer, near the termination of active vegetation, when buds have been formed and nutrient reserves stored up for next year's growth. And if foliage alone be attacked, the loss may perhaps amount only to temporary cessation of increment; when the roots and the cambium are badly injured, the sapling, pole, or tree usually wilts and soon dies.
The Prolificness of Injurious Insects varies greatly. Those which are most prolific are fortunately not the most injurious kinds. Hard winters are not fatal to most insects, and may even favour the increase of beetles (protected by their horny outer casing) by causing the death of many insectivorous birds. Naked larvæ, without hairy or protective covering, are sensitive to damp cold weather, and are killed off in large numbers when moulting their skins. Warm and dry weather, stumps remaining in the ground after heavy falls of timber, sickly crops of all ages, and badly-nourished, dominated, unhealthy trees with a weakly flow of sap, all form favourable breeding-places for injurious insects, which often increase with alarming rapidity in such material. Bark- and cambial-beetles lay their eggs in stems that have been thrown or broken by wind, or in winterfelled trees left lying till late spring or summer, or in those already rendered more or less sickly by attacks of other insects on the foliage. The large Pine-weevil seeks the stumps of recently felled trees as breeding-places, and feeds on neighbouring young plantations. Moths, in laying their eggs, usually prefer to attack backward crops growing on inferior soil, and therefore all the less able to recover from the injuries of the caterpillars. Such favourable breeding, and feeding-places form centres from which millions of noxious insects may easily spread to other woodlands. Hence the necessity for careful tending in all woods: neglect of any one part of a wooded estate is a danger to all of the
Natural Checks to Increase.—Wet, cold, raw weather while caterpillars are moulting their skins, and when beetles and moths are pairing, help to keep down the number of injurious insects. But before any of these can appear in unusual numbers, the balance of nature must somehow or another have been disturbed; and when left to nature the balance is usually restored in the course of about 3 to 4 years, after which bacterial and fungous diseases generally break out epidemically and then almost exterminate the insect. But to await this natural readjustment would usually mean entire loss of the timber crops. In mixed woods there are more insectivorous birds than in pure woods (of coniferous species especially), hence the tendency to excessive increase of noxious insects is checked by these their natural enemies. So far as other considerations permit (e.g., game preservation), all the natural enemies of injurious insects should be preserved. The chief of these are :
A. Mammals.-Bats, which devour cockchafers and moths ; moles, that destroy grubs and mole-crickets; shrew, hedgehog, weasel, pole-cat, stoat, badger, and fox, which devour large numbers of beetles and pupæ.
B. Birds. The most generally useful are (see chap. iii.) the cuckoo (the only bird that devours hairy caterpillars), the starling, flycatchers, titmice, tree-creepers, swallows, owls, and most song- birds ; then thrushes, blackbirds, rooks, gulls, plovers, the kestrel or wind-hover, buzzards, woodpeckers, sparrows and finches, crows, ravens, jackdaws and larks, which are only of minor utility.
C. Insects.—Predaceous and parasitic insects on the whole do far more than either mammals or birds to keep injurious kinds in check. The predaceous species prey, often both as larva and imago, on the ova, larvæ, pupæ, and imagines of noxious insects ; while the parasitic species generally lay their eggs on the ova and larvæ (less frequently on the pupæ or imagines) of the injurious kinds, on which the maggots feed when they hatch out. These useful insects of different kinds generally exist in woodlands in much larger numbers than might be expected ; and when the noxious kinds increase abnormally so also do these, their natural enemies, increase in equal or greater ratio, because they have then more food and more frequent opportunities of reproduction. The useful insects belong chiefly to the orders Coleoptera and Hymenoptera; then to the Diptera, Neuroptera, Hemiptera, and Orthoptera ; while the Lepidoptera contains no useful genus, just as the Neuroptera contains no injurious genus, so far as concerns the forester.
