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Injurious Insects

( Originally Published 1915 )



General observations on Insects and their feeding habits. Insecticides and their use. Earwigs. Destructive Caterpillars of Cabbage Butterflies, and the Currant, Winter, and Codling Moths. The Gooseberry Saw-fly. The Pea Thrips. Wireworms.

The next class to be considered is that of the INSECTS viewed from our present standpoint, they are of greater importance than the whole of the rest of the Animal Kingdom. Insects can be readily recognised by the presence of a pair of antennae or feelers, six pairs of legs, and the division of the body into head, thorax and abdomen. Either one or two pairs of wings are almost always present in adult Insects. The most remarkable feature in their life-history is the fact that they pass through a series of changes which we term metamorphosis.

On hatching from the egg, the first stage is the larva, which is succeeded by that of the pupa, and from the latter, emerges the perfect Insect. In many Insects, however, the pupa is absent, and then there is a gradual growth from the larva to the perfect Insect. Larva: are variously known as caterpillars, maggots, or grubs, and, being exclusively concerned with feeding, they are as a general rule more destructive than the perfect Insects. The pupa or chrysalis is purely a resting stage, no food is taken and during this period the organs and tissues the future insect are gradually built up. It is of great importance to understand the method by which an Insect feeds, whether it be in the larval or adult condition. Almost all remedial measures have to be based upon this factor. We can recognise three methods by which Insects feed : (1) By means of the biting action of their jaws. (2) By means of sucking, and in this case the mouth organs arc modified to form a auctorial apparatus. (3) By means of a combination of both of these methods.

Various chemical substances are used for destroying Insects and are known as Insecticides. Biting Insects are mainly destroyed by poisoning their food, while sucking Insects can usually only be destroyed by using contact insecticides—those which kill by means of surface contact. This method is adopted for the simple reason that we cannot poison the food when it consists of the internal juices of plants, as is the case with sucking Insects. When using insecticides the grower should proceed with caution until experience has been acquired. Also contact insecticides are liable tot injure the foliage under certain conditions. Many of the insecticides that have been recommended are inefficient, while others need scientific testing to fully determine their value. Insecticides are artificial methods of control, but it is necessary to point out that the utilisation of natural methods of control should not be neglected. Natural methods consist in the preservation and increase of those organisms which are directly beneficial to man, in that they destroy the injurious forms of animal life. This latter method will be dealt with in a subsequent lecture.

The first order of Insects that I shall deal with is the O rtho ptera, and the only member thereof that concerns us is the common Earwig (Forizcula auricularia) (Io). It is a good example of an Insect which undergoes incomplete metamorphosis. The adult Earwig lays its eggs in a group either beneath stones or in the soil. During the incubation period she guards the eggs until they hatch, exhibiting in this respect a rudimentary instance of parental care. The young larvae are minute white creatures, with very slender forceps and no traces of wings. After they have passed through several moults rudiments of wings appear, and subsequent growth chiefly consists of an increase in the size of the Insect and the gradual development ,of the wings. No pupa or resting stage is passed through. The Earwig is almost exclusively nocfaunal in its habits, and has been very seldom observed to use its wings in flight. During the day Earwigs hide away beneath the soil, among vegetation, under stones, bark, and in other dark situations. They cause some amount of harm to cultivated plants, particularly dahlias, but their diet may include animal matter also. Earwigs can be most readily got rid of by means of traps Plant pots tilled with straw or dead moss placed in an inverted position upon the ground, or upon stakes, are usually effective. The pots should be examined frequently, and the Insects shaken out into boiling water, or the straw and other rubbish containing them burnt.

