By far the most destructive insects in warm climates belong to the Hemiptera, especially to the Coccidae or scale insects. All fruit and forest trees suffer from these curious insects, which in the female sex always remain apterous and apodal and live attached to the bark, leaf and fruit, hidden beneath variously formed scale-like coverings. The male scales differ in form from the female; the adult male is winged, and is rarely seen. The female lays her eggs beneath the scaly covering, from which hatch out little active six-legged larvae, which wander about and soon begin to form a new scale. The Coccidae can, and mainly do, breed asexually (parthenogenetically). One of the most important is the San José scale (Aspidiotus perniciosus), which in warm climates attacks all fruit and many other trees, which, if unmolested, it will soon kill (fig. 4). These scales breed very rapidly; Howard states one may give rise to a progeny of 3,216,080,400 in one year. Other scale insects of note are the cosmopolitan mussel scale (Mytilaspis pomorum) and the Australian Icerya purchasi. The former attacks apple and pear; the latter, which selects orange and citron, was introduced into America from Australia, and carried ruin before it in some orange districts until its natural enemy, the lady-bird beetle, Vedalia cardinalis, was also imported.
| Fig. 4.—San José Scale (Aspidiotus perniciosus). A, Male scale insect; B, female; C, larva; D, female scale; E, male scale. |
After the Coccidae the next most important insects economically are the plant lice or Aphididae. These breed with great rapidity under favourable conditions: one by the end of the year will be accountable, according to Linnaeus, for the enormous number of a quintillion of its species. Aphides are born, as a rule, alive, and the young soon commence to reproduce again. Their food consists mainly of the sap obtained from the leaves and blossom of plants, but some also live on the roots of plants (Phylloxera vastatrix and Schizoneura lanigera). Aphides often ruin whole crops of fruit, corn, hops, &c., by sucking out the sap, and not only check growth, but may even entail the death of the plant. Reproduction is mainly asexual, the females producing living young without the agency of a male. Males in nearly all species appear once a year, when the last female generation, the ovigerous generation, is fertilized, and a few large ova are produced to carry on the continuity of the species over the winter. Some aphides live only on one species of plant, others on two or more plants. An example of the latter is seen in the hop aphis (Phorodon humuli), which passes the winter and lives on the sloe and damson in the egg stage until the middle of May or later, and then flies off to the hops, where it causes endless harm all the summer (fig. 5); it flies back to the prunes to lay its eggs when the hops are ripe. Another aphis of importance is the woolly aphis (Schizoneura lanigera) of the apple and pear: it secretes tufts of white flocculent wool often to be seen hanging in patches from old apple trees, where the insects live in the rough bark and form cankered growths both above and below ground. Aphides are provided with a mealy skin, which does not allow water to be attached to it, and thus insecticides for destroying them contain soft soap, which fixes the solution to the skin; paraffin is added to corrode the skin, and the soft soap blocks up the breathing pores and so produces asphyxiation.
| Fig. 5.—The Hop Aphis (Phorodon humuli). A, Winged female; B, winged male; C, ovigerous wingless female; D, viviparous wingless female from plum; E, pupal stage. |
Amongst Orthoptera we find many noxious insects, notably the locusts, which travel in vast cloud-like armies, clearing the whole country before them of all vegetable life. The most destructive locust is the migratory locust (Locusta migratoria), which causes wholesale destruction in the East. Large pits are dug across the line of advance of these great insect armies to stop them when in the larval or wingless stage, and even huge bonfires are lighted to check their flight when adult. So dense are these “locust clouds” that they sometimes quite darken the air. The commonest and most widely distributed migratory locust is Pachytylus cinerascens. The mole cricket (Gryllotalpa vulgaris) and various cockroaches (Blattidae) are also amongst the pests found in this order.
Of Neuroptera there are but few injurious species, and many, such as the lace wing flies (Hemerobiidae), are beneficial.
The Treatment of Insect Pests.—One of the most important ways of keeping insect pests in check is by “spraying” or “washing.” This method has made great advances in recent years. All the pioneer work has been done in America; in fact, until the South-Eastern Agricultural College undertook the elucidation of this subject, little was known of it in England except by a few growers. The results and history of this essential method of treatment are embodied in Professor Lodemann’s work on the Spraying of Plants, 1896. In this treatment we have to bear in mind what the entomologist teaches us, that is, the nature, habits and structure of the pest.
