Sidebilder
PDF
ePub

Flies having wings with many veins; fore-wings with one or two well-defined long bays near apex, and three or four rhomboidal bays at edge ; antennæ filiform or brush-like ; prothorax short ; abdomen consisting of 8 segments ; female with movable ovipositor. Larvæ very active, with 8, 18, or 22 legs, usually bright in colour, and moulting several times. Pupa soft ; pupation in an obloug or oval and leathery cocoon. Generation generally double, and sometimes treble in a year.

1. The Pine-Sawfly (Lophyrus pini).- This is one of the most destructive insects among conifer crops throughout Britain (see Fig. 165). It chiefly attacks the leaves of the Scots and Austrian Pine, devouring them down to the bottom of the leaf, and completely disfiguring the branches. The damage is done by the caterpillars, numbers of which appear on one branch, and then proceed to others until the tree is almost or entirely defoliated. They attack the trees about May, and continue until September (second brood). Their power of reproduction is wonderful ; and were it not for useful and predatory insects which feed on them, and fungous diseases which become epidemic among the caterpillars, they would soon entirely destroy extensive woods. They very often attack young plantations, and frequently confine themselves to one spot at a time; but they are also found upon older trees.

Appearance.—The wings of the female fly have a span of about s to of an inch. The antenna are short and bristle-like. The head is black, the body yellowish with black spots on thorax and abdomen, and with three black rings or sections on the abdomen. The male has a span of about 3 of an inch. The body is black, the legs yellowish, and the antennæ combed or double - feathered. The dirty yellowish-green tailed caterpillars have 22 legs, and a brown or black shining head, and black dots on the segments of the body. When touched they raise their heads like a snake. They are about 1 inch long when full-grown. All parts of the fly are already recognisable in the pupa. The pupal state is passed in a tough, oval, brownish-yellow cocoon formed in bark-fissures, or attached to twigs and needles, or under moss and dead foliage on the ground. The cocoon opens by a circular lid.

Life-history.—In dry warm seasons there may be two broods in the year. The sawflies appear late in April or during May, when the female lays about 120 eggs on the edges of the Pine-needles, after lacerating them with its saw-like ovidepositor. From 10 to 20 eggs are laid in one needle, each wound being sealed up with frothy slime. The caterpillars hatch out (in about a fortnight) in May and June, and collect in clusters on the whorls of young Scots Pine, especially near rides and green lanes, or on suppressed and dominated stems. At first the tiny caterpillars usually feed in pairs on each needle, eating the edges and leaving the midrib standing ; but as they get stronger and nearer their full growth (when they are about 1 inch long) they eat all the needle except a short stump. Fortunately they only feed on the old leaves, and not on the new ones of the last year. They moult several times, and leave their cast skins sticking to the twigs. In July they pupate in their peculiar leathery cocoons formed between bark-scales, twigs, and needles, &c. In two or three weeks the sawflies emerge, pair, and lay eggs for a second brood. The caterpillars of this second brood hatch out in August, and often feed until well on in autumn, when they descend and form cocoons under moss or dead leaves on the ground. Within these they hibernate as larvæ, and only become pupæ in the following spring, about three weeks before emerging as sawflies.

The above life-cycle is not constant and regular. Sometimes single broods remain for a whole year or longer in the cocoon before emerging as sawflies.

Prevention and Extermination.—Numerous animals prey on the Pine-Sawfly. Insectivorous birds devour the larvæ, and should be encouraged to breed in nestingcases ; squirrels and mice extract the larva or pupa from the cocoon ; and swine eat the caterpillars, but not the cocoons. Ichneumon-flies, parasitic flies (Tachinince),

[merged small][graphic][graphic][ocr errors][ocr errors][merged small][merged small][merged small]

and beetles (Carabido) attack the caterpillars, which are also rather sensitive to frosty, cold, damp, or unseasonable weather. Although it is not always easy to detect their dirty yellowish-green colour against the Pine- branches and

1 When the pupa has been devoured by an ichneumon-fly, the cocoon-lid does not open, but the fly issues through a small hole in the centre of what would have been the lid, thus O.