The next order of Insects which concern us is the Lepidoptera. Their larvae are known as caterpillars, while the perfect Insects are recognised as Butterflies and Moths. Butterflies can be readily distinguished from Moths by their feelers or antennae terminating in a club or knob, while those of Moths taper off to a point. Furthermore, Butterflies are diurnal while most Moths are nocturnal. Lepidoptera are only injurious in the caterpillar stage; the adults feed entirely upon the nectar and juices of flowers which they imbibe by means of a flexible sucking tube, and never pierce or injure the tissues of plants. Four wings are present and they are closely covered with microscopic scales which easily rub off, revealing the transparent wing membrane beneath. The eggs of Butterflies and Moths are almost always laid on or near the leaves of the plants which are to, serve as food for the future caterpillars. Very few Butterfly larvae are injurious, and only two species need concern us, viz., the Large and Small Cabbage Whites (Pieris brassicae and P. rapae) (10). The larvae of the former species are partial to the outer leaves of cabbages, while those of the smaller species also attack turnips. Both kinds frequently devour the leaves of nasturtiums " and other plants. When very abundant hand picking of the larvae is the best method. The pupae occur on palings, walls, and similar objects in the immediate vicinity, but a quick eye is needed to distinguish them, and for this reason their destruction is not likely to very materially reduce their numbers. The eggs of the Large Cabbage Butterfly are pale yellow and laid in clusters on the under sides of the leaves of the food plants. Every cluster destroyed means the reduction of a whole brood of the larvae. The eggs of the Small Cabbage Butterfly are laid singly and, consequently, their detection and destruction is too laborious to be worth while. Insecticides are of very little value against these two species. Among Moths, the larvae of the common Currant Moth (Abraxras grossulariata) (4) are very destructive in that they defoliate currants and gooseberries. The Moth is conspicuously spotted with black on a white ground, and is on the wing during July and August. The larva is similarly conspicuous, being deeply spotted with black on an ochreous-white ground, with an orange coloured line along each side. It is prevalent at the end of the summer and hibernates during the winter among dead leaves, in chinks of walls, under bark, etc. During the spring it recommences feeding and turns to the pupa in May or June. The pronounced colouration of this larva renders hand picking a very easy and effective measure. In extensive infestations spraying with lead arsenate at the end of the summer kills large numbers of the young larvae through poisoning their food. If they are still abundant during the following spring the operation should be repeated. Since lead arsenate is a poison it must not be applied later than four weeks before the fruit is to be gathered. The grower will do best to utilise Swift's arsenate paste rather than prepare his own compound. From 8 to 10 ozs. of the paste mixed in 10 gallons of soft water is a suitable strength; weaker solutions, however, are often equally effective. The Winter Moth (Cheimatobia brumata) (4) does immense damage to the foliage of apple, pear, plum and cherry trees and is universally common in this country. The male is a thin-bodied brown Moth, measuring 1 1/4iin. across the expanded wings. The female is wingless and spider-like in appearance. The Insect occurs from October until the beginning of January, the eggs are laid on or near the bases of the buds, and the green larvae belong to the type commonly known as " loopers." They commence feeding upon the leaf buds, and then the flower buds, which they spin together with the leaves to form shelters Later on they attack the foliage and even the fruit. During June they are fully fed and pass to the soil, where they change to the chrysalis a few inches below the surface. The most effective measure is " grease banding " the trees. Strips of grease-proof paper, 6 to 8 inches wide, and sufficiently long to encircle the trunks, should be tied tightly with string above and below, and placed on the trees during the first week in October. The most suitable height is from 2 to 4 feet from the ground. The paper is to be well smeared with cart grease, which must never be allowed to become dry. To ensure this, three applications during the season are usually sufficient. " Tree tanglefoot" may be used instead of cart grease, and has the advantage of not requiring renewal during the whole winter. By means of this device the wingless females are trapped in large numbers as they crawl up the tree trunks from the soil. If the grease bands be retained until the end of March, large numbers of females of the destructive March Moth (Anisopteryx aescularia) (4), which are likewise wingless, also meet with a similar fate. If the grease bands are neglected many of the female Moths succeed in making their way up to the buds to lay their eggs. When the larvae are very abundant the only measure is to spray with lead arsenate, using an ordinary knapsack sprayer, except for very large trees, which demand a more powerful instrument. The spray should be distributed as a fine mist, as all that is needed is to render the leaves poisonous. It is not advisable to spray during blossoming, and spraying with winter washes is useless. The Codling Moth (Carpocapsa pomonella) (4) is one of the most important of apple pests, attacking many varieties besides the Cod-ling, and also pears. Those types such as the Russet and Nonpareil, in which the "eyes" are more or less closed, are less susceptible than the Blenheim Orange and many others. The perfect Insect is a pretty brown Moth with coppery reflections, and measures about from tip to tip of the expanded wings. It flies during June and July, laying its eggs singly on the young fruit, but occasionally it may select the leaves. They hatch just about the time when the petals have fallen and the fruit set. The young larvae are whitish, pale yellow, or often pink, with the head and the shield immediately behind dark brown. They make their way to the calyx end of the fruits and gradually ,eat their way to the core. The entrance hole can always be detected, and through it the larvae ejects particles of excrement to the exterior, thereby avoiding contamination of its burrow. About midsummer they eat their way out of the fruit, and if the latter are still on the tree the larvae crawl down until they reach the trunk. In the case of fallen fruit the larvae make their way back to the trees and crawl up the trunk. In either case when the trunk is reached they spin cocoons among loose bark, moss or lichens, and there remain dormant .until the following spring, when they turn to the pupa and shortly afterwards give rise to the next generation of Moths. In a few instances two broods have been noticed to occur in one year. The attacks of this Insect cause the fruit to fall prematurely or decay rapidy when stored. As a remedial measure all loose bark,, moss, etc., should be scraped off the trunks, and artificial shelters in the form of one or more bands of loose straw or old sacking should be tied round the trees, not very far from the ground. It is safest to do this in June, and the bands can be examined at leisure during the winter and burnt. By this means large numbers of the cocoons containing the larvae are often destroyed. Fallen apples should be cleared away as soon as possible. Lofts and rooms utilised for storage should be well swept out, and the walls, floors, shelves, and window frames lime-washed. In severe attacks spraying the fruit-bearing portions of the trees with arsenate of lead is advisable, and should be carried out a few days after the petals have fallen. The larvae have to eat the coating of this mixture in order to make their way into the calyx, and are poisoned thereby, if the application has been successful.