1. The Predaceous Species include, among the Coleoptera, the tiger- or sand-beetles (Cicindelida); the predaceous ground-beetles (Carabida); the dungbeetles (Staphylinida); the carrion-beetles (Silphida); the nitid or shining-beetles (Nitidulida); the thread-beetles (Colydude); the soft-beetles (Malacodermata, including the gold-beetles, Clerida); and the lady-birds (Coccinellida).
Note.-Clerus formicarius and Rhizophagus depressus are two of the most useful beetles in British conifer woods, because they prey on the Pine - beetle (Hylesinus piniperda).
Clerus formicarius is easily recognisable by its large black head, and black antennæ with red-brown tips, and its red-and-black thorax. Its abdomen is red at base and black behind, with two well-marked transverse white bands across the wing.cases. The dark-headed larva is rose-red, and consists of twelve segments, the first three of which have legs. The first segment has a horny plate above, and the second and third have each two small horny spots; the last joint is covered with a horny shield, and ends in two small knobs.
Both beetle and larva feed on larvæ, pupæ, and imagines of destructive treebeetles ; and the larvæ bore into the bark to hunt for prey. The beetle always seizes its prey behind the head. It is also the most useful of insects in keeping down bark-beetles (Scolytido).
Rhizophagus depressus is only about of an inch long, bright rusty-red, with finely-punctured lines on the wing-cases. The larva is about 5 of an inch
Continental experience has shown that during bad attacks of moth-caterpillars there is first a year of minor damage, then three years of excessive destruction, and a succeeding year of minor damage, but that about the fourth year parasitic insects (Ichneumonida), fungous diseases, and insectivorous birds increase enormously, and soon decimate and finally exterminate the moths.
CHIEF GENERA AND SPECIES.
These useful Coleoptera may perhaps bo recognised by the following brief descriptions :-
LIFE-HISTORY. 1. Tiger Small, slender; abdomen 6 segments,
With 6 feet The beetles are active (both run and fly), Cicindela campestris is common in woods on beetles first 3 fused ; jaws powerful, with 3 teeth
broad and prefer warm sandy places. The larvæ sandy soil. (Cicindelidos). logs long, slender, with 5 tarsal joints ; and humped at dig holes in the ground and lie in wait at the antennæ filiform, 11-jointed.
entrance to seize passing insects and worms.
Both beetles and larvae are predatory. 2. Ground Varying in size, often large ; abdomen 6 With 6 feet, Most of the larger beetles only run, and Carabus (of which 3 species are common, C.
beetles to 8 segments, first 3 fused ; jaws 1-toothed long, cylindri- cannot fly. They hibernate under moss, granulatus, C.catenulatus, and C. cancellatus)
foliage, bark, stones, &c., pair in spring, and and the tree-climbing Calosoma sycophanta
ovideposit in ground. Larvæ live in and on and C. inquisitor devour moth-caterpillars.
beetles and larvæ are predatory.
carrion, humus, &c. They hibernate, and philus maxillosus.
pair in spring. The larvæ feed throughout
summer, and mostly pupate in autumn. Both
beetles and larvæ are predatory.
also feed on carrion and humus.
punctata (with yellow wing.cases and 2 black
spots on each) feeds on caterpillars on Oaks.
Both beetles and larvæ are predatory, Rhizophagus and Pityophagus. The former
Note on opposite page).
ing 6. Thread Small, thin, and elongated ; abdomen With 6 feet, The beetles and larvæ live in the bark of Colydium, of which the species C. elonga
beetles with 5 or 6 segments, the first 3 or 4 long, sometimes trees or in rotting wood, and feed on the tum frequents old Oaks. Colydiido). fused ; legs with 4 tarsal joints; antenna
having, horny larvæ of bark-beetles.
beetles pair in spring and lay eggs in Telephorus (black, brown, or yellow boetles,
beetles appears in the autumn, and hiber are several species. The chief is C. formi.
nates below the bark. Both beetles and carius, which is of great use in keeping down (Clerido).
larvæ are predatory.
the Pine-beetle, the Elm- and Ash- bark
beetles, &c. (see Note on opposite page).