For insects provided with a biting mouth, which take nourishment from the whole leaf, shoot or fruit, the poisonous washes used are chiefly arsenical. The two most useful arsenical sprays are Paris green and arsenate of lead. To make the former, mix 1 oz. of the Paris green with 15 gallons of soft water, and add 2 oz. of lime and a small quantity of agricultural treacle; the latter is prepared by dissolving 3 oz. of acetate of lead in a little water, then 1 oz. of arsenate of soda in water and mixing the two well together, and adding the whole to 16 gallons of soft water; to this is added a small quantity of coarse treacle. For piercing-mouthed pests like Aphides no wash is of use unless it contains a basis of soft soap. This soft-soap wash kills by contact, and may be prepared in the following way:—Dissolve 6 to 8 ℔ of the best soft soap in boiling soft water and while still hot (but of course taken off the fire) add 1 gallon of paraffin oil and churn well together with a force-pump; the whole may then be mixed with 100 gallons of soft water. The oil readily separates from the water, and thus a perfect emulsion is not obtained: this difficulty has been solved by Mr Cousin’s paraffin naphthalene wash, which is patented, but can be made for private use. It is prepared as follows:—Soft soap, 6 ℔ dissolved in 1 quart of water; naphthalene, 10 oz. mixed with 1½ pint of paraffin; the whole is mixed together. When required for use, 1 ℔ of the compound is dissolved in 5 to 10 gallons of warm water.
These two washes are essential to the well-being of every orchard in all climates. Not only can we now destroy larval and adult insects, but we can also attack them in the egg stage by the use of a caustic alkali wash during the winter; besides destroying the eggs of such pests as the Psyllidae, red spider, and some aphides, this also removes the vegetal encumbrances which shelter numerous other insect pests during the cold part of the year. Caustic alkali wash is prepared by dissolving 1 ℔ of crude potash and 1 ℔ of caustic soda in soft water, mixing the two solutions together, adding to them ¾ ℔ of soft soap, and diluting with 10 gallons of soft water when required for use. Another approved insecticide for scale insects is resin wash, which acts in two ways: first, corroding the soft scales, and second, fixing the harder scales to stop the egress of the hexapod larvae. It is prepared as follows:—First crush 8 ℔ of resin in a sack, and then place the resin in warm water and boil in a cauldron until thoroughly dissolved; then melt 10 ℔ of caustic soda in enough warm water to keep it liquid, and mix with the dissolved resin; keep stirring until the mixture assumes a clear coffee-colour, and for ten minutes afterwards; then add enough warm water to bring the whole up to 25 gallons, and well stir. Bottle this off, and when required for use dilute with three times its bulk of warm soft water, and spray over the trees in the early spring just before the buds burst. For mites (Acari) sulphur is the essential ingredient of a spray. Liver of sulphur has been found to be the best form, especially when mixed with a paraffin emulsion. Bud mites (Phytoptidae, fig. 6) are of course not affected. Sulphur wash is made by adding to every 10 gallons of warm paraffin emulsion or paraffin-naphthalene-emulsion 7 oz. of liver of sulphur, and stirring until the sulphur is well mixed. This is applied as an ordinary spray. Nursery stock should always be treated, to kill scale, aphis and other pests which it may carry, by the gas treatment, particularly in the case of stock imported from a foreign climate. This treatment, both out of doors and under glass, is carried out as follows:—Cover the plants in bulk with a light gas-tight cloth, or put them in a special fumigating house, and then place 1 oz. of cyanide of potassium in lumps in a dish with water beneath the covering, and then pour 1 oz. of sulphuric acid over it (being careful not to inhale the poisonous fumes) for every 1000 cub. ft. of space beneath the cover. The gas generated, prussic acid, should be left to work for at least an hour before the stock is removed, when all forms of animal life will be destroyed.
| Fig. 6.—Bud Mites (Phytoptidae). A, Currant Bud Mite (Phytoptus ribis); B, Nut Bud Mite (P. avellanae). |