leaves, the caterpillars, when collected in clusters, may be crushed by pulling the gloved hand firmly along the twigs from below upwards (so as not to damage the leaves), or perhaps even more expeditiously with a Büttner's double brush (see p. 19). Or the caterpillars may be shaken down, if the poles are strong enough to stand this. It is difficult to collect cocoons below moss on the ground, because, being small, they are not easily noticed. In parks and gardens ornamental trees may be freed from this pest by spraying with hellebore wash (1 oz. to 2 gallons of water), or arsenate of lead or Paris-green (1 oz. to 15 gallons of water); but these poisons require careful handling. Small ornamental plantations may also be cleared by sprinkling naphtha on the larvæ with a brush.

Board of Agriculture Leaflet No. 103 deals with L. pini and L. rufus.

2. The Fox-coloured Sawily (Lophyrus rufus) is often found working along with L. pini. It did great damage in Argyllshire in 1890, and in Ross-shire in 1898. Scots Pine 2 to 6 ft. high are most subject to attack.

Appearance.—The female is reddish brown or reddish-yellow, with black spots on the thorax, and with yellow to reddish-brown legs; the male is smaller, and glossy black, but the first abdominal ring and the feet (except the claws) are red or reddish-brown. It flies in August and September.— The larvæ are dusky greenish-grey with black heads, a pale longitudinal stripe along the back, and a dusky line with a pale one on each side of it above and below; the spiracles are placed in the lower pale line. The sucker, feet, and under side of the body are pale-green. When full-grown they are rather more than } an inch long. It has the same habit as L. pini of rearing its head when disturbed.

Life-history.The larvæ hatch out about the end of May, and feed till middle or end of June, when they pupate in an oval, pale yellowish-brown, parchment - like cocoon, formed amongst needles and heather, or in the earth, &c., beneath the trees. Like the common Pine-Sawfly, this species also collects in colonies, and two are usually at work on each needle. The cocoon is lighter in colour and not so tough as that of L. pini. On the Sawflies emerging, they pair, and lay their eggs in August and September in the needles, in the same way as L. pini, where they remain all winter and hatch out in May.

Preventive and Exterminative Measures.-As for Lophyrus pini.
B. Wood-Wasps (Siricide or Uroceride).

Wasps long and completely veined; body long and cylindrical ; antennæ filiform or brush - like ; abdomen consisting of nine segments ; ovipositor elongated. Larve 6-legged, soft, white, cylindrical. Pupæ soft and white. Generation never less than two years, and often more.

Wood-Wasps (Siricide)-among the largest and most striking of Hymenoptera—are technically injurious, as the deep borings of their large larvæ often honeycomb stems, and thus render timber unfit for many technical purposes. The two species common in Britain attack coniferous timber, but a third also bores into the Willow.

Life-history of Sirex Species.--The female, by her long and strong ovipositor, bores quickly (in about four minutes) through the bark and into the sapwood of sickly trees and felled or windfall timber, and lays her eggs singly in the wood of the stem, usually at the depth of the second or third last annual ring of wood. Rotten wood it avoids. The wasp only lives for about six or seven days, during which time it bores and lays from 100 to 150 eggs, one egg being laid in each borehole. The round whitish grub, which is over an inch long when full-grown, has three pairs of small thoracic feet, strong biting jaws, and a characteristic spine on

each eye.

the last segment. It bores first of all in the softer sapwood, but generally works in deeper and hibernates. During the second year it is stronger and bores still deeper, and again hibernates. In the spring following the second winter it bores its way towards the surface of the trunk, and forms a pupal chamber in the sapwood at a depth of about 1 an inch usually. Between July and September the complete wasp bores a round hole straight through to the surface, and emerges for pairing and reproduction. The borings are circular, like the larvæ.

Their generation is never less than two years, and is often longer, for the wasps occasionally emerge from beams and scantlings some considerable time after they have been converted and worked up.