Small, smooth, semi-globular, with red With 6 feet, The beetles hibernate under bark, leaves, Coccinella septempunctata and C. bipunctata,
in clusters on plants. The larvæ feed commonest species (larvæ bluish-grey, with antennæ 10 or 11 joints, very short and
warty knobs or
till July or August, then pupate, hanging 4 to 6 yellow spots). The genus Halyzia has clubbed.
from leaves for about 14 days. Both beetles some species only found in woods, and chiefly pitted. and larvæ prey largely on plant-lice and mites. in coniferous woods.
and clubbed. 5. Shining Small, and oval or oblong; abdomen
ed Coccinellide). abdomen with 5 segments; tarsi 3 joints
With 6 feet, long,
long; the head and prothorax are reddish, and all the other segments whitish above and reddish below. The last segment of the body is red-brown, with two knobs above and a small motor-appendage below.
Both beetle and larva live below the bark in Pine and Spruce, crawling along the tunnels bored by the injurious insects, and feeding on their larvæ and pupe. It also preys on the larvæ of longicorn beetles.
Among the Hymenoptera, the digging or fossorial wasps (Sphegidæ) are both predatory and parasitic. They kill or deaden caterpillars, beetles, plant-lice, &c., with their stings, drag them to their holes, and lay eggs inside them. Wasps in general (Vespidæ), and especially the hornet (Vespa crabro, itself injurious to Ash), prey on moths and flies; while ants (Formicidæ), which live in colonies of thousands within their breeding-mounds, devour an immense number of all sorts of injurious larvæ.
Among the Diptera, the predatory flies (Asilidæ) breed principally in sandy localities, and boldly attack and suck the vital fluid of many other kinds of insects ; while the leech-shaped larvæ of the humming- or hover-flies (Syrphidæ), commoner in orchards than in the woods, kill plant-lice by sucking their juice.
Among the Neuroptera the scorpion-fly (Panorpa), the camel-necked fies (Rhaphidia), the gold-eyed fly (Hemerobia perla), and the ant-lion (Myrmeleon), are all useful in preying on noxious insects (though the last also devours useful ants); and several among the dragon-flies (Libellulidæ) as larvæ, nymphæ, and imagines (and especially as perfect insects) kill many noxious insects, even including moths ; while among the Hemiptera, scaly and other bugs (Pentatomida and Reduviida) prey on plant-lice. But the predatory genera and species contained in these last two orders are nothing like so numerous or useful as those in the first-named three orders.
2. The Parasitic Species are certainly the chief enemies of noxious insects. The most important are the ichneumon-wasps (Ichneumonidæ) of the Hymenoptera, and the parasitic-flies (Tachininæ) belonging to the Muscido family of the Diptera.
The Ichneumonidæ vary in size, but are usually long and thin. The abdomen is often stalked, and the female has a long ovidepositor, consisting of a thin borer enclosed within two lateral sheaths. The wing-veins are few in number, and only form distinctly. closed cells when there is a submarginal vein. The antennæ are many - jointed, and usually thin, seldom clubbed. Tarsi have usually five joints. Larvae footless, smooth, generally white, soft, and tapering towards both ends. Pupa soft and white, with free limbs. Imagines usually fly from May till August, and generally lay their eggs in the larger species of larvæ of noxious insects. Ovideposition sometimes also takes place in pupæ, and occasionally in imagines; while minute species even ovideposit within eggs of injurious insects. Generation partly single, but often double, and sometimes manifold.
Of about 5000 known species of ichneumon - wasps, nearly 1000 are parasitic on noxious insects. By means of her long ovidepositor the female lays her eggs either singly for big species, or in large numbers for small species. The larvæ on hatching out suck the vital fluids of the hosts on which they are parasitic, and then eat their way out to the surface, where their cocoons often thickly stud the dying caterpillars. As the generation of the Ichneumonidæ is sometimes manifold, they multiply enormously when the number of hosts is large. The infested larvæ then become even more voracious than before to provide nourishment for their parasites, but die in the pupal state if they manage to live so long.