The order Hymenoptera is characterised by the presence of (1) two pairs of transparent wings provided with relatively few veins, 2) biting and sucking mouth organs, and (3) complete metamorphosis. The Sawflies are the only group that directly concerns us, and they may be easily. separated from other Hymenoptera by the absence of a " waist," or constriction of the body. The Gooseberry Sawfly (Nematus ribesii) (4), (ro), is very destructive to red currants and gooseberries, but seldom harms black currants. The perfect Insects appear in April and May; they have yellow bodies marked with black, and measure about 3/4in. in wing expanse. The eggs are laid in neat rows along the wins on the undersides of the leaves of the host plant. They hatch into bluish-green caterpillars spotted with black, and also marked laterally with blue and yellow. Unlike Moth caterpillars they possess ten pairs of feet, and when fully grown measure about long. The bushes may be very quickly stripped of their foliage by these larvae, and the fruit are not exempt from attack. About the beginning of June, they enter the soil beneath or near the bushes in order to spin their brown papery cocoons within which the pupal stage is passed. From ten to twenty-one days, according to the temperature, are passed beneath the ground until the flies emerge, and there are three broods during the year. The autumn larvae. pass the winter in their cocoons, turning to pupae early the following spring. Leaves bearing the eggs of this Insect should be destroyed whenever met with, while hand picking is an effective means of getting rid of the larvae if done thoroughly. In the autumn wholesale removal of the surface soil beneath and around the bushes to a depth of five inches is valuable. It needs to be buried in a deep hole dug for the purpose. By this means the winged Insects are buried beneath the earth and perish on emergence from the pupae. Fresh soil and manure should be placed round the bushes. Spraying with arsenate of lead is an effective poison for the larvae, and can be applied any time they are abundant after the fruit has been gathered.

The Thysanoptera form a very small order of Insects, comprising only those minute forms which arc known as " Thrips." They arc provided with four strap-like wings with long " fringes" all round, and are entirely suctorial in their feeding habits. The Pea Thrips (Kakothrips robustus) (11) is a dark brown Insect, about 1 1/4in. long, attacking edible peas and broad beans, often causing much damage. The adults occur from May until August, and the eggs are laid within the flowers on the stamen sheath or on the young developing pods. The larvae resemble the adults with the .exception of having no wings; when fully fed they descend to the ground, penetrating to a depth of 3-12in. They remain in the soil until spring, when the adults emerge from the pupae, there being thus only one brood in the year. Both the larvae and the adults are injurious, and in bad attacks no pods are formed or are curled and undersized. The terminal buds and shoots may also be infested, and damage is stated to be most severe in light soil. This Insect sometimes spoils a whole crop, and no varieties appear to be immune, but it has not so far been found on sweet peas or scarlet runner beans, though they are mentioned as host plants in France. Control is difficult to achieve, but early sown plants are less severely attacked. Spraying is useless when the insects are in the flowers as it does not reach them in that situation. When they feed openly on the pods in large numbers, spraying by means of contact insecticides is then likely to be effective. A mixture of 1lb. soft soap in 10 gallons of water is a cheap remedy and worthy of a trial, or better still 3 lbs. tobacco powder (or i lb. of Voss° tobacco extract), lb. soft soap in to gallons of water. Treatment of the soil during the winter does not offer much promise on account of the depth to which the Insect descends.

Coleoptera or Beetles are characterised by the anterior pair of wings being modified to form horny sheaths which usually cover the upper side of the abdomen. They are exclusively biting in their feeding habits and pass through a direct metamorphosis. Wireworms (12) are the larva-of Click Beetles (family Elateridh) and are known to attack almost any kind of crop. They are more especially pests of the agriculturalist, though tomatoes, strawberries, potatoes, and other vegetables arc liable to suffer injury when grown in gardens or allotments. Three years and even more are believed to be spent in the larval stage and, owing to their resistant coats, these larv,w are notoriously difficult to destroy, no effective remedy having so far been discovered. Lures in the form of slices of potato, carrot, or beet buried an inch or more beneath the soil when Wire-worms are prevalent, often attracts considerable numbers which can then be readily destroyed. The lures should be examined twice a week and the spots marked with pegs. In bad infestations crude powdered naphthaline dug well into the soil in the autumn or early spring is worthy n f trial. Gas lime, lime, or salt are of very little use. In the case of a badly infested potato crop there is no remedy beyond digging it up. Infested soil should be well turned over, exposed, and broken up. Birds then have easy access to the Wireworms and material benefit is often derived by adopting this measure.

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