1. The Giant or Yellow Wood-Wasp (Sirex gigas) bores into Spruce, Silver Fir, Larch, and rarely Pine. It is blackish in colour, with a yellow patch behind

In the female, which may attain 1} inch in length, the first two and the last three segments of the abdomen are yellow, while in the smaller male all the rings of the abdomen are reddish-yellow, except the first and the last, which are black,

2. The Steel-blue Wood-Wasp (Sirex juvencus) bores chiefly in Pine (Fig. 166), but also in Larch. The female has a steel-blue body with reddish legs, and varies up to 1} inch in length, while the male is usually smaller in size, and with yellowish-red rings from the fourth to the seventh segments of the abdomen.

Preventive and Exterminative Measures of any practical use against woodwasps consist merely in at once cutting out any sickly or damaged stems likely to serve as breeding-places, and in keeping the woods properly thinned and supervised.

The handsome Ichneumon, Rhyssa persuasoria, is parasitic on Sirex juvencus larvæ. Rhyssa bores with its long ovipositor into a tree where the wood-wasp larvæ are at work, and lays an egg in the tunnel of the wood-wasp. When the Rhyssa grub hatches, it pro. ceeds to feed on the grub of the wood-wasp, which is thus destroyed. The ovipositor of Rhyssa is very long, and sometimes this Ichneumon is captured fixed to the tree, from which it has been unable to withdraw its ovipositor (MacDougall, in Trans. Roy. Scot. Arbor, Socy., vol. xv., part iii., 1898, p. 312).

3. The Willow Wood-Wasp, Sirer (Xiphydria) dromedarius, sometimes bores into Willows in the fen district. The wasps begin to emerge from the wood in June. It is a rare insect, and it is not yet known if it attacks sound wood, or merely sickly and moribund trees.

C. Gall-Wasps (Cynipide).

Gall-wasps, with fore.wings having six or eight bays, with one long bay at apex and two or three rhomboidal bays at edge—but sometimes with aborted wings only, or none at all ; abdomen compressed and shorter than wings (when present); antennæ filiform ; male usually much smaller than female. Larvue generally thick and fleshy, smooth, whitish, and incurved. Pupa thick, smooth, and whitish. Several gall-wasps are parasitic on the larvæ of injurious insects.

The Cynipidæ are of great interest in having, like many fungi and also the plantlouse Chermes, an alternative generation. Winged insects develop a wingless, hibernating, sexless form, which reproduces itself parthenogenetically (“ virgin birth”), and whose brood reproduces the original form, in which the pairing of male and female is necessary before ovideposition. Thus, Cynips fecundatrix produces C. pilosa, whose virgin-brood reproduce C. fecundatrix ; Cynips terminalis breeds C. aptera, which, after forming galls on the roots of Oak-trees, lays virgin-eggs from which C. terminalis is reproduced. Here C. terminalis, produced at the tips of the crown twigs, migrates to the roots to lay her

1 This spine and the cylindrical form of the grub distinguish it from the flattened and spineless grubs of the long-horn beetles (Cerambycida, see p. 99).

eggs, while the sexless C. aptera migrates back to the summit of the crown to reproduce the sexual form there. A winged sexual generation thus alternates with a wingless parthenogenetic generation to complete the full life-cycle.

The Gall - wasps chiefly attack Oaks of different species.

There are many species (some 50 being said to occur on the Oak), and they are all provided with an ovipositor, which is entirely concealed except at the extremity. They bore holes with this into the buds, leaves, or small

[merged small][graphic][ocr errors][ocr errors]

Borings of the Steel-blue Wood-Wasp (Sirex juvencus) in Scots Pine-natural size.

a. Larval borings, partly filled with bore-dust (B).
b. A circular exit-hole.

branches, and lay their eggs there much in the same way as is done by the Sawfly. Some time afterwards, when the young larvæ hatch out, the disturbance they cause leads to “oak-galls ” being formed. These enclose the larvæ, which remain and feed within the gall until they emerge as wasps.

The Oak-Apple Gall-Wasp, Cynips (Dryophanta) querci, is the chief and the commonest of Gall-wasps that attack Oak foliage. It occasions the well-known large green and red “Gall-apples,” about as large as a cherry, found on the lower side of Oak-leaves.

« ForrigeFortsett »