The Tachininæ, easily distinguishable from other flies by the rough brush-like hairs on their abdomen, are chiefly parasitic as larvæ on the larvæ and pupæ of moths and sawfies. The principal species is Tachina (Echinomyia) fera, which destroys large numbers of insects. It generally pupates outside the host, in or on the ground, the pupa being round or oval, and brown or black.
Enormous swarms of noxious insects might possibly in course of time be suppressed by Ichneumonidæ and Tachinince, but the normal balance of nature existing before any plague of insects breaks out is usually restored naturally by epi. demic diseases produced among the larvæ by Botrytis, Micrococcus, Isaria, &c.
Prevention of Insect attacks. -As injurious insects increase most in backward or sickly crops, careful clearing and weeding of young plantations, and thinning and tending of all older woods, are necessary to remove suppressed, diseased, or damaged plants or trees before they become breeding-places for noxious insects. Such measures are all the more necessary in plantations made on land not properly drained, or where the soil and situation are not well suited to the kinds of trees that have been planted. Danger from insects is least in mixed woods of broad-leaved species and conifers (wherever possible, under given conditions of soil and situation); and it is greatest in pure coniferous crops, or those consisting of conifers only. All woods and plantations should be inspected frequently, and thinnings should be made regularly. Dead branches and débris of thinnings should be removed, and not left on the ground. Fallen branches and small thinnings are often left to rot on the ground, although such dead and decaying material forms breeding-places for many injurious insects.
Damage done by frost, snow, ice, wind, or fire should be rectified as far as, and as soon as, may be possible ; and damaged timber should be barked or removed from the woods as soon as convenient, to prevent bark- and cambialbeetles breeding in them. It is best to fell conifer crops in summer, and at once bark them (see Fig. 136). Trees should be felled as low as possible, and the bark peeled from the stumps. But when winter fellings have to be made, then some of the logs should be left here and there with the bark on as decoy stems. These should be peeled in the following May, and the bark burned to destroy the broods of bark-beetles they contain.
It is not at all unusual to find that the trees felled during winter are left in the wood or its neighbourhood till far into summer. Each Scots Pine that is so left attracts hundreds of Pine-beetles, besides other noxious insects, and in the month of June and July sends forth thousands upon their work of destruction. Even in woods that are regarded as being well managed this state of things may be seen, and the whole might be almost as easily prevented as permitted. When a timber sale is held in spring, the buyer is often allowed his own time to lift the wood, provided he has paid for it, whereas he ought to be bound to have it removed not later than the 15th of June, or otherwise to cause it to be peeled before that date. The same applies to timber designed for estate work. It is, of course, not enough simply to cart the wood to the sawmill and there allow it to lie. Such a course would do nothing to reduce the number of Pine-beetles. It must either be peeled or cut up, and in the latter case the slabs should be burned or peeled. If it can be managed, it is well to delay stripping the bark till near the middle of June, for there is but little danger of the young beetles escaping before that time, and by so doing time will be allowed for a larger number of beetles to utilise the trees for breeding purposes, and thus they and their larvæ can be destroyed in much greater numbers (Somerville, in Trans. High. and Agri. Socy., Scot., 1891, p. 41).
Wherever a market for firewood makes it possible to grub up coniferous stumps, this should be done immediately after the fall of the timber. No suitable breeding-places are then offered to the dangerous weevils and cambial-beetles. Even palings made of unbarked Larch, Spruce, or Pine poles will show how many beetles find breeding-places in the bark and cambium.
At the same time, measures should be taken to preserve and increase the natural enemies of injurious insects by encouraging the presence of insectivorous birds, and especially in dark, sombre, coniferous woods, where they occur in far fewer numbers than in broad-leaved and mixed woods. And among insectivorous birds none deserves encouragement so much as the starling, for whose protection